1. ACCELERATOR LABORATORY ______________________________________________________________________________________________________

1.1. PREFACE


A noteworthy factor which has had a significant impact on the Accelerator Laboratory activity this year has been the rather good level of outside funding. This has enabled us to support postgraduate students, expand and strengthen our activity in traditional fields of our expertise. Materials research has further enforced its significance in the laboratory's research profile. Also the research in nuclear physics, applied physics, nonlinear laser spectroscopy and molecular physics has been continued successfully. Several new students have joined the research groups during this year aiming at M.Sc. and Ph.D. degrees.

In addition to several extensive domestic and international cooperation projects which have progressed during the year also some equipment renovations and developments have taken place. For example, the Accelerator Mass Spectrometry (AMS) project is being started at our laboratory by amending the EGP-10-II accelerator to certain extent. Also the upgrading of the laboratory isotope separator has engaged significant financial and man power efforts. In addition to the laboratory's own investment outside funding from various sources has helped us to realise the bringing up to date of the electrical power supplies and the improvement of the apparatus's vacuum systems.

Jyrki Räisänen


1.2. LABORATORY OPERATION AND DEVELOPMENT

Armas Fontell, Raimo Ingren, Kim Kirjavainen, Heikki Sepponen, Birger Ståhlberg and Kim Wahlström

The main three accelerators of the laboratory are the 2.5 MV Van de Graaff (VdG) accelerator, the 60 kV isotope separator and the 5 MV tandem accelerator EGP-10-II. During the period covered by this annual report these devices completed their 39th, 32nd and 14th year in service, respectively. The VdG and EGP accelerators operated with a reliability compared with that of the previous years while the isotope separator showed many malfunctions.

The EGP maintenance has required one long shut-down lasting from August 12 to September 2. The pressure tank was opened for service of the inner parts of the EGP. All the high voltage resistors were measured and the broken ones renewed. All the bearings were renewed, the corona needles were renewed, service of the stripper gas equipment was carried out and the whole machinery was polished and cleaned. In addition to the maintenance of the inner parts of the EGP most of its vacuum pumping systems were removed for service.

Among the technical improvements made to the EGP measurement stations the most noteworthy was the one to the Elastic Recoil Detection Analysis (ERDA) system. A cooling and heating device was constructed to the ERDA target assembly. This device allows cooling of the target close to the liquid nitrogen temperature and heating of it to a temperature of 1000 °C.

The VdG required 30 days of maintenance. In January the pressure tank was opened for service of the ion source. The tank was again opened in August for renewal of the charging sheets and service of the ion source. In September service of the ion source and replacement of the ion source oscillator tube was carried out. This was done again twice in November. Altogether the pressure tank was opened five times.

The EGP and the VdG provided total beam times of 1386 h and 1892 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 distributions 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 sputtering and the duoplasmatron ion sources of the EGP are summarized in Table 1.

At the VdG protons were accelerated for 54 %, deuterium for 4 % and 4He + for 42 % of the total beam time. Similarily to the previous years the laboratory isotope separator was in active sample production. The ion collection energy varied from low energies up to 110 keV. Altogether 433 samples were produced from 22 different isotopes. During the year new power supplies and vacuum pumps with peripheral equipments were purchased. The construction work to modernize the isotope separator was initiated.

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

Fig. 2. Distribution of beam hours divided into 0.5 MV intervals of the terminal voltage.

Table 1. Beam hours for various ions from EGP-10-II.

COMPUTER SERVICES

Sisko Vikberg, Kai Arstila, Antti Kuronen and Jura Pulkkinen

Our computer system (Fig. 3) consists of 10 client PC's (ranging from a 40 Mhz 386 to a 166 MHz Pentium) connected by an Ethernet cable to a server machine (a 166 MHz Pentium with 4 GB disk space and 64 MB of main memory). Both the server and the client machines are running the Linux operating system. Client machines are essentially used as X window terminals running usually only the X window server program. Most of the application programs are run in the server machine.

For numerical simulations we also have a Digital AlphaStation 200 4/166 with 5 GB of disk space and 64 MB of main memory.

Most of our system and application software is freely distributable and collected from the Internet (including the Linux operating system itself). Text is produced using the TeX/LaTeX program and in data analysis (in addition to in-house codes) the PAW system is used.

Fast network connection allows us to use the com-puter resources at the Physics Computation Unit, the Computer Center of the University and the Center of Scientific Computing of Finland. Practically all heavy computing is presently performed using computers in these centers and our own Digital AlphaStation.

Client machines can also be booted to run DOS /Windows and to connect to the Novell servers at the University. In the DOS/Windows environment our DECstation 3100 acts as a NFS disk server.

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

Fig. 3. The computer system of the Accelerator Laboratory

PUBLIC RELATIONS ACTIVITY

Birger Ståhlberg and Olli J. Marttila

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.


1.3. NUCLEAR PHYSICS

LIFETIMES OF 32P LEVELS

Aki Kangasmäki, Pertti Tikkanen, Juhani Keinonen, W.E. Ormand* and S. Raman**

The study on the properties of excited states in 32P has been completed [1]. Mean lifetimes of 22 bound levels and upper lifetime limits of four more levels in 32P below an excitation energy of 6.4 MeV were deduced from the Doppler-shift-attenuation (DSA) of gamma rays produced in the inverse reaction 2H( 31P,p gamma). Of these 26 levels, the lifetimes of six levels are reported for the first time. Experimental DSA data from lifetime measurements reported previously in the literature were critically evaluated and reanalyzed. The resulting revised lifetime values are in most cases in good agreement with the current values. The low-lying portion of the level scheme, the level lifetimes, and the g-ray branchings and reduced transition prob-abilities were compared with shell-model predictions. The overall agreement is reasonably good. A novel feature of the model calculations is that in addition to the full sd shell configurations, one-particle­one-hole excitations to the next major oscillator shell are included.

1. A. Kangasmäki et al., submitted to Phys. Rev. C.

* INFN Sezione di Milano,via Celoria 16, 20133 Milano, Italy and Oak Ridge National Laboratory, Oak Ridge, TN 37831

** Oak Ridge National Lab., Oak Ridge, TN 37831

SPECTROSCOPY OF LEVELS IN 32S

Aki Kangasmäki, Pertti Tikkanen, Juhani Keinonen, W.E. Ormand*, S. Raman**, Zs. Fülöp***, Á. Z. Kiss*** and E. Somorjai***

A study on the properties of excited states in 32S is in progress. In this work we are concentrating mainly on the measurement of mean lifetimes of the levels. A critical evaluation of the experimental Doppler-shift-attenuation (DSA) data from lifetime measurements reported in the literature revealed the available data to show a large variation. The only exception is the first excited state. Therefore, reliable lifetime data for a meaningful test of modern shell-model calculations was called for. In the current work, mean lifetimes of 18 bound levels below an energy of 8.2 MeV have been deduced from the Doppler-broadened g ray lineshapes produced in the reactions 2H( 31P,n gamma), 28Si( 6Li,pn gamma) and 31P(p, gamma). The use of these three reactions yields both overlapping and complementary lifetime data. In the case of the (p, gamma) reaction, branching ratios for primary and secondary g rays of two resonances have also been determined. The experimental level properties (excitation energies, mean lifetimes, gamma-ray branchings and mixing ratios) for both positive- and negative-parity states are compared with the results of large-space shell-model calculations.

* INFN Sezione di Milano, 20133 Milano, Italy and Oak Ridge National Laboratory, Oak Ridge, TN 37831

** Oak Ridge National Lab., Oak Ridge, TN 37831

*** Institute of Nuclear Research of the Hungarian Academy of Sciences, Debrecen, Hungary

SPECTROSCOPY OF LEVELS IN 24Na

Pertti Tikkanen, Aki Kangasmäki, Juhani Keinonen and S. Raman*

The large density of states of 24Na (69 known bound levels) makes this nucleus a suitable testing labora-tory for model calculations in this mass region. A recent study on excited states in 24Na with the 23Na( nthermal, gamma) reaction has produced an almost complete level scheme up to 6.70 MeV excitation energy [1]. The data of that work consist of accurate level energies and gamma-ray branchings, and as such are superior to the data reported previously in the literature. However, reliable data on the mean lifetimes of the levels in 24Na are still missing. Therefore, we have started a study where accurate lifetime values are deduced from Doppler-broadened gamma-ray lineshapes produced in the kinematically inverted 2H( 23Na,p gamma) reaction. Data from a singles gamma-ray measurement revealed that overlapping of the gamma-ray peaks prevents a reliable lineshape analysis in many instances, and it is necessary to perform proton-gamma coincidence measurements in the future.

1. E. T. Jurney and J. W. Starner, private communication.

* Oak Ridge National Laboratory, Oak Ridge, TN 37831

1.4. MATERIALS PHYSICS

MOLECULAR DYNAMICS SIMULATION OF LOW-ENERGY ION BEAM INDUCED POINT DEFECTS ON THE Si(100)-(2x1) SURFACE

Jura Pulkkinen, Kai Nordlund, Antti Kuronen and Juhani Keinonen

Whilst the size of semiconductor devices keeps getting smaller and smaller, the number of lattice defects they have becomes a more and more relevant issue. Ion implantation is a widely used technique in producing semiconductor devices. Ion bombardment causes self-ion recoils inside the target producing defects in the lattice. The production of point defects on the silicon surface under self-bombardment was studied in this work.

The study was done by applying classical molecular dynamics simulation technique. Because of the limitations of the method used, the work concentrated on low energies (0-200 eV). The aim was to study defect production and defect (vacancy, interstitial) profiles during the first 1 ps of the collision cascades in the first few atomic layers of the sample. Also, the threshold energy, associated with the model potential used in the simulations [1], for producing a Frenkel pair was determined both on the surface and in the bulk. Furthermore, the melting point of silicon was determined. The results from the simulations were compared with those given by H. Hensel and H. Urbassek [2] showing the divergences arising from the differences in the model potential and the methods used for analyzing the defects.

The net sum of the defects reaches its maximum at 0.1-0.2 ps, depending on the energy of the ion. After that the surroundings of the collision path cools down and approximately a little less than half of the vacancies and interstitials recombine. The simulations show that the number of defects is a super-linear function of the energy of the incident ion.

1. J. Tersoff, Empirical Interatomic Potential for Silicon with Improved Elastic Properties. Phys. Rev. B 38 (1988) 9902

2. H. Hensel and H. Urbassek, Disordering and An-nealing of a Si Surface under Low-Energy Si Bom-bardment. To be published in Rad. Eff. & Def. Sol.

ELECTRONIC STOPPING POWERS FOR HEAVY IONS

Kai Arstila, Pertti Tikkanen and Juhani Keinonen

The project for combining the Doppler shift attenuation (DSA) and transmission methods for accurate stopping power measurements has been continued.

The work carried out during 1996 includes the devel-opment of the position-sensitive particle detector system, continuation of the development of target preparation, and extensive work with the analysis software.

For transmission measurements the elastic recoil detection (ERD) method was chosen. A good energy resolution in ERD measurements can be achieved only by using well-collimated particle detector, which reduces the detection yield and results in time consuming measurements. This problem can be overcome by using a large position-sensitive detector, which gives high detection yield while maintaining good angular resolution.

The aim of target preparation development was to produce a very thin deuterium layer on the top of a metal film. The parameters of consecutive argon, carbon and hydrogen implatations was optimized to achieve this.

For analysis of the gamma-ray line shapes the Monte Carlo simulation code was completely rewritten. This resulted in a speedup of a factor of four and a greater modularity and reliability. The effect of the detector response on the gamma-ray line shape was simulated with EGS4 code in order to achieve realistic description of the detector angular efficiency and the shape of the background of the gamma spectra.

TOF-ERDA SPECTROSCOPY

Janne Jokinen and Timo Sajavaara

The time-of-flight elastic recoil detection analysis (TOF-ERDA) spectrometer constructed last year [1,2,3] has been extensively used in applied research. The method can be used for profiling sample elemental composition as a function of depth. Thin films grown with atomic layer epitaxy for electroluminescent displays have been studied in collaboration with Helsinki University Department of Chemistry and Planar International Ltd. Composition analyses have also been performed for several other projects and the technique has become a routine analysis method at the laboratory. Analytical methods have also been devel-oped further. Ion stopping powers needed in the analysis were studied and determined. A new target holder with heating and liquid nitrogen cooling was introduced. Analysis software was further developed and an additional detector for analyzing hydrogen was constructed.

1. Annual Report 1995, Department of Physics, University of Helsinki (Helsinki University Press, Helsinki, 1996)

2. J. Jokinen, J. Keinonen, P. Tikkanen, A. Kuronen, T. Ahlgren and K. Nordlund, Nucl. Instr. and Meth. in Phys. Res. B119 (1996) 533

3. J. Jokinen, P. Haussalo, J. Keinonen, M. Ritala, D. Riihelä and M. Leskelä, Thin Solid Films 289 (1996) 159

H-IMPLANTED SiO2, REDISTRIBUTION OF HYDROGEN AND AMORPHISATION Ý

W. Bolse*, Marcus Gustafsson, F. Harbsmeier* and Juhani Keinonen

Damage accumulation, migration and annealing behaviour of hydrogen in ion-irradiated silicon dioxide, have been studied as a continuation to studies on amorphisation of ion-irradiated ceramics [1].

SiO2 samples were implanted at liquid nitrogen temperature with 5 and 10 keV protons to fluences of 1015-1017 H+cm­2.

Hydrogen implantation induced damage and amorphisation of SiO2 was measured at liquid nitrogen and room temperature with Rutherford backscattering spectrometry and channelling using the 500 kV isotope separator at the University of Göttingen.

Concentration distributions of implanted H were measured at liquid nitrogen and room temperature with the Nuclear Resonance Broadening technique utilizing 6.4 MeV 15N2+ beams from the tandem accelerator EGP-10-II of the University of Helsinki.

Preliminary studies on diffusion of implanted light elements in SiO2 have also been done.

Ý This work is part of a bilateral Finnish-German project (SEBIM).

* Zweites Physikalisches Institut, Univ. Göttingen, Bunsenstr.7-9, D-37073 Göttingen, Germany

1. "Amorphisation Kinetics in Ion-Irradiated Ceramics." Invited talk given by W. Bolse at the 14th CAARI-conference in Denton, 1996.

ION BEAM STUDIES OF III-V AND II-VI COMPUND SEMICONDUCTORS (EPIMATTER) Ý

Tommy Ahlgren, A. Kazantsev*, Juhani Keinonen, J. Lammasniemi*, J. Likonen**, Markko Rajatora, Eero Rauhala, Jyrki Räisänen, K. Saarinen***, Reeta Salonen, Jonatan Slotte, Kyösti Väkeväinen

The project is part of the extensive national Epimatter (Advanced Epitaxial Materials for Microelectronic Applications) project aiming at better understanding of the physical properties of III-V and II-VI compound semiconductor materials. The activity report related to this project is summarized below.

The diffusion of Au from electrical contacts on un-doped ZnSe has been studied by ion beam techniques in the temperature region of 400 - 700 °C. The lattice location of the diffused Au in ZnSe has been deter-mined by channeling measurements with 2.5 MeV 4He ions. Angular yield profiles for <100> and <110> axial directions for ion scattering from both Au and the substrate atoms at the same depth were determined. The lattice location of the Au atoms was then determined by comparing the calculated profiles obtained by a computer program and the measured profiles. The activation energy and frequency factor for the diffusion process were deduced. After annealing gold grain formation has been observed on the ZnSe surface and also the activation energy for this process has been determined.

Defect formation and annealing behavior of InP im-planted by low energy 15N ions has been investigated. N-type (100) single crystal S-doped InP has been implanted at room temperature with 30 keV 15N ions with doses 1014-1016 Ncm­2. The implanted samples were subjected to isochronal vacuum-annealing in the temperature range 450 - 650 °C. The annealing behavior of the distribution of the nitrogen atoms, the implantation induced displacements of the In atoms, vacancy-type defects and damage annealing have been studied by NRB, SIMS, RBS/channeling and slow positron annihilation techniques. The correlations between the damage, vacancy and nitrogen concentrations as well as the identification and dose and tem-perature dependence of vacancy types have been established.

Concentration dependent and independent Si diffusion in ion implanted GaAs has been studied by SIMS, NRB and RBS/channeling techniques in the temperature region 650 - 850 °C. The concentration independent diffusion, observed as a broadening of the initial implanted distribution, is slow and is assigned to Si atoms diffusing interstitially. The concentration dependent diffusion with low solubility is decribed by a model where Si atoms occupy either Ga or As sites and the substitutional Si atoms then diffuse via Ga and As vacancies. The activation energies and frequency factors have been deduced for the various diffusion models. Solid solubility of Si in GaAs has been determined in the investigated temperature region.

Irradiation of GaAs, GaInP and GaAs/GaInP-tandem solar cells by 3 MeV protons has been conducted for doses of 1010 - 1013 protons/cm2 to simulate space environment. An irradiation setup providing homogeneous beam profiles on the solar cell samples was constructed. The efficiency of the solar cells was followed as a function of the irradiation dose. Significant deviations in the irradiation resistance of the various solar cell types was observed.

Ý The Epimatter project is funded by the Academy of Finland under the National Programme in Materials and Structure Research.

* Department of Physics, Tampere University of Technology, FIN-33101 Tampere, Finland

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

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

PROTON ELASTIC SCATTERING CROSS SECTIONS NEAR AND ABOVE THE COULOMB BARRIER

V. Zazubovich * , Eero Rauhala, Reijo Lappalainen and Jyrki Räisänen

The measurements of proton elastic scattering cross sections for Mo, Sn, Ag and Au have been performed at the angles of 135° and 165°. The goal of the study is to determine the cross section behaviour in the energy region where they begin to deviate from the values predicted by the Rutherford model. The measurements have been so far conducted upto the energy value of 5.5 MeV and they will be continued to higher energies. The knowledge of the non-Rutherford threshold energies is of significant importance in backscattering spectrometry carried out with protons.

* Institute of Physics, University of Tartu, EE-2400 Tartu, Estonia

4He(p,p)4He SCATTERING CROSS SECTIONS IN THE ENERGY REGION 1.4 - 6.0 MeV

Arto Nurmela, Eero Rauhala, Kyösti Väkeväinen and Jyrki Räisänen

Cross sections for the elastic scattering of protons from 4He have been determined in the energy region 1.4 - 6.0 MeV, in 100 keV intervals. The measure-ments were performed at laboratory angles of 110°, 120°, 130°, 140°, 150°, 160°, and 170° for energies 1.4 - 2.7 MeV and at angles of 140° and 170° for energies 2.7 - 6.0 MeV. Helium targets made by implanting 100 keV 4He ions in tantalum foils (1.5 mm thick) were used. Several implantation doses were employed to investigate the dose-dependence of the accumulated helium concentration. The target helium concentrations were determined also via ERD measurements by 28Si ions. Accurate non-Rutherford scattering cross section values are needed in depth profiling of helium by proton backscattering.

STOPPING POWERS OF Ti AND TiN FOR 4He IONS

A. Climent-Font*, Jyrki Räisänen and Eero Rauhala

The energy transfer of light ions traversing matter to electrons of the target medium is the main interaction in the slowing down of energetic particles in solids in the low-MeV range. In analytic applications of ion beams the precise knowledge of the rate of the energy loss is essential. In preliminary comparisons with theoretical calculations, significant deviations from measured stopping powers have been found, for ex-ample, in the case of 4He in titanium. A co-operation project with the Universidad Autónoma de Madrid has been started to obtain accurate experimental Ti and TiN stopping powers for 4He.

* Departamento de Física Aplicada C-12, Universidad Autónoma de Madrid, Campus de Cantoblanco, E-28049 Madrid, Spain

STOPPING CROSS SECTIONS OF ZnSe, Zn AND Cu FOR 1H, 4He AND 4Li IONS

Kyösti Väkeväinen

The stopping cross sections of the II-VI semiconduc-tor material ZnSe for 0.4-2.5 MeV 1H, 4He and 7Li ions have been studied by the Rutherford backscatter-ing technique. The stopping of Zn and Cu for lithium ions were measured by the backscattering and trans-mission techniques for comparison and reference purposes. The stopping cross sections of ZnSe obtained for hydrogen ions were in good agreement with the semiempirical calculations based on the ZBL-85 model. For helium and lithium ions the stopping cross sections were 4-10 % and 1-9 % smaller than predicted by the model, respectively. For lithium ions, the stopping cross sections of Zn were systematically 5 % below calculations based on the model. The stopping data of Cu agreed well with the literature data.

EXFOLIATION PROCESS OF InP CAUSED BY 1H AND 4He ION IMPLANTATION AND ITS APPLICATION TO RANGE DETERMINATIONS

Kyösti Väkeväinen, Markko Rajatora, Tommy Ahl-gren, Eero Rauhala and Jyrki Räisänen

Exfoliation of crystalline InP by 0.6-2.1 MeV 1H and 1.0-2.6 MeV 4He ion implantation at random orienta-tion has been studied. The profiles of the produced craters have been determined by a profilometer. The correlation between the crater depths and ion range parameters has been established by measuring the implanted 4He ion depth distribution by elastic pro-ton backscattering. The deduced projected ranges are compared with calculated values based on the ZBL-85 model.

AMORPHOUS DIAMOND FILM DEPOSITION

Asko Anttila, Reijo Lappalainen, Marko Hakovirta, Panu Pekko and Veli-Matti Tiainen

Amorphous diamond films (sp3 bonding fraction 78%) have been deposited using plasma accelerator methods. By using the high energy plasma beams the films with optimal adhesion were produced. Films have been deposited for tribological tests in laboratory and for industrial applications. New set-ups for testing of diamond-coated hip joints and hip joint materials in liquids have been constructed and used for preliminary tests in laboratory. Promising results with the amorphous diamond coatings have been achieved, e.g. in medical implants and cutting tools. The optimum conditions to eliminate the internal stress of the deposited diamond films by intermediate layer techniques have been determined for different substrate materials. New arc discharge and evaporation units for metal deposition have been constructed.

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* and Janne Jokinen

SrS:Ce3+ is a promising candidate for the blue phosphor in full-color thin film electroluminescent displays. At present the blue color 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 films prepared by various deposition techniques. An important, though not thoroughly understood factor in respect of the EL properties of SrS:Ce is the effect of impurities.

In order to get a basis for future studies on the rela-tionships between the impurities and the luminescent properties, we have analyzed in detail all the relevant impurities in the state-of-the-art SrS:Ce films prepared with different techniques using several nuclear methods. In additon, we have used ion implantation to add dopants and impurities in the EL structure in a well-controlled way. The implanted ions include F, Na, P, Cl, K, Cu, Ga and Ce. We have found out that e.g. by optimized F-implantation photoluminescence intensity of the EL structure didn't decrease signifi-cantly but the spectrum shifts towards blue. It should be emphasized that even this small blue shift (about 10 nm) is very significant if that can be reproduced in a real EL device.

* Department of Chemistry, P.O. Box 55, Univ. Helsinki, FIN-00014 Helsinki, Finland

1.5 APPLIED PHYSICS

12C, 14N AND 16O ION INDUCED GAMMA-RAY YIELDS

Eero Rauhala, Jyrki Räisänen , Z. Szökefalvi-Nagy*, Ulrika Backman and Anni Seppälä

The determination of absolute thick-target gamma-ray yields induced by 12C, 14N and 16O ions has been completed for elements with Z=3-30. The study was a co-operation project with the Research Institute for Particle and Nuclear Physics, Budapest, Hungary.

* KFKI Research Institute for Particle and Nuclear Physics, Konkoly Thege u. 29-33, H-1121 Budapest, Hungary

AMORPHOUS OR NANOCRYSTALLINE THIN FILMS, MULTILAYERS AND FIBERS

Pauli Torri , Reijo Lappalainen and Juha-Pekka Hirvonen*

We have studied nanocrystalline ceramics in the form of thin films and fibers 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 distri-bution 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 special properties such as superplasticity and high resistance for oxidation, corrosion and wear. Mechanical properties of the films are tested with pin-on disc method, scratch tester, nanoindenter etc.

Based on our earlier studies we have prepared new multilayered MoSi2/SiC, MoSi2N3/SiC, MoSi2/MoSi2N3/SiC/MoSi2N3 and MoSi2-N samples for oxidation, stress relaxation and diffusion studies. We have shown that microstructure determined by deposition parameters and heat treatment has an important role in oxidation behaviour and thermal stabilIty. Furthermore, we have found excellent stability in thin film structures stabilized with nitrogen. This same concept has been preliminary tested with amorphous diamond coatings doped with nitrogen and boron.

* Technical Research Center of Finland, Manufactur-ing Technology, FIN-02044 VTT, Finland

ANNEALING STUDY OF HELIUM AND HYDROGEN IMPLANTED METALS

Pekka Haussalo

The annealing behaviour of Ta, Nb, W and AISI 316L-stainless steel samples implanted with 20 keV helium ions and either with 15 keV hydrogen or 12 keV deuterium ions was studied.

Half of the trapped hydrogen and deuterium was re-leased at the temperature of 488 K in steel, 583 K in W, 648 K in Ta, and 553 K in Nb. There was no isotope effect in the hydrogen release: hydrogen and deuterium escape at exactly the same temperature. It was also found that implanted hydrogen migrates during the room temperature implantation so that the range distribution resembles that of implanted He.

The recovery temperatures of implanted and trapped hydrogen isotopes found are lower than the recovery temperatures of solid soluble hydrogen migrating from the metal bulk after the helium implantation, deduced earlier [1].

1. P. Haussalo, J. Keinonen, U.-M. Jäske, and J. Sievinen, J. Appl. Phys. 75 (1994) 7770

STOPPING OF 5 - 100 keV HELIUM IN CHROMIUM, NICKEL, COPPER, AND MOLYBDENUM &

Pekka Haussalo, Elizaveta Vainonen and Jussi Sillanpää

The stopping power of Cr, Ni, Cu, and Mo for He ions at velocities below the Bohr velocity has been deduced by comparing the ranges of 5 to 100 keV He+-ions determined with the elastic-recoil-detection-analysis method with those obtained in molecular dynamics simulations.

The nuclear slowing down was treated through the use of molecular dynamics calculations and a potential obtained from density-functional theory calculations.

The comparisons showed that the electronic stopping powers given by J. F. Ziegler, J. P. Biersack, and U. Littmark [1], had to be multiplied with a factor of 1.2 for Cr, 1.0 for Ni, 1.0 for Cu, and 1.3 for Mo. The uncertainty of the factors and hence the scaled stopping power values is ±5 %. The total stopping powers were obtained from molecular dynamics simulations in which the modified electronic stopping powers were used.

1. J. F. Ziegler, J. P. Biersack, and U. Littmark, The Stopping Powers and Ranges of Ions in Matter (Pergamon, New York, 1985), Vol. 1

& Supported by Association Euratom-TEKES

ELEMENTAL ANALYSIS OF BIOMEDICAL SAMPLES USING ION BEAM TECHNIQUES

Tiina Kupila-Rantala, Mervi Hyvönen-Dabek and Jan Dabek*

Proton induced X-ray emission analysis (PIXE) was used for studies of loosely bound copper and adjacent elements in human blood plasma and cerebrospinal fluid. Loosely bound copper is associated in progres-sion of some neurological diseases and some forms of cancer. In order to enable determination of very low elemental concentrations in human fluid samples, several measurement parameters were optimised and new sample preparation techniques were developed. These included optimisation of the absorber in front of the detector, use of helium as an ambient gas, employing ultrafiltration in the sample matrix reduction, and investigation of properties of supporting foils needed when studying fluid samples. Using ultrafiltration and 20 mg/cm2 thin Formvar resin foils resulted in detection limits of 6.3, 4.1, 8.5 and 18 ppb (part per billion) for iron, copper, zinc and bromine, respectively, in human cerebrospinal fluid, thus allowing confident determination of these elements. Using supporting grids prepared from 8 mm thick Kapton foil with a pulsed Nd:YAG laser also im-proved the detection limits by homogenising the sample surface.

* Department of Clinical Chemistry, Helsinki University Central Hospital

SEM-EDS AND PIXE ANALYSES OF MEDIEVAL GLASS FROM THE MUSEUM ABOA VETUS IN TURKU

Pirkko Kuisma-Kursula

Aboa vetus is a museum of archaeology and history in Turku. Over 30,000 objects have been discovered in the excavations on the area of the museum. The first finds are from the early 13th century. The excavations were particularly intensive in 1994 and then, for example, over 30 glass vessels and hundreds of glass fragments were found. This is a large glass find also on a European scale and will change our view of the use of glass in medieval times in Finland. In this study 65 samples excavated from the oldest layers have been analyzed. The aim of this study is to identify major, minor and trace elements of these glass fragments, and hence to help archaeologists to reconstruct the vessels and to obtain information about the origin of the glasses.

The major and minor elements have been analyzed by scanning electron microscopy (SEM) with an energy-dispersive spectrometer (EDS) and trace elements by the PIXE method.

1.6 LASER PHYSICS

The research associated with lasers has continued very much according to the pattern of the previous year. We have developed visible diode laser systems to be used both for nonlinear spectroscopy and optical metrology. The labora-tory facilities and equipments have been utilized extensively during a laser physics course and for educational demon-strations to groups visiting the laboratory. During the year 1996 Åsa Lindberg has had a post doc position in the Huygens Laboratory, University of Leiden, The Netherlands.

Birger Ståhlberg

SATURATION SPECTROSCOPY OF 22Ne AT 640 nm

Stefan Eriksson and Birger Ståhlberg

The 640 nm 1s5(J = 2) ­ 2p9(J = 3) transition in neon is a closed transition the lower level (1s5) of which is a metastable state. Therefore this transition interacts very strongly with resonant light. In order to perform saturation spectroscopy using this transition we have constructed a single mode tunable diode laser based on an external grating feedback. The single mode operation of the laser can be maintained up to a power of about 2 mW. This power is, however, too low to saturate the 20Ne isotope 1s5 ­ 2p9 transition in a DC discharge. Because natural neon gas contains only about 9 % of the isotope 22Ne the laser is strong enough to saturate the 22-isotope transition. We have investigated the Doppler free collision- and power-broadened signal as a function of the neon pressure, laser power and discharge current. The best signal-to-noise ratio is achieved in a 4 cm long discharge with the lowest gas pressures (tens of Pa) and lowest discharge current (~ 1 mA) values allowed by our equipment.

DIODE LASER INTERACTION WITH 127I2 AT 637 nm

Stefan Eriksson and Birger Ståhlberg

Because affordable diode lasers in the red wavelength region are commercially available, a renewed interest in studies of laser interaction particularly with iodine has grown. The goal of these studies are development of new realizations of the metre. The hyper fine components of the 7-4, R(39) transition of the B-X system of 127I2 at 637 nm are very promising transitions to be used to frequency-stabilize extended-cavity diode lasers. The wavelength region (637 nm) makes the hyperfine components excellent for commercial diode lasers, the nominal wavelength of which are 635 nm. We have performed saturation spectroscopy of the 7-4 R(39) transition using the same diode laser system as in the spectroscopic studies of 22Ne. Our preliminary studies show very promising results.

LASER INTERACTION WITH LIQUIDS

Nina Widén, Tomas Fellman* and Birger Ståhlberg

Laser beam interaction with optically absorptive liquids is interesting both from the fundamental and from the educational/entertaining viewpoint. When a laser beam is focused into a liquid the traversing light intensity distribution easily show ring structures. These ring structures depend on the beam power, the interaction length of the liquid and the position of the beam focal point in the liquid. We have studied the ring formations with several kinds of liquids and lasers. The experiments carried out with liquids having a low specific heat capacity and a low thermal conductivity show ring formation with a few mW beam powers provided that the liquid is dyed appropriately to efficiently absorb the applied radiation.

* Oy LM Ericsson Ab, Electronics Department, Jorvas, Finland

MEASUREMENTS OF THE STABILITY OF THE He-Ne LASERS INCORPORATED IN THE ABSOLUTE GRAVIMETERS OF THE FINNISH GEODETIC INSTITUTE

Birger Ståhlberg and Jaakko Mäkinen*

Absolute gravimeters mostly employ polarization stabilized or iodine stabilized He-Ne lasers as light sources. We have continued to investigate the frequency stability of a polarization stabilized He-Ne laser incorporated in the gravimeter FGI. We have done the measurements at a few months intervals during the year. During autumn we have also perfo-med the same kind of measurements with a second polarization stabilized He-Ne laser and an iodine stabilized He-Ne laser which has been constructed to be utilized in absolute gravimeters. The measurements have been carried out by heterodying the three lasers against an iodine stabilized He-Ne meter laser. The polarization stabilized lasers show similarly to the measurements of the previous years a few MHz (i.e. a few parts in 10­9) differences between the measurements. The iodine stabilized He-Ne gravimeter laser shows a few tens of kHz difference in its frequency from the iodine stabilized He-Ne meter laser.

* The Finnish Geodetic Institute, Helsinki

1.7. MOLECULAR PHYSICS

The Molecular Physics Laboratory, historically, has evolved around the Raman spectrometer. The regular research staff of the laboratory increased significantly at the beginning of the year when doc. Niklas Meinander returned to the department. It now comprises Assoc. Prof. Folke Stenman, doc. Niklas Meinander, and Dr. 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 are Lic. Phil. Olli-Pekka Sievänen, M. Sc. Jere T. Koskinen and M. Sc. Leena Wallenius. Ms. Johanna Blomqvist has joined the group in the final stage of her M. Sc. thesis work. Three Master's theses were completed in the laboratory during 1996. 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 during the autumn term in an effort to attract new students to the laboratory. The course was well received by the students taking it. We plan to offer similar courses at regular intervals in the future. 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.

BINARY INTERACTIONS IN DILUTE GASES

Niklas Meinander

The focus of this work is on the empirical interaction energy function and polarizability function of interact-ing pairs of small symmetric molecules. Experimental data utilized are mainly the interaction-induced light scattering (CILS) spectrum of the gas, the re-fractivity 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. Presently the potential energy and polarizability functions of the mercury dimer are under investigation. Modelling of temperature dependent effective isotropic potential energy functions for SF6 and CH4 is planned.

Part of this research is done in collaboration with several people at the Department of Physics of Uni-versità di Firenze and at the Istituto di Elettronica Quantistica, CNR, in Florence, Italy and with with prof. George Tabisz at the Department of Physics of the University of Manitoba in Winnipeg, Canada. Recent publications and conference contributions are:

1. N. Meinander, G. C. Tabisz, F. Barocchi, M. Zoppi, ÒRevisiting the collision-induced light scatter-ing spectrum of gaseous CH4Ò, Molec. Phys. 89 (1996) 521-531

2. N. Meinander, L. Ulivi, G. Pratesi, G. Cirnigliaro, and F. Barocchi, "Bound dimer contribution in the collision-induced pair spectrum of mercury vapour", Contribution at the 13th International Conference on Spectral Line Shapes, Firenze, Italy, June 16-21, 1996 (Page C-23 of the abstract book).

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

Niklas Meinander

Two-dimensional potential energy surfaces of the form are optimized to the observed IR- and Raman-transitions between the vibrational levels of coupled large amplitude vibrational modes of small ring molecules. The potential energy surface determines the equilibrium conformation of the molecule. 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.

Our contribution in Texas has in recent years mainly been to write the FORTRAN programs presently used for the optimizations. In these calculations the Schrödinger equation has traditionally been solved in a prediagonalized harmonic basis. It was observed, however, that the results obtained were rather sensi-tive to the choice of harmonic frequencies in the case of asymmetric potential functions, and that the optimum choice was not easy to identify. During the summer of 1996 we therefore decided to use suitably distributed Gaussian functions as a basis, and started to modify the computer programs accordingly. The optimization of one-dimensional potentials was suc-cessfully implemented during the summer. The com-puted energy levels are quite insensitive to the details of the basis within a fairly wide range of parameter choices for the basis. An optimal choice of basis functions for two- and higher-dimensional potentials is not as straightforward, but a functioning procedure was developed during the autumn. One paper resulting from this collaboration was published this year [3], one is in print [4] and several manuscripts are in preparation [5,6] or planned.

3. C. M. Cheatham, M. Huang, N. Meinander, M. B. Kelly, K. Haller, W.-Y. Chiang, and J. Laane, "A computer-controlled apparatus for laser-induced fluo-rescence spectroscopy in a supersonic jet", J. Mol. Struct. 377 (1996) 81­92

4. J. Choo, N. Meinander, J. R. Villarreal, and J. Laane, "Far-infrared spectra and two-dimensional potential energy surface for the ring-bending and ring-twisting vibrations of 5,6-dihydro-4H-thiopyran", Microchimica Acta (1997)

5. Soo-No Lee, Paul Sagaer, N. Meinander, and J. Laane, "Far-infrared spectra and the ring-bending and ring-twisting vibrations of tetrahydrofuran-3-one" (presented at the Southeast regional ACS meeting in Houston in October 1996)

6. Sachie Sakurai, N. Meinander, and J. Laane, "Two-dimensional potential energy surface for the out-of-plane ring modes of phtalan".

GAS PHASE ANALYTICS WITH FTIR SPECTROSCOPY

Niklas Meinander, Stefan Söderholm and Jyrki Kauppinen

The possibility to use the FTIR gas analyser, GASMET, manufactured by Temet Instruments Oy, for precise (better than 10-4 relative accuracy) determi-nations of the carbon isotopic composition in atmospheric CO2 is being investigated. This work, which involves development of the data analysis and the elimination of various instabilities of the instrumentation, has been carried out by M. Sc. Stefan Söderholm in the laboratory of prof. Jyrki Kauppinen at the Department of Applied Physics at the University of Turku. The results will be published during 1997.

RAMAN SPECTROSCOPIC STUDY OF AMORPHOUS SUGARS

Niklas Meinander, Stefan Söderholm and Yrjö Roos

A study of phase transitions in amorphous sugars using Raman spectroscopy was started during the autumn. 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 which aims at understanding the rôle of sugar as a food preservative. M. Sc. Stefan Söderholm will write his PhD-thesis on the results of this project.

MOLECULAR MODELLING AND SPECTROSCOPY

Johanna Blomqvist, Jere T. Koskinen, Berit Mannfors and Tom Sundius

The research in this project aims at the construction of realistic potential energy functions for atomistic simulations of polymers, catalysts, drugs, and bio-logically active species. The computational methods used are quantum mechanics, molecular mechanics, molecular dynamics, and Monte Carlo simulations, the experimental methods are Raman and infrared vibrational spectroscopy and structure determination by X-ray crystallography. Major applications are protein and drug design, and optimization of the com-position of new polymer based materials. Theoretical calculations have been performed using facilities at the Center for Scientific Computing (CSC-Tieteellinen laskenta Oy., Espoo, Finland) and the Physics Computation Unit, University of Helsinki. In our group the main purpose of molecular modelling using the molecular mechanics method is to develop potential energy functions for torsions around flexible single bonds and for nonbonded (van der Waals and coulombic) interactions. Torsional and nonbonded interactions determine to a large extent the conformational properties of the molecules.

The research is performed in collaboration with prof. Samuel Krimm (University of Michigan, USA, mo-lecular mechanics and dynamics, proteins), prof. Jacques Weber (University of Geneva, Switzerland, quantum mechanics, organometallics), Dr. I. Ignatyev (Russian Academy of Sciences, the Institute for Silicate Chemistry, Russia, quantum mechanics, silicon compounds), and the modelling group at VTT Chemical Technology (Lic. Phil. Lisbeth Ahjopalo and Dr. Lars-Olof Pietilä, molecular mechanics and Monte Carlo simulations, polymers). The experimental part of the research has been done in collaboration with Dr. Ilpo Mutikainen at the Inorganic Chemistry Laboratory (Department of Chemistry, University of Helsinki, X-ray crystallography) and at MAX-lab at the University of Lund, Sweden (FTIR using synchrotron radiation).

During 1996 studies on the application of different quantum mechanical ab initio and density functional methods to reproduce molecular conformational prop-erties of some conjugated systems were completed [7]. Quantum mechanical studies on pharmacologically significant molecular and ionic systems [8,9] and organometallics [10] continue. Cationic com-pounds containing silicon have also been studied quantum mechanically [11]. The crystal and molecular structures of some compounds, which are significant as starting materials in drug design, were determined [9,12]. As part of the development of potential energy functions, quantum mechanical calculations have been performed for some amides in order to model confor-mational deformations in amino groups [13]. Some olefines have been investigated with the molecular mechanics method [14]. More detailed studies on olefines are in progress. Potential energy functions for several polar molecules containing oxygen and nitrogen will be developed in collaboration with the group at the University of Michigan. The first part of this project is the investigation of different quantum mechanical methods for modelling of the molecular charge distributions.

7. B. Mannfors, J.T. Koskinen, L.-O. Pietilä and L. Ahjopalo, ÒDensity functional studies of conforma-tional properties of conjugated systems containing heteroatomsÒ, J. Mol. Struct. (Theochem) (accepted)

8. B. Mannfors, J.T. Koskinen and L.-O. Pietilä, "Conformational study of glyoxal bis(amidinohydrazone) by ab initio methods", J. Mol. Struct. (submitted)

9. J.T. Koskinen, M. Koskinen, I. Mutikainen, B. Mannfors and H. Elo, "Experimental and computa-tional studies on aminoguanidine free base, monoca-tion 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. C (accepted)

10. P. Mayor, B. Mannfors and J. Weber, "Density functional studies of electrophilic reactions of ferrocenes", manuscript in preparation

11. I.S. Ignatyev and T. Sundius, "Potential Energy Surface of the Dimethylsilylium Cation and Mechanism of the Isomer Interconversion", Organometallics 15 (1996) 5674-5677

12. J.T. Koskinen, M. Koskinen, I. Mutikainen, B. Mannfors and H. Elo, "Ethyl-methyl-glyoxal- bis(amidinohydrazonium)dichloride - water (1:2)", Acta Cryst. C (in press)

13. J. Blomqvist, J.T. Koskinen, L.-O. Pietilä and B. Mannfors, "Coupling of the amino torsion and the out-of-plane deformation in conjugated 3-center p-electron systems: formamide, formamidine, viny-lamine", manuscript in preparation

14. B. Mannfors, T. Sundius, K. Palmö, L.-O. Pie-tilä and S. Krimm, "Molecular mechanics energy functions from vibrational spectroscopy and quantum chemistry", J. Mol Struct (Theochem) (in press)

INVESTIGATION OF COMPLEX KINETICS USING RAMAN SPECTROSCOPY

Sisko Maria Eskola, Olli -Pekka Sievänen, Päivi Nevalainen, Kari O. Eskola and Folke Stenman

The investigations of complexes of iodine with benzene [15, 20], toluene [16], o- and p- xylene [17], mesitylene [18] and durene [19] have been concluded in their present form. The two-state model formulated in Ref. [15] is now being extended to comprise an intermediate state and more accurate measurements are being performed to serve that purpose.

15. S. M. Eskola and F. Stenman, Molec. Phys. 88 (1996) 1187

16. S. M. Eskola, Molec. Phys. 88 (1996) 1205

17. S. M. Eskola, Molec. Phys. 88 (1996) 1211

18. S. M. Eskola and F. Stenman, Molec. Phys. 88 (1996) 1471

19. A. Lindroos, S. M. Eskola and F. Stenman. Molec. Phys. 88 (1996) 1217

20. K. O. Eskola, M. Sc. Thesis, Department of Physics 1996

NUMERICAL METHODS FOR DATA ANALYSIS

Sisko Maria Eskola, Olli-Pekka Sievänen and Folke Stenman

The method originally sketched in Ref. [21] for analyzing small spectral shifts using the shift theorem of the discrete Fourier transform has been brought into final form [22]. Typically, it allows shifts between peaks to be determined to within better than 0.2 % of the peak width if the S/N ratio is around 50 and the peak is recorded with 10 - 15 points in the half width. The work on resolution enhancement of mo-lecular spectra using linear prediction has resulted in an efficient method based on following the true mean square prediction as function of filter length [23]. A typical application is the resolving of the 509 cm-1 doublet in the Raman spectrum of solid naphthalene (separation 0.8 cm-1), where ordinary peak fitting procedures were unable to reveal the existence of two peaks at 1.5 cm-1 experimental resolution. The linear prediction method clearly resolves these peaks. The new fast algorithm for the bit reversal in the Fast Fourier Transform has been finalized [24]. Spectral analysis methods have been developed and employed for analyzing and simulating formation of surface structures [25 - 27].

21. S. M. Eskola, Ph. D. Thesis, University of Helsinki, Report Series in Physics HU-P-D60, Helsinki 1995

22. S. M. Eskola and F. Stenman, Appl. Spectrosc., in press

23. O. - P. Sievänen, Appl. Spectrosc., in press

24. S. M. Eskola and F. Stenman, Appl. Spectrosc., in press

25. I. Koponen, M. Hautala, and O. - P. Sievänen, Phys. Rev. B 54 (1995) 13502

26. I. Koponen, M. Hautala, and O. - P. Sievänen, submitted for publication

27. I. Koponen, M. Hautala, and O. - P. Sievänen, IBMM-96, Conference Proceedings

RAMAN SPECTROSCOPY OF AROMATIC CRYSTALS

Olli-Pekka Sievänen, Leena Wallenius, Päivi Ne-valainen, Kari O. Eskola, Sisko Maria Eskola and Folke Stenman

The study of high-resolution spectra of aromatic crystals has been revived through the use of a closed-cycle helium sample cooling system combined with very careful refocusing and alignment of the double monochromator used. The first spectra of naphthalene exhibit considerable improvement of spectral definition and resolution over earlier measurements [28]. The results are now being subjected to further analysis using linear prediction for enhancing spectral resolution.

28. A. Karonen and F. Stenman, University of Hel-sinki, Report Series in Physics HU-P-182, 1982

OPTIMIZATION OF HOLOGRAPHIC DIFFRACTION GRATINGS

Folke Stenman

An efficient and fast method for designing aberration-corrected holographic diffraction gratings has been developed. The method has been applied to the devel-opment of several spectrometers as alternative base-line concepts for the European COALA ozone detection instrument [29, 30].

29. F. Stenman, Design of the COALA Grating Spectrometer - Preliminary Report, Report issued to the COALA Project, May 1, 1996.

30. F. Stenman, COALA Grating -Intermediate re-port, Report issued to the COALA Project, June 3, 1996.