1. ACCELERATOR LABORATORY

1.1. PREFACE

During 1998 our researchers have successfully pursued established lines of scientific activity: materials physics using ion beam and plasma accelerator techniques, computational, laser, molecular and nuclear physics studies. The accelerator team has focused on the understanding of basic ion-solid interactions, processing and applications of semiconductor and optical technology materials. The goal is to develop new processes and materials using the potentials of ion beam methods.

A central theme was getting insight into the role of surfaces on ion irradiation phenomena. The effect of surface roughness on elastic recoil spectra was studied using ion beam and electron and atomic force spectroscopy. This led to development of a simulation method by which the effect of surface roughness on spectra can be accurately examined and controlled. Atomistic simulation methods revealed unexpected aspects in the microscopic mechanisms of irradiation damage production at surfaces. In semiconductors, a superlinear dependence of defect production on energy was demonstrated at energies just above the damage threshold, while in metals shear displacement of atoms towards the surface was observed for the first time. Furthermore, grain formation at the surface and diffusion of ohmic metal contact materials, diffusion mechanisms and lattice location of dopant atoms were studied in II-VI compound semiconductors. In an effort to understand the processes between hydrogen isotopes and the wall material of a fusion reactor, the diffusion of deuterium in diamond-like carbon films was investigated. A model to describe the experimental diffusion profiles was created.

As a recognition of their work with amorphous diamond coatings, the biomaterials group was elected as a member of the National Centre of Excellence by the Academy. The non-linear laser physics team has strived for a better knowledge of interactions between light and matter, stabilization and noise properties of laser light. The molecular physics group has investigated molecular crystals by Raman spectroscopy and developed numerical methods. Molecular calculations have been used to predict and model properties of materials and intermolecular interactions.

Our technical staff has accomplished the installation of the new helium ion source in the main accelerator. The test runs completed by the end of the year showed the expected performance. The higher energy helium beams now feasibly open up new interesting prospects, e.g., in materials analysis. In connection with the current AMS project, the stripper gas pumping and beam throughput in the accelerator terminal have been improved.

Eero Rauhala

1.2. LABORATORY OPERATION AND DEVELOPMENT

Raimo Ingren, Heikki Sepponen, Birger Ståhlberg, Pertti Tikkanen and Kim Wahlström

The 5MV EGP-10-II tandem accelerator and the 2.5 MV VdG accelerator operated reliably through the period covered by this annual report. The EGP and the VdG completed their 16th and 41st year in service, respectively.

A couple of months shutdown of the EGP was required because of a major maintenance and a reconstruction work. This work was carried out during May 5th and August 24th. In addition to normal maintenance work the following tasks were completed:
- The sputtering source side of the EGP injector was reconstructed; in front of the injector magnet a new electrostatic lens and an electrostatic deflector chamber with three input ports were installed. Now the possible number of ion sources of the EGP is increased to four.
- A new He ion source (Alphatross manufactured by NEC) was installed to the injector of the EGP. The other ion sources are the old off-axis duoplasmatron and the old sputtering ion source.
- The gas pressures and the mechanical dimensions of the HV-terminal stripper channel were measured in order to obtain base data for the reconstruction of the stripper.
- An electronic equipment shield box was installed to the high voltage terminal. The box is made of copper plated 5 mm steel and it makes possible to use sensitive electrical devices in the hostile environment of the terminal.
- An electrostatic steerer was installed into the high voltage terminal after the stripper tube.
- The wiring and power distribution system of the high voltage terminal was mainly rebuilt; the new wiring is installed inside EMI screening conduits.

The VdG required altogether 25 days of maintenance. Repairs of the analyzing magnet power supply and the charging power supply were carried out. The VdG pressure tank was opened four times during the year. In April (5 days shutdown), in May (4 days shutdown) and in June (2 days shutdown) the openings were due to the service of the ion source. In November the opening (8 days shutdown) was due to pressure tests of the tank and the peripheral equipments.

The EGP and the VdG provided total beam times of 1320 h and 1356 h, respectively. The utilization of the accelerators per month is shown in Fig.1. In July the EGP was shut down due to the annual vacations of the personnel. The distribution of the beam hours divided into 0.5 MV intervals of the terminal voltage of the accelerators is shown in Fig. 2.

The beams provided by the duoplasmatron, the alphatross and the sputtering ion sources of the EGP are summarized in Table 1. At the VdG protons were accelerated for 51 %, deuterons for 2 % and 4He+ for 47 % of the total beam time.

Some technical improvements were directed to the upgrading of the laboratory’s isotope separator. The vacuum system of the isotope separator was connected to the automation system of the device. A new noise isolation wall was built between the isotope separator facilities and the laser laboratory. The operator control point of the isotope separator was moved to the EGP accelerator main control room mainly due to the high noise levels close to the separator.

In the sample production the isotope separator ion collection energy was varied from 100 eV up to 120 keV. Altogether 314 samples were produced from 25 different isotopes.
 
 


 
 

Fig.1. Utilization per month of the EGP-10-II and the VdG accelerators based on the charging system timers
 


 
 

Fig.2. Distribution of the beam hours divided into 0,5 MV intervals of the terminal voltage
 

Table 1. Beam hours for various ions from EGP-10-II
 
 
Ion Ion source a Beam hours Percentage
p D 131 9.9
4He A 10 0.8
6Li SP 50 3.8
7Li SP 212 16.0
11B SP 50 3.8
12C SP 101 7.6
14N D 61 4.6
15N D 50 3.8
19F SP 71 5.3
35Cl SP 40 3.1
48Ti SP 40 3.1
63Cu SP 10 0.8
81Br SP 10 0.8
127I SP 423 32.0
197Au SP 61 4.6

____________________________________________________
ª  D= Duoplasmatron, A = Alphatross, SP = Sputtering
 

COMPUTER SERVICES
Sisko Vikberg, Kai Arstila, Marcus Gustafsson, Jura Tarus and Kai Nordlund.

During the recent year, the laboratory computer system grew both in the number of client machines and performance power (our server was upgraded to a 450 MHz dual-PII). An installment of a NetUPS to secure the server in case of power failure was done. A new Lexmark printer which does double-sided printing was acquired to decrease paper spending. The old 10 Mb/s connection to the outside world was also replaced by a 155 Mb/s ATM. In conjunction with this new xylan-switches were installed.

Another improvement was the replacing of the old DOS/Win3.x-operating systems with NT. The primary OS which all the client machines run is still the freely distributable Linux operating system (currently Red Hat 5.1). Samba software is used to enable the NT-machines to use the Linux as a fileserver (35 GB soft-RAID).

The client machines (30 PC's ranging from 120 MHz Pentiums:s to 450 MHz dual-PII) are connected through a 100 Mb/s network connection to our Linux-server ``beam'' (450 MHz dual-PII) and also to four separate computers (200 MHz dual-PPro:s) devoted only to heavy numerical simulations and capable of making use of parallell programming.

Most of the application software is free and collected from the Internet. The word-processing is done by using the TeX/LaTeX program (and to a minor extent StarOffice and WordPerfect). Commercial software includes programs such as Mathematica, Matlab and MicroStation.

The main programming languages used are C, Fortran and different script-languages.

The fast network connections allow us to use the computer resources at the Physics Computation Unit (their DLT-drive is used for making the daily system backup), the Computer Center of the University and the Center of Scientific Computing of Finland.

The machines use ssh (Secure Shell) for secure communication.

The address of our WWW-server is http://beam.helsinki.fi/.
 
 

PUBLIC RELATIONS ACTIVITY
Birger Ståhlberg and Kai Nordlund

The Department of Physics has continued the policy to invite students leaving the upper secondary school to visit the laboratories of the Department. In addition, an opportunity to get acquainted with working in a research institute has been offered to a few interested students in the last form of lower secondary schools.

In 1998 the Kumpula premises of the Accelerator laboratory were visited by 40 groups, 700 guests in all. Thirty-four groups were from higher or lower secondary schools, 2 groups from technical colleges and 4 groups from the University of Helsinki.
 

1.3. NUCLEAR PHYSICS

SHORT LIFETIMES IN sd-SHELL NUCLEI FROM KINEMATICALLY INVERTED (d,p) REACTIONS
Aki Kangasmäki, Pertti Tikkanen, Juhani Keinonen and S. Raman*

The deuteron stripping reactions have been one of the major tools of the nuclear structure physics in the past. On the other hand, the utilization of the same reactions in gamma spectroscopy has not been equally popular because of the high gamma yield from the contaminating deuteron-induced reactions on materials surrounding the target. This difficulty can be overcome to a large extent by the use of inverted kinematics in the deuteron stripping reactions, combined with carefully prepared implanted targets. Reactions on only the desired projectile-target combination can be achieved by this technique. A nuclear reaction is kinematically inverted in the sense that the roles of projectile and target nuclei are interchanged.

When kinematically inverted reactions are used in conjunction with our improved data analysis, it is possible to detect the desired excited nuclear state in a singles gamma-ray measurement. The data analysis includes (i) determination of the effective angular distribution of the emitted particle - a factor that can significantly affect the gamma-ray line shape, (ii) utilization of both low-  and high-stopping power targets, and (iii) use of experimentally verified electronic stopping powers. These advances in the analysis enables reliable lifetime determination of especially short-lived states (tau < 100 fs) where the literature data is scarce.

Our previous work with the inverted deuteron stripping reactions includes lifetime studies in 28Al, 32P, and 32S, and lifetime studies combined with the (nthermal, gamma) measurements in 15N and 20F. Presently, ongoing work is being done with the 2H(23Na,p gamma)24Na and 2H(32S,p gamma)33S measurements, both of which are supplemented by gamma-ray energy and branching ratio measurements done by the (nthermal, gamma) reaction at ORNL. The 24Na and 33S nuclei are especially suitable testing laboratories for the shell model calculations because of the high level densities.

The present work is aimed partially to supplement the best level energy and gamma-ray branching ratio measurements available. With the most accurate level energies, branching ratios and lifetimes at hand, the testing of the predictions of the nuclear shell model can be done in more detail that has been possible so far.

* Oak Ridge National Laboratory
 

1.4. MATERIALS PHYSICS

RECOILS, FLOWS AND EXPLOSIONS: SURFACE DAMAGE MECHANISMS IN METALS AND SEMICONDUCTORS DURING 1-100 KEV ION BOMBARDMENT
Kai Nordlund*,** Jura Tarus*, Juhani Keinonen*, M. Ghaly** and R. S Averback**

Recent molecular dynamics computer simulations have shown that defect production mechanisms are far richer in physical content than binary collision calculations would suggest. Ever since the very first MD simulations which revealed replacement collision sequences, each new generation of simulations carried to higher and higher energies has led to new mechansisms for producing defects. Furthermore, these simulations have shown that types of atomic bonding and surfaces add an additional new elements in the way damage is created in irradiated materials. With massively parallel machines we have now reached a watershed in radiation damage research in that energies are now attainable that enable direct simulation of experiments.

We examined collision cascades between 30 and 100 keV in a variety of materials, including both metals and semiconductors and looked at events where energetic recoils are initiated in the bulk and by ion implantation.  We showed that a variety of damage mechanisms are possible, starting with simple Frenkel pair production [1], through viscous flow [2], microexplosions [3], and recently for the first time, shear deformation [4]. These simulations were compared with existing experimental data obtained by field ion microscopy, transmission electron microscopy and diffuse x-ray scattering.

* Accelerator Laboratory, P.O. Box 43, 00014 University of Helsinki, Finland
** Materials Research Laboratory, University of Illinois, Urbana, IL 61801, USA

1. J. Tarus, K. Nordlund, A. Kuronen, and J. Keinonen, Effect of Surface on Defect Creation by Self-ion Bombardment of Si (001), Phys. Rev. B 58 (1998) 9907
2. M. Ghaly and R.S. Averback, Effect of Viscous Flow on Ion damage near Solid Surfaces, Phys. Rev. Lett. 72 (1994) 364
3. M. Ghaly, K. Nordlund, and R.S. Averback, Molecular dynamics investigations of surface damage produced by keV self-bombardment of solids, Phil. Mag. A (1998), in press
4. K. Nordlund, J. Keinonen, M. Ghaly, and R.S. Averback, Coherent displacement of atoms during ion irradiation, Nature 398 (1999) 49
 

INVERSE KIRKENDALL MIXING IN COLLISION CASCADES
Kai Nordlund*,** and R. S Averback**

The forced mixing of atoms across interfaces is of both technological and scientific interest for ion implantation in layered structures, precipitation dissolution in reactor materials, and processing of materials by high strain conditions. Theoretical approaches to treating materials under such driving conditions has recently been reviewed by Martin and Bellon. An as yet unknown element in these formulations, however, is an atomistic description of the mixing events at the interface. Such a description, specifying the configuration of mixed atoms and any defects that are created, is important as these details strongly influence the resulting microstructures. The kinetic roughening of interfaces is for instance expected to be sensitive to the average number of atoms forced to cross the interface as a result of a single cascade.

We studied ion beam mixing of metallic bilayer interfaces using classical molecular dynamics simulations of 5 keV collision cascades in the vicinity of Co/Cu and Ni/Cu (111) interfaces [1]. We found that the production of vacancies and interface roughening is asymmetrical. On average, more Cu is introduced into the Co or Ni parts than vice versa, and more vacancies are produced in the Cu, indicative of an inverse Kirkendall effect in collision cascades. The effect was explained by the difference in melting points leading to different recrystallization rates of the two materials.

* Accelerator Laboratory, P.O. Box 43, 00014 University of Helsinki, Finland
** Materials Research Laboratory, University of Illinois, Urbana, IL 61801, USA

1. K. Nordlund and R.S. Averback, Inverse Kirkendall mixing in collision cascades, Phys. Rev. B 59 (1999) 20
 

THE ROLE OF SELF-INTERSTITIAL ATOMS ON THE HIGH TEMPERATURE PROPERTIES OF METALS
Kai Nordlund*,** and R. S Averback**

It is well known that vacancies are the predominant point defect in metals at elevated temperatures. Most diffusion-mediated properties of metals are indeed explained with reasonable accuracy solely in terms of vacancy mechanisms. Less attention, outside the field of radiation damage, is usually paid to the other point defect, the self-interstitial atom. The high mobility of this defect and its large relaxation volume and diaelastic effect, however, may give interstitials influence on the properties of metals far beyond what their concentrations might suggest. For example, the curvature observed in Arrhenius plots of self-diffusion is usually attributed to divacancies, but it has also been suggested for Cu and Pt that interstitial diffusion may be the underlying cause [1]. Central to these questions about the influence of interstitial atoms on materials properties is therefore obtaining a reliable estimate of their equilibrium concentrations.

The hope for experimentally measuring interstitial concentrations is not promising, primarily because vacancies are always present in far greater numbers, dominating measurements. On the other hand, modern semiempirical interatomic potentials have been found to be rather reliable in predicting point defect properties in metals. We thus turned to computer simulations using molecular dynamics (MD) to calculate the concentration of interstitials and assess their influence on high-temperature properties. Using these methods we found that the concentration of interstitials at the melting temperature in metals like Cu is surprisingly high, $\sim 1 \times 10^{-6}$ at equilibrium and even higher during rapid heating, with implications for several aspects of material behaviour [2].

* Accelerator Laboratory, P.O. Box 43, 00014 University of Helsinki, Finland
** Materials Research Laboratory, University of Illinois, Urbana, IL 61801, USA

1. A.V. Granato, Interstitialcy Model for Condensed Matter States of Face-Centered-Cubic Metals, Phys. Rev. Lett. 68 (1992) 974
2. K. Nordlund and R.S. Averback, The role of self-interstitial atoms on the high temperature properties of metals, Phys. Rev. Lett. 80 (1998) 4201
 

ROLE OF ELECTRON-PHONON COUPLING ON COLLISION CASCADE DEVELOPMENT IN Ni, Pd AND Pt
Kai Nordlund*,** L. Wei**, Y. Zhong** and R. S Averback**

The understanding of ion irradiation effects in solids has increased dramatically in recent years owing in large part to progress in computer simulations of energetic displacement cascades using molecular dynamics (MD) [1]. With the development of realistic interatomic potentials it is now possible to perform MD simulations of cascades in many semiconductors and metals. These potentials, however, are classical representations of atomic collisions in solids, and accordingly they neglect any possible coupling between the excited vibrational system of a material with the conduction electrons.

In some important materials, such as Fe, Ni, Pd and Pt, theoretical considerations suggest that electron-phonon coupling (EPC) should have a strong influence on cascade dynamics [2]. Previous studies have not been able to conclusively determine the magnitude of the coupling, however. By directly comparing ion beam mixing experiments and molecular dynamics simulations of collision cascades in Ni, Pd and Pt, metals in which the coupling is believed to be most important, we showed that the influence of electron-phonon coupling on mixing can be no more than about 30 %, roughly an order of magnitude less than the most widely used models predict [3].

* Accelerator Laboratory, P.O. Box 43, 00014 University of Helsinki, Finland
** Materials Research Laboratory, University of Illinois, Urbana, IL 61801, USA

1. R.S. Averback and T. Diaz de la Rubia, in Solid State Physics, edited by H. Ehrenfest and F. Spaepen (Academic Press, New York, 1998), Vol. 51, pp. 281-402
2. I. Koponen, Atomic mixing in ion-bombardment-induced temperature spikes in metals, J. Appl. Phys. 72 (1992) 1194
3. K. Nordlund, L. Wei, Y. Zhong, and R.S. Averback, Role of electron-phonon coupling on collision cascade development in Ni, Pd and Pt, Phys. Rev. B (Rapid comm.) 57 (1998) 13965
 

CONCENTRATION DEPENDENT DEUTERIUM DIFFUSION IN DIAMOND-LIKE CARBON FILMS
Tommy Ahlgren, Elizaveta Vainonen, J. Likonen* and Juhani Keinonen

Diffusion of deuterium (D) in diamond-like carbon films has been studied. The deuterium concentration profiles in D ion implanted films were measured by Secondary Ion Mass Spectrometry. A model is proposed to describe the experimental depth profiles. In this model it was assumed that atomic D is the diffusing species whereas D in clusters is immobile, leading to concentration dependent diffusion. The diffusion coefficients obtained for atomic D resulted in an activation energy of 2.9 eV. The solid solubility of D was observed to decrease with increasing temperature.

* Technical Research Centre of Finland, Chemical Technology, P.O. Box 1404, FIN-02044 VTT, Finland
 

THE ELECTRONIC STOPPING FROM A 3D CHARGE DISTRIBUTION
Jussi Sillanpää, Kai Nordlund and Juhani Keinonen

Description of the slowing down of energetic ions penetrating matter is a long-standing problem of considerable theoretical and practical interest. Despite much intensive work during the last 80 years, the models describing the slowing down of an ion by collisions with electrons (electronic stopping) may still give results with errors of several tens of percents [1, 2]. We present a model for the electronic stopping at low velocities (v < $v_0$) [2,3]. By using molecular dynamics and calculating the electronic stopping from a 3D charge distribution without using any free parameters, we obtain accurate range distributions on a solid physical basis. Our electronic stopping model is based on the Brandt-Kitagawa (BK) theory, in which the electronic stopping of a heavy ion is the electronic stopping of a proton scaled by the square of the effective charge. We calculate the stoppings of different ions in silicon, for which both accurate electron distributions and experimental range distributions are available. We first test the model for hydrogen ions, to determine whether a basis exists for the scaling hypothesis, and then for heavier ions. The results are compared with experimental range profiles and show good agreement, much better than that achieved by using standard (nonlocal) electronic stopping models. Our model has a sound physical basis, no free parameters and can be used for all ion-target combinations.

1. J. Sillanpää, E. Vainonen-Ahlgren, P. Haussalo and J. Keinonen, Stopping of 5 - 100 keV helium in molybdenum, chromium, copper and nickel, Nucl. Instrum. and Meth. in Phys. Res. B 142 (1998) 1-8
2. J. Sillanpää, K. Nordlund and J. Keinonen, Channeling in Manufacturing Sharp Junctions: a Molecular Dynamics Study, Physica Scripta T79 (1999) 272-274
3. J. Sillanpää, K. Nordlund and J. Keinonen, to be published.
 

HEAT SPIKE AND BALLISTIC CONTRIBUTIONS TO MIXING IN SI
Jura Tarus, Kai Nordlund, Jussi Sillanpää and Juhani Keinonen

Molecular dynamics (MD) simulations have proved to be a valuable tool for examining ion-solid interactions. The reliability of the results of MD simulations depend on the quality of the interatomic potential used. Testing the validity of these potentials for ion-solid interactions is problematic, since few experimentally measurable quantities depend directly on the outcome of collision cascades. One such quantity is the ion beam mixing, which has been measured in a wide range of materials [1]. Recent simulations have shown that mixing caused by very low-energy recoils and heat spike mixing can indeed be significant for atom relocation both in amorphous and crystalline Si due to the nature of the tetrahedral bonding [2]. Therefore comparison between mixing experiments and MD simulations may prove illuminating on the role of ballistic collisions and heat spike on mixing in Si, and serve as a test of the interatomic potential models.

A direct comparison of ion beam mixing experiments and molecular dynamics simulations has not been achieved before because of the high beam energies used in most experiments. We have developed and used a scheme which combines efficient range calculations and simulations of low energy (< 5 keV) collision cascades. A similar method has been succesfully applied for metals [3]. The Tersoff potentital [4] was chosen to model the silicon as it is fitted, in addition to the diamond structure, to over- and under-coordinated configurations.

The simulations reproduce the experimental results to within the uncertainties, showing that the MD simulations give a reasonable description of the ballistic properties of the collision cascade. Most of the experimental mixing was found to derive from high-energetic primary recoils, with at most 1/3 of the mixing deriving from the heat-spike phase of the cascade.

1. See e.g. B. M. Paine and R. S. Averback, Nucl. Instr. and Meth. in Phys. Res. B 7/8 (1985) 666
2. K. Nordlund, M. Ghaly, and R.S. Averback, J. Appl. Phys. 83 (1998) 1238
3. K. Nordlund, L. Wei, Y. Zhong, and R.S. Averback, Phys. Rev. B (Rapid comm.) 57 (1998) 13965-13968
4. J. Tersoff, Phys. Rev. B 38 (1988) 9902
 

LIGHT ELEMENT DIFFUSION IN SiO2 AFTER IMPLANTATION
Marcus Gustafsson and Juhani Keinonen

As an independent continuation on the previous work [1,2] done on SiO2 in collaboration with others*,**, light element diffusion in irradiated a-Si/SiO2 bilayers has been studied.

While the previous work has focused on noble-gas and alkaline-ions, this study focuses on nitrogen in particular, with some references to carbon and oxygen-18.

The physics of the Si-SiO2 interface has always been an important issue in the semiconductor industry and the increased use of opto-electronics and high speed device technology today has made it even more important.

a-Si films of 4-350 nm thicknesses were grown on amorphous SiO2-samples. These samples were then irradiated with energies of 1, 5, 20, 60 and 100 keV with doses of 1015-1017 ions/cm2. The energies were chosen to match the film thicknesses, giving the ion distributions depth ranges both at and slightly deeper than the interfaces.

The samples were measured with time-of-flight recoil detection analysis (TOF-ERDA) and then subsequently annealed to a temperature of 800°C in steps of 200°C.

The measurements accumulated large amounts of data that will be used to thoroughly study the diffusion mechanisms at work.

* Zweites Physikalisches Institut, Universität Göttingen, Bunsenstr. 7-9, D-37073 Göttingen, Germany
** Institut für Strahlenphysik, Universität Stuttgart, Allmandring 3, D-70569 Stuttgart, Germany

1. M. Schwickert, K.P. Lieb, W. Bolse, M. Gustafsson, J. Keinonen, Nucl. Instr. and Meth. in Phys. Res. B147 (1999) 238-243
2. K.P. Lieb, M. Schwickert, W. Bolse, M. Gustafsson, J. Jokinen, J.Keinonen, Nucl. Instr. and Meth. in Phys. Res. B148 (1999) 951-956
 

EFFECTS OF SURFACE ROUGHNESS AND MULTIPLE SCATTERING ON RESULTS IN ELASTIC RECOIL DETECTION MEASUREMENTS
Timo Sajavaara, Kai Arstila, Antti Laakso* and Juhani Keinonen

In ion beam analysis the sample surfaces are usually assumed to be flat. This assumption is not often true since for instance film growth, oxidation, ion beam irradiation and sample preparation may result a rugged surface. Since elastic recoil detection analysis (ERDA) is performed using small angles relative to the surface, a rough surface leads to misinterpretations of film thicknesses and atomic concentrations.

To study multiple scattering in ERDA a Monte Carlo simulation program was written. The program was tested by comparing simulated results to experimental ones. The results agreed well and the multiple scattering effects in the sample and detector system could be reproduced.

*Laboratory of Physics, Helsinki University of Technology, 02150 Espoo, Finland
 

DETECTION EFFICIENCY OF TIME-OF-FLIGHT ENERGY ELASTIC RECOIL DETECTION ANALYSIS SYSTEMS
Yanwen Zhang*, Harry J. Whitlow*, Thomas Winzell*, Ian F. Bubb**, Timo Sajavaara, Kai Arstila and Juhani Keinonen

The detection efficiency of recoils with masses ranging from H up to Nb at energies from 0.05 - 1 MeV per nucleon has been investigated for Time-of-Flight Energy Elastic Recoil Detection (ToF-E ERD) systems. It is observed that the detection efficiency for the ToF-E detector telescope depends on the stopping power in the carbon foils, which in turn relies upon the recoil mass and energy. Furthermore, the limits of this behaviour depend on the setting of the discriminator thresholds. The detection efficiency of a time detector could be fitted to a universal curve that can be described by a simple empirical formula as a function of recoil electronic stopping power in the carbon foil. This formula can be used to predict the detection efficiency by recoil energy for N, O and other elements, for which it may not be easy to prepare suitable reference samples containing only that element.

*Department of Nuclear Physics, Lund Institute of Technology, Box 118, S-22100 Lund, Sweden
**Department of Applied Physics, Royal Melbourne Institute of Technology, GPOBox 2476V, Melbourne 3001, Australia
 

ENERGY AND ANGULAR DISTRIBUTIONS OF SCATTERING CROSS SECTIONS AND OPTICAL MODEL PARAMETERS
Arto Nurmela, Petteri Pusa, Eero Rauhala and Jyrki Räisänen*

Elastic scattering cross section measurements of 6Li and 7Li ion scattering by natural aluminium, silicon and titanium have been completed [1]. The measurements were performed at scattering angles of 140° and 170° for energies below 12 MeV. The threshold energies where the scattering cross section deviates 4 % from Rutherford value have been determined. The obtained results have been compared with available literature data. The models predicting the thresholds yield systematically lower values than our experimental data.

The series of scattering cross section experiments has been continued by determining the forward recoil cross sections for the reactions 4He(Ni,4He)Ni, 7Li(Ni,7Li)Ni, 11B(Ni,11B)Ni and 12C(Ni,12C)Ni at scattering angles of 20°, 30° and 40°. The recoil cross sections were obtained by measuring the corresponding backscattering reactions and reversing them kinematically. The cross section energy ranges were from 4 to 130 MeV and from 2 to 25 MeV, respectively. The threshold energies were determined for the forward and backscattering reactions.

A new optical model code was developed for the analysis. The experimental cross sections were used to determine optical model parameters. With the code the angular and energy distributions of scattering cross sections can be predicted.

* University of Jyväskylä, Department of Physics, P.O. Box 35, 40351 Jyväskylä, Finland

1. A. Nurmela, E. Rauhala and J. Räisänen, Nucl. Instrum. and Meth. in Phys. Res. B, in press
 

1.5. APPLIED PHYSICS

THERMAL STABILITY OF THE Au/ZnSe HETEROSTRUCTURE
Jonatan Slotte, Antti Laakso*, Reeta Salonen, Tommy Ahlgren, Eero Rauhala, Jyrki Räisänen, Juhani Keinonen, P. Uusimaa**, A. Salokatve**, M. Pessa**, A. Simon+, I. Uzonyi+, A.Z. Kiss+ and E. Somorjai+

Gold is a promising contact material for compound semiconductors, such as ZnSe, and it is therefore essential to study the thermal stability of the Au/ZnSe heterostructure.

We applied Rutherford backscattering spectrometry (RBS) and channeling measurements to study the diffusion of gold in ZnSe. The samples were prepared at Tampere University of Technology. A thin gold layer (10 - 100 nm) was deposited onto a 1.4-2.0 mm thick molecular beam epitaxy grown ZnSe layer. The samples were then annealed in argon atmosphere in the temperature interval 400-800 °C and gold depth profiles were determined from the annealed samples with 4He and 12C ion backscattering. The crystal quality of the ZnSe was investigated with 4He channeling. We found that gold diffusion in ZnSe depended on the crystal quality of the ZnSe, the activation energy for the diffusion was 1.7 eV [1].

Atomic force microscopy (AFM) and scanning electron microscopy studies of the samples showed that gold grains were formed on the surfaces of annealed samples. Since such grains can make the interpretation of RBS spectra difficult we developed a novel analysis method, with which we were able to determine correct depth profiles from the annealed samples. AFM was used to determine the sample topography and this information was used to calculate the influence of the grains on the RBS spectra. The results were verified by measuring RBS spectra from grain and grain free areas of the samples with a nuclear microprobe at the Institute of Nuclear Research in Debrecen.

This work has been partly funded by the Academy of Finland under the EPI-2 project.

* Laboratory of Physics, Helsinki University of Technology, P.O. Box 1100, FIN-02015 HUT, Finland
** Laboratory of Physics, Tampere University of Tehchnology, P.O. Box 692, FIN-33101 Tampere, Finland
+ Institute of Nuclear Research of the Hungarian Academy of Sciences, P.O. Box 51, H-4001, Debrecen, Hungary

1. J. Slotte, R. Salonen, T. Ahlgren, E. Rauhala, J. Keinonen, J. Räisänen, P. Uusimaa, A. Salokatve, M. Pessa, A. Laakso, J. Appl. Phys. 85 (1999) 799-802
 

ANALYSIS OF ZnSe EPITAXIAL THIN FILMS BY ION CHANNELING %
Anni Seppälä, Reeta Salonen, Tommy Ahlgren, Eero Rauhala and Jyrki Räisänen*

Initial efforts to develop short-wavelength diode lasers had focused on zinc selenide (ZnSe) which has a large bandgap needed for blue or green lasing. Much research has been done since to make a commercial blue green laser diode. One crucial task in this work is the preparation of ohmic contacts.

We have studied the lattice sites of contact metal atoms in epitaxial ZnSe films. The ZnSe samples were grown by molecular beam epitaxy at Tampere University of Technology. In this work we have also studied the special features of channeling techniques and applied this knowledge in the analysis of measured results. Especially the analysis of channeling results measured with particle induced X-ray emission technique (PIXE) has been improved.

% This work has been partly funded by the Academy of Finland under the EPI-2 project.

*University of Jyväskylä, Department of Physics, P.O. Box 35, FIN-40351 Jyväskylä, Finland
 

GEOMETRY OF ETCHED ION TRACKS IN POLYMERS
Eero Rauhala, Anni Seppälä, Jiri Vacik*, Vratislav Perina* and Vladimir Hnatovicz*

Irradiation of polymers with high energy ion beams causes formation of tracks in the material. Chemical etching of irradiated polymers has been used to produce nuclear membranes with desired properties. Etched tracks have many practical applications in environmental, chemical and biological sciences.

By ion transmission measurements, information on the pore geometry can be found. We have measured the transmission of 12C ions through the irradiated and etched polycarbonate foils. The information on the geometry can be obtained by simulations of particle transmission through the system of interlacing etched tracks.

*Nuclear Physics Institute, CZ-250 68 Rez near Prague, Czech Republic
 

AMORPHOUS DIAMOND FILM DEPOSITION
Asko Anttila, Reijo Lappalainen, Marko Hakovirta, Panu Pekko, Veli-Matti Tiainen and Esa Alakoski

Amorphous diamond films (sp3 bonding fraction about 80 %) have been deposited using plasma accelerator methods. New set-up allows simultaneous deposition with two pulsed arc discharge units. Furthermore, sputtering unit can be used to deposit proper intermediate layers. The optimum conditions and multilayer structures to achieve perfect corrosion and wear resistance for any metal are developed and used for long-term corrosion and wear testing. Films have been deposited for tribological laboratory tests and industrial applications. Promising results with the amorphous diamond coatings have been achieved, e.g. in medical implants and cutting tools. By using AD coating, the wear of conventional hip joint materials in hip simulator has been decreased by a factor of a million. The steady state wear rate of the coated material is practically zero and the corrosion rate is at least four orders of magnitude lower than for the best metallic implant materials.
 

GROWTH AND MODIFICATION OF LUMINESCENT THIN FILMS USED IN FULL-COLOR ELECTROLUMINESCENT DISPLAYS BY USING ION BEAM METHODS

Reijo Lappalainen, Juhani Keinonen, M. Leskelä*, M. Ritala*, W.-M. Li*, Milja Karjalainen, Ritva Serimaa, Timo Sajavaara and Kai Arstila

SrS-based materials are widely developed for the blue phosphor in full-color thin film electroluminescent displays. At present the blue colour is achieved using a filter with the sacrifice of the luminescence intensity. Large efforts have been devoted to improve the luminescence efficiency and chromacity of SrS:Ce and SrS:Cu films prepared by various deposition techniques.

In this study, ion implantation is used to add dopants and impurities in SrS:Ce and SrS:Cu based EL structures in a well-controlled way. The implanted ions include F, Na, Al, P, Cl, K, Cu, Ga, Ag and Ce. We have found out that e.g. by optimised F, K or Ag implantation photoluminescence characteristics of these ALE-grown thin films can be significantly improved. Furthermore, these properties were related to diffusion profiles of dopants and microstructural changes determined by nuclear methods and X-ray diffraction.

* Department of Chemistry, P.O. Box 55, University of Helsinki, FIN-00014 Helsinki, Fin-land
 

AMORPHOUS OR NANOCRYSTALLINE THIN FILMS AND MULTILAYERS
Pauli Torri , Reijo Lappalainen and Juha-Pekka Hirvonen*

We have studied nanocrystalline ceramics in the form of thin films consisting of homogeneously mixed components or multilayers. The materials include Me-MeOx, SiC-Si3N4 and Me-Si-(C,N) ceramics prepared by sputtering or pulsed arc discharge. These methods provide a powder free route for the preparation of specimens with a homogeneous distribution of constituents and give flexibility in the choice of the chemical composition. By the control of crystallization and microstructure it is possible to achieve materials with superior properties compared to normal bulk materials.
We have shown that layer thicknesses and microstructure determined by deposition parameters and heat treatment has an important role in oxidation behaviour, thermal stability and mechanical strength. Excellent stability in thin film structures was achieved using nitrogen.

* European Commission, Joint Research Centre, Institute of Advanced Materials, 1755 LE Petten, Netherlands
 

MECHANICAL TESTING AND EVALUATION OF MATERIALS USED IN ORTHOPAEDIC SURGERY
Reijo Lappalainen, Y.T. Konttinen*, Sanna Lehti, Panu Pekko, S. Santavirta** and Asko Anttila

Both artificial and natural materials are commonly used in orthopaedic surgery. Especially, in the case of load bearing applications, e.g. in spine and neck, mechanical strength is a very relevant issue. Recently, the failure of commercial bonegrafts used in neck surgery has resulted in of four-limb paralysis of a few patients in Finland. Therefore, mechanical characterisation and evaluation of different types of materials in medicine is essential.

We have tested mechanical properties of several types of biomaterials (graphite, bonegrafts, bone cement etc.) and the bonding strength of bone cement using tensile, compressive and creep testers. It has turned out that the mechanical properties of the commercial bonegrafts used for the patients mentioned above deteriorate within a few days leading to a total failure of bonegraft in creep testing. Therefore, new substitutes for the commercial bonegrafts are under study. In the case of bone cement, amorphous diamond coating was found to be very effective to protect the implant surface against ceramic particles commonly used to make bone cement radiopaque. Also in clinical use, this kind of a coating should decrease significantly the amount of wear particles due to micromotion between the implant and the bone.

* Department of Anatomy, P.O. Box 9, University of Helsinki, FIN-00014 Helsinki, Finland
** Department of Orthopedics and Traumatology, Helsinki, University Central Hospital, Topeliuksenkatu 5, FIN-00260 Helsinki, Finland
 

1.6. LASER PHYSICS

We have continued to investigate coherence and noise properties of laser light and to perform laser spectroscopy. Mainly we have utilized diode lasers in the red wavelength range. During the spring and the summer a wall was constructed between the isotope separator facilities and the facilities for laser based research. The wall has shown good ability to effectively damp acoustical disturbances stemming mainly from the isotope separator electrical generator. Such a minimization of external technical noise sources is needed when performing noise measurements of lasers, as described below. Similarly to the previous years the laboratory facilities and equipments have been utilized for educational demonstrations to groups visiting the laboratory. For our research activities we acknowledge financial support from Svenska Kulturfonden and Magnus Ehrnrooth Foundation.

Birger Ståhlberg and Åsa Lindberg
 

ANALYSIS OF EXTENDED CAVITY DIODE LASER FREQUENCY FLUCTUATIONS
Stefan Eriksson, Åsa Lindberg and Birger Ståhlberg

The laser linewidth is an important parameter of a laser. In some cases it is also important to know the lineshape. The primary spectral width and spectral characteristics of a laser’s output are determined by frequency fluctuations. The transmission slope of a Fabry-Pérot interferometer can be used as a means to convert the frequency fluctuations of laser light to amplitude fluctuations of the photocurrent in a photodetector. The power spectral density of the photocurrent is then recorded with a spectrum analyzer. We have applied this method to study the frequency fluctuations of an extended cavity diode laser constructed in the laboratory. The frequency noise of the laser follows a 1/f form up to about 50 kHz. Above this frequency the fluctuations are governed by white noise corresponding to a quantum limited linewidth of a few 100 kHz.
 

SATURATION SPECTROSCOPY OF 20Ne AND 22Ne AT 640 nm
Stefan Eriksson, Åsa Lindberg and Birger Ståhlberg

We have continued the work with the closed 1s5 (J=2) - 2p9 (J=3) transition at 640 nm in neon. In earlier work we constructed an extended cavity diode laser for laser spectroscopy of this transition. During that work we found that at pressures in the order of 100 Pa the attainable laser output power was insufficient to saturate the transition in both isotopes of natural neon gas. We have replaced the conventional DC-discharge with a radio-frequency (RF) discharge as the mechanism to provide population on the excited neon states. With a RF-discharge neon pressures as low as 10-2 Pa can be accessed. At pressures below about 20 Pa the transition becomes saturable with 2 mW of power in the laser beam and we have succesfully carried out saturation spectroscopic measurements with the extended cavity diode laser [1]. The laser can readily be stabilized to the transition by Dopplerfree techniques. Because the atoms interact very strongly with resonant light at this particular transition wavelength, the resonance fluorescence can be used as a guide for wavelength tuning of the laser. Transitions at nearlying wavelengths do not exhibit similar properties. We found that a precision wavemeter was not needed to confirm interaction with the selected transition.

1. S. Eriksson, Å. Lindberg and B. Ståhlberg, submitted to Optics Comm.
 

OBSERVED FACTORIZATION OF EXCESS QUANTUM NOISE THAT IS DUE TO BOTH POLARIZATION AND SPATIAL MODE NONORTHOGONALITY
G.P.Karman*, Åsa M. Lindberg and J.P. Woerdman*

When the eigenmodes of a laser cavity are nonorthogonal, the quantum-limited linewidth of the laser is larger by an excess-noise factor than the standard Schawlow-Townes expression. Mode nonorthogonality can exist in the spatial domain, as in unstable-cavity resonators, but also in the polarization domain when the two polarization eigenmodes are nonorthogonal. We show experimentally [1] that these two contributions are independent of each other, i.e., that the excess-noise factor factorizes as a product of the spatial and polarization excess-noise factors. As a result, one achieves a situation in which a large fraction of the spontaneous emission is channeled into the lasing mode. Such a combination on excess-noise factors may be an alternative route to the long-sought threshold-less laser.

* Huygens Laboratory, Leiden University, P.O.Box 9504, 2300 RA Leiden, The Netherlands

1. G.P.Karman, Å.M. Lindberg and J.P. Woerdman, Optics Letters 23 (1998) 1698-1700
 

MEASUREMENTS OF THE STABILITY OF THE POLARIZATION STABILIZED He-Ne LASERS INCORPORATED IN THE ABSOLUTE GRAVIMETERS OF THE FINNISH GEODETIC INSTITUTE
Birger Ståhlberg and Jaakko Mäkinen*

We have continued to measure the stability of the polarization stabilized two-mode He-Ne lasers incorporated in the gravimeters FG I and FG II. The measurements have been carried out three times with a few months intervals during the year. We have used a primary standard iodine stabilized He-Ne meter laser as a reference in the calibration of the gravimeter lasers. Similarly to the previous years starting from the year 1985 the results show a few MHz (i.e. a few parts in 10-9) differences between the measurements [1].

*The Finnish Geodetic Institute, Masala

1. J. Mäkinen and B. Ståhlberg, Long-term frequency stability and temperature response of a polarization-stabilized He-Ne laser, Measurement, Journal of the International Measurement Confederation 24 (1998) 179-185
 

MINIMIZATION OF LASER SERVO ELECTRONICS SENSITIVITY TO RADIO FREQUENCY FIELDS
Birger Ståhlberg, Stefan Eriksson and Åsa Lindberg

In some laser experiments the utilization of radio frequency (RF) oscillators is unavoidable. Often these oscillators affect the laser control devices; especially the servo electronics of stabilized lasers. Such lasers are, for example, primary standard meter gas lasers and extended cavity diode lasers. These lasers require quite many electronic servo circuits. One of the most sensitive circuits to RF sources is the temperature servo electronics. Particularly the servos based on integrated circuit temperature sensors are extremely sensitive to RF sources. We have performed tests with different sensors and alternative circuit designs of these servos in a RF field environment. The RF power have extended up to a few watts at a frequency of about 250 MHz. We have found that a circuit design based on thermistors connected to precision voltage references and followed by active integration directly after the sensor, is the best alternative to minimize RF field sensitivity.
 

1.7. MOLECULAR PHYSICS

MOLECULAR PHYSICS LABORATORY

The Molecular Physics Laboratory, historically, has evolved around the Raman spectrometer. At present there are three research groups active in the laboratory, headed by  Prof. Folke Stenman, doc. Niklas Meinander, and doc. Berit Mannfors. The research interests of doc. Tom Sundius closely associates him with the laboratory. Dr. Sisko Maria Eskola, doc. Kim Palmö (U. of Michigan) and doc. Lars-Olof  Pietilä (VTT), who did their PhD-theses in this laboratory, are also actively engaged in the research projects of the laboratory. Graduate students working in the laboratory were  Lic. Phil. Olli-Pekka Sievänen, Lic. Phil. Stefan Söderholm, M. Sc. Johanna Blomqvist, M. Sc. Leena Wallenius and M.Sc. Virpi Korpelainen. Three Master's theses were completed in the laboratory during 1998. Several collaborative projects with research groups both in Finland and abroad are going on.

A 2 h/week course presenting the research being done in the laboratory was offered for the third time during the autumn term. Courses on molecular physics, molecular spectroscopy, spectroscopy of liquids, intermolecular forces, molecular modeling, optics and digital spectral analysis are given at more or less regular intervals as part of the senior undergraduate and graduate program of the department.

Niklas Meinander
 

BINARY INTERACTIONS IN DILUTE GASES
Niklas Meinander

The focus of this work is on the empirical interaction energy function and polarizability function of interacting pairs of small symmetric molecules. Experimental data utilized are mainly the interaction-induced light scattering (CILS) spectrum of the gas, the refractivity of the gas, equation of state data and transport properties, such as the viscosity. The molecules under investigation have been the noble gases, mercury, and molecules with tetrahedral and octahedral symmetry, such as CH4 and SF6. The effect of the anisotropy of the intermolecular potential on the refractivity virial coefficient was investigated for CH4 and found to be small [1]. Preliminary calculations were done to model a temperature dependent effective isotropic potential energy function for SF6 [2].

1. Virpi Korpelainen, "Metaanin moolirefraktion toisen viriaalikertoimen orientaatioriippuvuuden laskeminen", Laudaturtyö, 23.1.1998.
2. Ulrika Backman, "Viskositeten och virialkoefficienten för SF6 med temperaturberoende parametrar ", Laudaturarbete 24.11.1998.
 
 

TWO-DIMENSIONAL POTENTIAL ENERGY SURFACES FOR LOW FREQUENCY LARGE AMPLITUDE VIBRATIONS OF SMALL RING MOLECULES
Niklas Meinander

Two-dimensional potential energy surfaces are optimized to the observed vibrational levels of coupled large amplitude vibrational modes of small ring molecules. The potential energy surface determines the equilibrium conformation of the molecule. The molecules are studied both in the electronic ground state and in electronically excited states. This is a collaboration with prof. J. Laane at the Department of Chemistry of Texas A & M University, where the experimental work is carried out.

Two papers on this wortk were published in 1998. [1,2] The ongoing work was presented at several conferences during the year. [3,4]

1. Sachie Sakurai, Niklas Meinander, and Jaan Laane, "Two-dimensional potential energy surface for phtalan: The effect of large coupling on vibrational quantum states''. J. Chem. Phys. 108 (1998) 3537-3542
2. Soo-No Lee, Niklas. Meinander, Paul Sagaer, Deb N. Nath and Jaan Laane, ``Far-infrared spectra and two-dimensional potential energy surfaces for the out-of-plane ring vibrations of tetrahydrofuran-3-one in its S0 and S1(n,B*) electronic states", J. Chem. Phys. 108, (1998) 8884-8890
3. J. Laane, S. Sakurai, T. Klots, N. Meinander, K. Morris, W. Y. Chiang and E. Bondoc, "Vibrational potential energy surfaces for phtalan and 1,3-benzodioxole in their S0 and S1(B,B*) states".  XXIV European Congres on Molecular Spectroscopy, Prague 23-26.8.1998, Book of Abstracts 223
4. Sachie Sakurai, Niklas Meinander, and Jaan Laane, ``Two-dimensional potential energy surface for phtalan: The effect of large coupling on vibrational quantum states''. Proceedings of the XXXII Annual Conference of the Finnish Physical Society, March 19-21, 1998, Tampere, Contribution 2.27
 

RAMAN SPECTROSCOPIC STUDY OF AMORPHOUS SUGARS

Niklas Meinander, Stefan Söderholm and Yrjö Roos

The glass transition in amorphous sugars is studied using Raman spectroscopy. This is a collaboration with Dr. Yrjö Roos at the Department of Food Technology, University of Helsinki (Elintarviketeknologian laitos). The project is part of a large collaboration financed by EU [1] which aims at understanding the role of sugar as a food preservative. Lic. Phil. Stefan Söderholm will write his PhD-thesis on the results of this project.

The Raman spectra of the amorphous phase of glucose and fructose have been measured as a function of the amount of water in the sample [2] and as a function of temperature. [3] The crystallization process of glucose has been studied with Raman microscopy [4]. The project has been presented at various meetings during the year. [4-6]

1. Commission of the European Communities, Agriculture and Fisheries (FAIR) specific RTD programme, CT96-1085, Enhancement of Quality of Food and Related Systems by Control of Molecular Mobility
2. Stefan Söderholm, Yrjö H. Roos, Niklas Meinander, and Matti Hotokka, "Raman spectra of fructose and glucose in the amorphous and crystalline states", to appear in J. Raman Spectrosc.
3. S. Söderholm, Y. H. Roos, N. Meinander, and M. Hotokka, "Characterisation of Biomaterials using FT-Raman Spectroscopy", XIth International Conference on Fourier Transform Spectroscopy, Athens, Georgia, USA, 1997, AIP Conference Proceedings 430, Editor James A. de Haseth, 1998, pp. 316-319
4. S. Söderholm, N. Meinander, Y. H. Roos, and M. Hotokka, "Characterisation of Carbohydrates using FT-Raman Spectroscopy", Proceedings XVIth International Conference on Raman Spectroscopy, September 6-11, 1998, Cape Town, South Africa, Ed. A. M. Heyns, pp. 842-843
5. S. Söderholm, Y. H. Roos, and N. Meinander, "Characterisation of the glass transition in amorphous sucrose using FT-Raman spectroscopy", Proceedings of the XXXII Annual Conference of the Finnish Physical Society, March 19-21, 1998, Tampere, Contribution 2.28
6. S. Söderholm, Y. H. Roos, and N. Meinander, "Characterisation of the effects of water on molecular mobility in amorphous sucrose using FT-Raman spectroscopy", ISOPOW 7, International Symposium on Water Management in the Design and Distribution of Quality Foods, 30th May - 4th June 1998, Helsinki Finland, Book of Abstracts, p. 89, and Proceedings of the Poster Sessions, pp. 104-107

STUDIES OF STRUCTURE BASED PROPERTIES OF LARGE MOLECULES

Johanna Blomqvist, Virpi Korpelainen, Jere Koskinen, Berit Mannfors and Tom Sundius

In this project atomistic simulations (quantum mechanics and force field based methods) are used for prediction of molecular properties based on structure in different types of molecular systems. The results obtained are also utilized in the investigation and development of (spectroscopic and molecular mechanics) force field methods. The research, described below, is divided between two research groups.
 

FORCE FIELD BASED MODELLING

Johanna Blomqvist, Virpi Korpelainen, Jere Koskinen and Berit Mannfors

This project aims at the construction of realistic potential energy functions for force field based simulations of polymers, catalysts, drugs, and biologically active species. The computational methods used are based on quantum mechanics, molecular mechanics, molecular dynamics, and statistical methods like Monte Carlo simulations. Experimental data consist of Raman and infrared vibrational spectra and structure determinations by X-ray crystallography. Major applications are various polymer based materials, protein and drug design. Theoretical calculations have been performed using the facilities at the Center for Scientific Computing (CSC/Tieteellinen laskenta Oy., Espoo, Finland).

The research is performed in collaboration with prof. Samuel Krimm (University of Michigan, USA, molecular mechanics and dynamics, proteins), prof. Jacques Weber (University of Geneva, Switzerland, quantum mechanics, organometallics), and the modelling group at VTT Chemical Technology (Dr. Lars-Olof Pietilä, molecular mechanics and RIS simulations, synthetic polymers). The experimental part of the research has been done in collaboration with Dr. Ilpo Mutikainen at the Laboratory for Inorganic Chemistry (Department of Chemistry, University of Helsinki, X-ray crystallography), and with Dr. Ritva Serimaa (Department of Physics), connected to the MATRA project (Academy of Finland, coordinated by prof. Franciska Sundholm (Department of Chemistry, Univ. of Helsinki)).

The quantum chemical ab initio and density functional studies on some aliphatic, conjugated and ionic molecular systems of pharmaceutical interest were presented in the thesis "Quantum Chemical, Structural and Spectroscopic Studies on Glyoxal Bis(amidinohydrazone) and Selected Structurally Related Model Molecules" by Jere Koskinen in 1998 [1], the first graduate student of the group.

During 1998 the quantum chemical density functional studies on the electrophilic substitution reactions of ferrocene were completed and published [2]. Also the quantum chemical ab initio calculations, aimed for the development of a molecular mechanics force field for alkenes and their different conformations, were completed [3].

The main focus of the research during 1998 has been in the amorphous cell simulations on some aliphatic polyesters and polyfluorides [4,5]. The pcff force field, generally used to generate the conformational states of polymer chains, was reoptimized for this purpose using results from quantum chemical ab initio and density functional calculations on some aliphatic esters and fluoroalkanes [6-8]. Also the RIS Metropolis Monte Carlo results with the reoptimized pcff force field on various properties of aliphatic polyester chains were found to be in excellent agreement with experimental data [9].

1.  J.T. Koskinen, Quantum Chemical, Structural and Spectroscopic Studies on Glyoxal Bis(amidinohydrazone) and Selected Structurally Related Model Molecules (thesis, University of Helsinki, 1998). The thesis is based on the following papers:
 1.  J.T. Koskinen, M. Koskinen, I. Mutikainen, B. Mannfors and H. Elo, Ethylmethylglyoxal Bis(amidinohydrazonium) Dichloride-Water (1:2),  Acta Cryst. C52, 3167-3169 (1996)
 2. J.T. Koskinen, M. Koskinen, I. Mutikainen, B. Mannfors and H. Elo, Experimental and Computational Studies on Aminoguanidine Free Base, Monocation and Dication, Part I: The Crystal and Molecular Structure of Aminoguanidine Monohydrochloride and the Ab Initio Structure of the Endiamine Tautomer of Aminoguanidine Free Base, Z. Naturforsch. 51b, 1771-1778 (1996).
 3. B. Mannfors, J.T. Koskinen, L.-O. Pietilä and L. Ahjopalo, Density Functional Studies of Conformational Properties of Conjugated Systems Containing Heteroatoms, J. Mol. Struct. (Theochem) 393, 39-58 (1997)
 4. B. Mannfors, J.T. Koskinen and L.-O. Pietilä, Conformational Study of Glyoxal Bis(amidinohydrazone) by Ab Initio methods, J. Mol. Struct. 415, 135-151 (1997).
 5. J.T. Koskinen, M. Koskinen, I. Mutikainen, P. Tilus, B. Mannfors and H. Elo, Experimental and Computational Studies on Aminoguanidine Free Base, Monocation and Dication, Part II: Acid-Base Properties, Gas Phase Protonation Energies and Total Energies of Two Tautomers of the Free Base, Z. Naturforsch. f 52b, 1259-1272 (1997).
 6. M. Koskinen, I. Mutikainen, J.T. Koskinen and H. Elo, Crystal and Molecular Structure of Methylpropylglyoxal Bis(amidinohydrazone) Sulphate Monohydrate, Z. Naturforsch.  52b, 1114-1118 (1997).
 7. J.T.Koskinen, Experimental and Computational Studies on Aminoguanidine Free Base, Monocation and Dication, Part III: Proton Affinities of Guanidine, Aminoguanidine and Glyoxal Bis(amidinohydrazone), Z. Naturforsch. 53b, 386-392 (1998).

2.  M.J. Mayor-Lopez, J. Weber, B. Mannfors and A.F. Cunningham, Jr., Density Functional Study of the Protonated, Acetylated and Mercurated Derivatives of Ferrocene: Mechanism of Electrophilic Substitution Reaction, Organometallics 17, 4983-4991 (1998).
3.  B. Mannfors, T. Sundius, K. Palmö, L.-O. Pietilä and S. Krimm, Spectroscopically Determined Force Fields for Macromolecules. 3. Alkene Chains, manuscript in preparation.
4. J. Blomqvist, Amorphous Cell Studies of Biogradable Polyglycolic and Polylactic Acids, manuscript in preparation.
5. K. Jokela, V. Korpelainen, R. Serimaa, B. Mannfors, and S. Vahvaselkä, The Liquid Structure of Polyvinylidenefluoride by Molecular Modelling and X-Ray Scattering, manuscript in preparation.
6. J. Blomqvist, L. Ahjopalo, B. Mannfors and L.-O. Pietilä, Studies on Aliphatic Polyesters I: Ab Initio, Density Functional and Force Field Studies of Esters with One Carboxyl Group, J. Mol. Struct. (Theochem), accepted for publication.
7. J. Blomqvist, B. Mannfors and L.-O. Pietilä, Studies on Aliphatic Polyesters II: Ab Initio, Density Functional and Force Field Studies of Esters with Two Carboxyl Groups, submitted for publication.
8. V. Korpelainen, B. Mannfors and L.-O. Pietilä, Ab Initio, Density Functional and Force Field Studies of Some Fluoroalkanes, manuscript in preparation.
9. L. Ahjopalo, J. Blomqvist, B. Mannfors and L.-O. Pietilä, RIS Metropolis Monte Carlo Studies of Some Aliphatic Main Chain and Side GroupPolyesters, manuscript in preparation.
 

A THEORETICAL STUDY OF SOME MODEL COMPOUNDS FOR BERLINITE
Tom Sundius

In collaboration with Igor Ignatyev (Russian Academy of Sciences, St. Petersburg) the structure and vibrational spectra of the X3AlOPX3 (X=F,Cl) molecules have been theoretically investigated. This work is a continuation of our previous study of the hexahalodisiloxanes [1].

The electronic structure and the vibrational spectrum of X3AlOPX3 have been predicted by quantum chemical methods, including correlated methods such as MP2 and density functional theory (B3LYP). Our interest in the dynamics of this molecule stems from its role as the simplest model of the AlOP bridge occurring in "-berlinite and molecular sieves, which are important in zeolite chemistry.

The calculations predict the AlOP bond angle in Cl3AlOPCl3 to be larger than 140°, in agreement with recent experimental results. The theoretical force fields of the X3Al.OPX3 complexes at the SCF level have been scaled using scale factors refined for the AlX3 and X3P=O molecules. Both the scaled SCF and unscaled  B3LYP/6-31G(2df) frequencies predicted for the experimentally characterized Cl3Al.OPCl3 complex agree well with the experimental spectra available for this complex. The theoretical force fields of F3SiOSiF3 and F3AlOPF3 have also been compared.

1.  I.S. Ignatyev, T. Sundius, J. Mol. Struct. (Theochem) 343, 69 (1995)