We investigate chaotic ionization of highly excited hydrogen atom in crossed electric and magnetic fields (Rydberg atom) and intra-molecular relaxation in planar carbonyl sulfide (OCS) molecule. The underlying theoretical framework of our studies is dynamical systems theory and periodic orbit theory. These theories offer formulae to compute expectation values of observables in chaotic systems with best accuracy available in given circumstances, however they require to have a good control and reliable numerical tools to compute unstable periodic orbits. We have developed such methods of computation and partitioning of the phase space of hydrogen atom in crossed at right angles electric and magnetic fields, represented by a two degree of freedom (dof) Hamiltonian system. We discuss extensions to a 3-dof setting by developing the methodology to compute unstable invariant tori, and applying it to the planar OCS, represented by a 3-dof Hamiltonian. We find such tori important in explaining anomalous relaxation rates in chemical reactions. Their potential application in Transition State Theory is discussed.

Observations from the 1930s until the present have established the existence of dark matter with an abundance that is much larger than that of luminous matter. Because none of the known particles of nature have the correct properties to be identified as the dark matter, various exotic candidates have been proposed. The neutralino of supersymmetric theories is the most promising example. Such cold dark matter candidates, however, lead to a conflict between the standard simulations of the evolution of cosmic structure and observations. Simulations predict excessive structure formation on small scales, including density cusps at the centers of galaxies, that is not observed. This conflict still persists in early 2007, and it has not yet been convincingly resolved by attempted explanations that invoke astrophysical phenomena, which would destroy or broaden all small scale structure. We have investigated another candidate that is perhaps more exotic: Lorentz-violating dark matter, which was originally motivated by an unconventional fundamental theory, but which in this dissertation is defined as matter which has a nonzero minimum velocity. Furthermore, the present investigation evolved into the broader goal of exploring the properties of Lorentz-violating matter and the astrophysical consequences – a subject which to our knowledge has not been previously studied. Our preliminary investigations indicated that this form of matter might have less tendency to form small-scale structure. These preliminary calculations certainly established that Lorentz-violating matter which always moves at an appreciable fraction of the speed of light will bind less strongly. However, the much more thorough set of studies reported here lead to the conclusion that, although the binding energy is reduced, the small-scale structure problem is not solved by Lorentz-violating dark matter. On the other hand, when we compare the predictions of Lorentz-violating dynamics with those of classical special relativity and general relativity, we find that differences might be observable in the orbital motions of galaxies in a cluster. For example, galaxies – which are composed almost entirely of dark matter – observed to have enlarged orbits about the cluster center of mass may be an indication of Lorentz violation.

Vikram Sarabhai Space Centre,Trivandrum / In the space surrounding the earth there are two major regions where orbital debris causes concern. They are the Low Earth Orbits (LEO) up to about 2000 km, and Geosynchronous Orbits (GEO) at an altitude of around 36000 km. The impact of the debris accumulations are in principle the same in the two regions; nevertheless they require different approaches and solutions, due to the fact that the perturbations in the orbital decay due to atmospheric drag effects predominates in LEO, gravitational forces including earth’s oblateness and luni solar effects dominating in GEO are different in these two regions. In LEO it is generally known that the debris population dominates even the natural meteoroid population for object sizes 1 mm and larger. This thesis focuses the study mainly in the LEO region. Since the first satellite breakup in 1961 up to 01 January 2003 more than 180 spacecraft and rocket bodies have been known to fragment in orbit. The resulting debris fragments constitute nearly 40% of the 9000 or more of the presently tracked and catalogued objects by USSPACECOM. The catalogued fragment count does not include the much more numerous fragments, which are too small to be detected from ground. Hence in order to describe the trackable orbital debris environment, it is important to develop mathematical models to simulate the trackable fragments and later expand it to untrackable objects. Apart from the need to better characterize the orbital debris environment down to sub millimeter particles, there is also a pressing necessity of simulation tools able to model in a realistic way the long term evolution of space debris, to highlight areas, which require further investigations, and to study the actual mitigation effects of space policy measures. The present thesis has provided newer perspectives for five major issues in space debris modeling studies. The issues are (i) breakup modeling, (ii) environment modeling, (iii) evolution of the debris environment, (iv) collision probability analysis and (v) reentry prediction. The Chapter 1 briefly describes an overview of space debris environment and the issues associated with the growing space debris populations. A literature survey of important earlier work carried out regarding the above mentioned five issues are provided in the Chapter 2. The new contributions of the thesis commence from Chapter 3. The Chapter 3 proposes a new breakup model to simulate the creation of debris objects by explosion in LEO named “A Semi Stochastic Environment Modeling for Breakup in LEO” (ASSEMBLE). This model is based on a study of the characteristics of the fragments from on orbit breakups as provided in the TLE sets for the INDIAN PSLV-TES mission spent upper stage breakup. It turned out that based on the physical mechanisms in the breakup process the apogee, perigee heights (limited by the breakup altitude) closely fit suitable Laplace distributions and the eccentricity follows a lognormal distribution. The location parameters of these depend on the orbit of the parent body at the time of breakup and their scale parameters on the intensity of explosion. The distribution of the ballistic coefficient in the catalogue was also found to follow a lognormal distribution. These observations were used to arrive at the proper physical, aerodynamic, and orbital characteristics of the fragments. Subsequently it has been applied as an inverse problem to simulate and further validate it based on some more typical well known historical on orbit fragmentation events. All the simulated results compare quite well with the observations both at the time of breakup and at a later epoch. This model is called semi stochastic in nature since the size and mass characteristics have to be obtained from empirical relations and is capable of simulating the complete scenario of the breakup. A new stochastic environment model of the debris scenario in LEO that is simple and impressionistic in nature named SIMPLE is proposed in Chapter 4. Firstly among the orbital debris, the distribution of the orbital elements namely altitude, perigee height, eccentricity and the ballistic coefficient values for TLE sets of data in each of the years were analyzed to arrive at their characteristic probability distributions. It is observed that the altitude distribution for the number of fragments exhibits peaks and it turned out that such a feature can be best modeled with a tertiary mixture of Laplace distributions with eight parameters. It was noticed that no statistically significant variations could be observed for the parameters across the years. Hence it is concluded that the probability density function of the altitude distribution of the debris objects has some kind of equilibrium and it follows a three component mixture of Laplace distributions. For the eccentricity ‘e’ and the ballistic parameter ‘B’ values the present analysis showed that they could be acceptably quite well fitted by Lognormal distributions with two parameters. In the case of eccentricity also the describing parameter values do not vary much across the years. But for the parameters of the B distribution there is some trend across the years which perhaps may be attributed to causes such as decay effect, miniaturization of space systems and even the uncertainty in the measurement data of B. However in the absence of definitive cause that can be attributed for the variation across the years, it turns out to be best to have the most recent value as the model value. Lastly the same kind of analysis has also been carried out with respect to the various inclination bands. Here the orbital parameters are analyzed with respect to the inclination bands as is done in ORDEM (Kessler et al 1997, Liou et al 2001) for near circular orbits in LEO. The five inclination bands considered here are 0-36 deg (in ORDEM they consider 19-36 deg, and did not consider 0-19 deg), 36-61 deg, 61-73 deg, 73-91 deg and 91- 180 deg, and corresponding to each band, the altitude, eccentricity and B values were modeled. It is found that the third band shows the models with single Laplace distribution for altitude and Lognormal for eccentricity and B fit quite well. The altitude of other bands is modeled using tertiary mixture of Laplace distributions, with the ‘e’ and ‘B’ following once again a Lognormal distribution. The number of parameter values in SIMPLE is, in general, just 8 for each description of altitude or perigee distributions whereas in ORDEM96 it is more. The present SIMPLE model captures closely all the peak densities without losing the accuracy at other altitudes. The Chapter 5 treats the evolution of the debris objects generated by on orbit breakup. A novel innovative approach based on the propagation of an equivalent fragment in a three dimensional bin of semi major axis, eccentricity, and the ballistic coefficient (a, e, B) together with a constant gain Kalman filter technique is described in this chapter. This new approach propagates the number density in a bin of ‘a’ and ‘e’ rapidly and accurately without propagating each and every of the space debris objects in the above bin. It is able to assimilate the information from other breakups as well with the passage of time. Further this approach expands the scenario to provide suitable equivalent ballistic coefficient values for the conglomeration of the fragments in the various bins. The heart of the technique is to use a constant Kalman gain filter, which is optimal to track the dynamically evolving fragment scenario and further expand the scenario to provide time varying equivalent ballistic coefficients for the various bins. In the next chapter 6 a new approach for the collision probability assessment utilizing the closed form solution of Wiesel (1989) by the way of a three dimensional look up table, which takes only air drag effect and an exponential model of the atmosphere, is presented. This approach can serve as a reference collision probability assessment tool for LEO debris cloud environment. This approach takes into account the dynamical behavior of the debris objects propagation and the model utilizes a simple propagation for quick assessment of collision probability. This chapter also brings out a comparison of presently available collision probability assessment algorithms based on their complexities, application areas and sample space on which they operate. Further the quantitative assessment of the collision probability estimates between different presently available methods is carried out and the obtained collision probabilities are match qualitatively. The Chapter 7 utilizes once again the efficient and robust constant Kalman gain filter approach that is able to handle the many uncertain, variable, and complex features existing in the scenario to predict the reentry time of the risk objects. The constant gain obtained by using only a simple orbit propagator by considering drag alone is capable of handling the other modeling errors in a real life situation. A detailed validation of the approach was carried out based on a few recently reentered objects and comparison of the results with the predictions of other agencies during IADC reentry campaigns are also presented. The final Chapter 8 provides the conclusions based on the present work carried together with suggestions for future efforts needed in the study of space debris. Also the application of the techniques evolved in the present work to other areas such as atmospheric data assimilation and forecasting have also been suggested.

X-ray binaries are binary stellar systems containing a compact object and a normal companion star which are gravitationally bound and rotate about a common center of mass. The compact object accretes matter from the companion star. The accreted matter may have a high angular momentum and hence follow a Keplarian orbit about the compact object. It slowly spirals inward as its angular momentum is redistributed via viscous forces and forms an accreting disk before being finally accreted onto the compact object. The compact object that is accreting matter may either be a neutron star or a black hole. X-ray binaries can be broadly classified into two classes depending on the mass of the companion star. Low Mass X-ray Binaries (LMXBs) have companion star masses and accrete mass via Roche lobe overflow of the companion star. High Mass X-ray Binaries (HMXBs) have companion star masses and in these systems the compact object accretes matter from the high velocity stellar winds of the companion star. For the work and results that are presented in the thesis we have studied the orbital evolution, apsidal motion and long term flux variations in High mass X-ray binaries which have a neutron star compact object with very high magnetic field of the order of B ~ 1012 G. Due to the high magnetic field, the accretion disk is disrupted at the Alfven radius where the magnetic field pressure equals the ram pressure of the infalling matter. From that boundary, the flow of the infalling matter will be guided by the magnetic field lines. The infalling matter will follow these lines, finally falling onto the magnetic poles with velocity nearly equal to the free fall velocity and form an accretion column over the magnetic poles. A hot spot is formed at both the magnetic poles and high energy photons are emitted from these regions. Inverse Compton scattering of these photons by high energy electrons in the accretion column can produce hard X-rays. If the optical depth of the accretion column is low, the radiation comes along the magnetic axis forming a pencil beam whereas if the optical depth is high, radiation escapes tangential to the accretion column forming a fan beam. Since the neutron star is rotating about its rotation axis, the radiation beam directed along magnetic axis non-aligned with the rotation axis will sweep across the sky. Whenever this beam of rotating radiation is aligned with the line of sight, a pulse of X-ray radiation is detected. Hence these systems are also called X-ray Binary Pulsars (XBP). These pulses are emitted at equal intervals of time, where the time between the emission of two pulses is the spin period of the neutron star. But since the neutron star is in a binary orbit, the arrival time of pulses as recorded by an observer will be delayed or advanced due to the motion of the neutron star. When the neutron star is moving towards the observer, the pulses arrive faster and when the neutron star is moving away from the observer, the pulses are delayed. These delays or advances of the arrival time of pulses can be measured accurately which allows us to measure the orbital elements (ax sin i, Porb, e, ω, Tω ) of the neutron star orbit. The neutron star orbit may evolve with time due to mass loss from the system, mass transfer from the companion star onto the neutron star and due to tidal interaction between the neutron star and the companion star. Gravitational wave radiation may also cause orbital evolution. However, in HMXBs this effect is likely to be much weaker compared to the effect of mass loss, mass exchange and tidal interaction. Rossi X-ray Timing Explorer (RXTE) is an X-ray astronomy satellite launched in 1995 by NASA. It has two pointed instruments, the Proportional Counter Array (PCA) and the High Energy X-ray Timing Experiment (HEXTE). PCA has a large effective area of 6500 sq cm and works in the energy range of 2-60 keV. It has a very good time resolution of 1 microsec. HEXTE observes in the energy range of 15-250 keV and has a time resolution of 8 microsec. RXTE also has an All Sky Monitor (ASM) which scans 80% of the sky in 90 minutes. We have used RXTE-PCA data for timing and spectral studies and ASM data for the long term flux variation studies of Cen X-3. The thesis presents details of our work, the analysis of the data, results of the analysis and our conclusions from these results. The first chapter of the thesis gives an overview of X-ray binaries, their orbital evolution and the instrument details of RXTE. In the second chapter we have presented our work of timing analysis of three persistent sources, namely Cen X-3, SMC X-1 and 4U 1538–52. For the SMC X-1 system, we have for the first time measured the eccentricity and the angle of periastron (ω). We found that the accuracy of pulse timing analysis is limited by the dependence of pulse profile on orbital phase. The new measurement of the orbit ephemeris of Cen X-3 when combined with the previous measurements of orbit ephemeris obtained by observations from other X-ray missions, gave an improved measurement of the rate of orbital decay P˙orb/Porb ~ -1.8 x 10−6yr−1 . A long observation of SMC X-1 made by RXTE in 2000 during the high state of SMC X-1 allowed us to measure the very small orbit eccentricity e ~ 0.00021 in this system. SMC X-1 was again observed for a long time by RXTE during 2003 during its low state. The SMC X-1 pulse fraction depends on the flux state of the source such that the pulse fraction decreases with decrease in the source flux. Thus the 2003 observations of SMC X-1 have higher error in measurement of pulse arrival times compared to the 2000 observations and could not be used to measure the eccentricity of the orbit. But combining the orbital ephemeris of SMC X-1 measured using the 2000 and the 2003 observation with the epoch history allowed us to improve the measurement of rate of orbit decay by an order of magnitude compared to previous observations P˙orb/Porb ~ - 3.4 x 10−6yr−1 . We observed 4U 1538–52 with RXTE under the guest observer program to measure the orbital evolution of this system. From observations of this system with BeppoSAX , a circular orbit similar to the SMC X-1 system was inferred. 4U1538–52 was observed with RXTE again in 1997 and analysis of this observation showed it to have eccentric orbit with a marginal evidence for an orbital decay. Our analysis carried out using the 2003 RXTE observation data confirmed that the orbit is eccentric with e ~ 0.18. But the new orbital ephemeris measured clearly shows that the orbit is not evolving with time as reported earlier. We have derived an upper limit on the rate of change of orbital period of this system to be P˙orb/Porb = 2.5 x 10−6yr−1 . 4U 1538–52 is similar to SMC X-1 in many respects, both have similar orbital period of Porb(SMC X -1) = 3.89 days and Porb(4U1538 - 52) = 3.72 days and companion star mass. But tidal interactions between the neutron star and the companion star have almost circularised the orbit of SMC X-1 where as the orbit of 4U 1538–52 is quite eccentric. Therefore we conclude that 4U 1538–52 is a young system and hence the orbit has not circularised by tidal interaction. The neutron star orbit also precesses due to tidal interaction and rotation of the companion star, which causes the longitude of periastron ω to change with time. The rate of change of ω can be measured by comparing the orbital elements of the neutron star orbit measured at different epochs of time. This rate of change of ω is directly related to the mass distribution of the companion star and hence the apsidal motion constant that are predicted by the theoretical models for stellar structure. Therefore measuring ˙ω will be a direct test for the stellar structure models. But ω can be measured only when the orbit is eccentric and for this purpose the Be-star/X-ray binary pulsars are the most suitable objects. The Be-star/X-ray binary pulsars are transient systems and have wide eccentric orbits of Porb > 10 days. The Be-stars are fast rotating stars with rotational velocity near to the break-up velocity. They eject matter along their equator in a circumstellar disk. When the neutron star intercepts this circumstellar disk during its periastron passage, the rate of mass accretion increases and the system becomes bright in X-rays. These short outburst are called the type-I X-ray bursts. The Be-star also has episodes of high mass ejection when the neutron star may accrete a larger amount of matter and can be seen over several binary orbits. These long duration outbursts are called type-II X-ray bursts. In the third chapter of the the thesis we have reported the analysis and results of three Be-/X-ray binary pulsars we have studied, namely 4U 0115+62, V0332+52 and 2S 1417-624 which were observed by RXTE during their respective type-II bursts. The X-ray pulse profiles of the Be-/X-ray systems evolve as a function of the source flux. Generally a simple single peaked pulse profile is seen during the onset of the outburst, which evolves into a more complex multiple peaked pulse profile as the source flux increases. The pulse profile again returns to the simple single peaked profile as the outburst fades off and the source flux decreases to persistent X-ray flux levels. Also due to varying mass accretion rate, the spin period evolves during the outburst. Both these factors together reduce the accuracy of measuring the arrival time of pulses. Hence we have used the instantaneous spin period measurements to deduce the orbital parameters of these system. The apparent spin period (Pspin) of the neutron star is modified by the radial velocity of the neutron star due to Doppler effect. The radial velocity of neutron star is dependant on the neutron star orbit and hence measurement of the spin period of the neutron star at different orbital phases allows us to determine the orbital elements. 4U 0115+63 was observed with the RXTE during two of its recent type II outbursts in 1999 and 2004. We measured the orbital parameters during both these outbursts independently. We combined the previous measurements of ω with our two measurements and measure the rate of apsidal motion of the system to be ˙ω ~ 0o .06 yr−1. V0332+52 was seen in outburst during 2004. During its previous outburst of 1983 only nine spin period measurements had been obtained and the orbital parameters measured from them were erroneous. We have measured the orbital parameters of this system accurately and determined the correct projected semi-major axis ax sin i and orbital period. The new orbit parameters can now be used to compare with future orbital element measurements to estimate any apsidal motion and/or orbital evolution in this system. We also used the 1999 outburst of 2S 1417–624 to accurately measure the orbital parameters of this system. We have also investigated the long term flux variations in the X-ray light curves of X-ray Binaries. Our studies on the flux variations observed in Cen X-3 are described in the fourth chapter of the thesis. Long term light curves of X-ray binaries show variations due to many reasons. Periodic variations of few milliseconds to a few hours in the light curve are seen due to spin of the neutron star. Light curves show variations due to motion of the neutron star in its orbit at timescales of few minutes to several days. Many sources also show quasi periodic variations in their X-ray light curves at timescales smaller than the neutron star orbital period which are believed to arise due to some material inhomogeneity orbiting the neutron star. These variations are called quasi periodic oscillations (QPOs). QPOs in X-ray binaries are observed between a frequency range of few millihertz to a few kilohertz. Long term X-ray light curves of many sources also reveal flux variations at time scales greater than the respective orbital period of the source. These variations are called superorbital variations. Systems like Her X-1, LMC X-4, 2S0114+650, SS 433, XTE J1716–389, 4U 1820–303 and Cyg X-1 show periodic superorbital variations whereas other systems like SMC X-1, GRS 1747-312, Cyg X2, LMC X-3 and the Rapid Burster show quasi periodic superorbital flux variations. These superorbital flux variations are understood as arising either due to a changing mass accretion rate which could be aperiodic in nature or as due to obscuration of the central X-ray source by a tilted, warped and precessing accretion disk. Many theoretical models have been proposed to explain the disk precession. The long term flux variations in the X-ray light curves of bright persistent X-ray binaries like Her X-1, SMC X-1 and LMC X-4 have been understood to be due to a periodic (in case of Her X-1 and LMC X-4) or a quasi periodic (for SMC X-1) precession of a warped accretion disk. We analysed the light curves of Cen X-3 obtained with the RXTE-ASM. The Cen X-3 light curves show aperiodic X-ray flux variations in all the energy bands of 1.5-3, 3-5 and 5-12 keV. The high and low states last for a few to upto a hundred days. The source also shows two spectral modes during the observations carried out with the ASM. The source was in a hard state during December 2000 to April 2004. At first look the aperiodic variations seen in Cen X-3 light curves seem to be arising due to a changing mass accretion rate. To investigate the cause of these aperiodic flux variations of Cen X-3 we studied the orbital modulation and the pulsed fraction as a function of source flux state. In the high state, the eclipse ingress and egress are found to be sharp whereas in the intermediate state, the transitions are more gradual. In the low state, instead of eclipse ingress and egress, the light curve shows a smooth intensity variation with orbital phase. The orbital modulation of the X-ray light curve in the low state shows that the X-ray emission observed in this state is from an extended object. The intensity dependent orbital modulations indicate that the different intensity states of Cen X-3 are primarily due to varying degree of obscuration. Measurements of the pulsed fraction in different intensity states are consistent with the X-ray emission of Cen X-3 having two components, one highly varying component with a constant pulsed fraction and a relatively stable component that is unpulsed and in the low state, the unpulsed component becomes dominant. The observed X-ray emission in the low state is likely to be due to scattering of X-rays from the stellar wind of the companion star. Though we can not ascertain the origin and nature of the obscuring material that causes the aperiodic long term intensity variation, we point out that a precessing accretion disk driven by radiative forces is a distinct possibility. We also studied the QPOs in Cen X-3 that are seen at 40 mHz. The QPOs are explained by the Beat Frequency Model (BFM) as arising due to the beat between the Keplarian frequency of the inner accretion disk and the spin of the neutron star. Thus when the mass accretion rate is high the inner disk radius decreases, increasing the Keplarian frequency and hence the observed QPO frequency and vise versa when the mass accretion rate decreases. Thus if the flux variations of Cen X-3 were due to a changing mass accretion rate then the observed QPO frequency should have a positive correlation with the observed X-ray flux of the source. But we find in our study that the QPO frequency does not have any correlation with the observed X-ray flux and the QPO frequencies does not follow the Frequency-Flux relation as expected in the Beat frequency model. Thus the QPO behaviour is in agreement that the observed X-ray flux does not indicate the true X-ray intensity state and hence the mass accretion rate in Cen X-3. Therefore, we conclude that X-ray variations of Cen X-3 are not due to changing mass accretion rate but due to varying obscuration of the central X-ray source, possibly by an accretion disk which precesses aperiodically. The conclusions from our studies presented in chapter 2, 3 and 4 of the thesis are summarised in the final chapter. The improved measurements of the rate of change of orbital periods from our work can now help us to detect any small departures from a constant period derivative in the persistent HMXB systems. The improved measurements of the orbital elements of Be-/X-ray binaries can now be used to study orbital evolution and apsidal motion in these system. New outbursts of the transient systems observed by future satellites providing good timing accuracy and large effective area, like LAXPC (Large Area X-ray Proportional Counter) of the ASTROSAT mission will facilitate such studies. The long term X-ray light curves study as done for Cen X-3 can be extended to other X-ray binary systems observed by All Sky Monitor. The method of source flux state dependent studies developed to study the Cen X-3 system can be easily extended to other systems that show long term superorbital flux variations. These kind of studies can be done by future proposed X-ray missions like ASTROSAT which will have a Sky Monitor similar to ASM dedicated to monitor X-ray sources. More sensitive measurements of long term X-ray light curves with the MAXI mission will allow similar studies of a large number of X-ray binaries and we will be able to see if aperiodically precessing accretion disk is present in many X-ray binaries.

No description available.

During the last decades the specific manipulation of matter on the (sub-) nanometer scale, also known as nanoscience, became possible by technologies such as the scanning tunneling microscope. Nanocatalysts, i.e. catalytic active structures of up to a few nanometers in size, belong to this rather new class of materials. Unlike ordinary ’macroscopic’ catalytic materials, the performance of nanocatalysts does not simply scale, for instance, with the surface to volume ratio of the active material. In this Thesis model nanocatalysts are investigated by means of ab-initio density functional theory calculations. In paper I, we explain the experimentally observed catalytic characteristics of small gold clusters, Au1-4, on a regular magnesium oxide terrace towards the oxidation of carbon monoxide by thoroughly studying the adsorption of CO and O2 on these clusters. In the subsequent paper II, we study the feasibility of a catalytic water-mediated CO oxidation reaction on Au1-4/MgO and find that this reaction mechanism is not assessable for Au2,4/MgO and unlikely for Au1,3/MgO. Papers III and IV concentrate on the reactivity of clusters in the gas phase. Particularly, we focus on the relative stability of Au13 isomers and its potential for O2 dissociation (paper III). We find the lowest energy isomers, which contain a triangular prism at their center surrounded by a ring of the remaining seven atoms, to be generally stable upon O2 adsorption. The dissociation of O2 at certain sites of Au13 is found to be exothermic. In paper IV we performed scans of the Born-Oppenheimer potential energy surfaces of neutral and charged Cu3, Ag3, and Au3 to explore the thermally excited vibrations of these trimers. While the Born-Oppenheimer surface of Cu3 exhibits one fairly deep energy minimum, it is comparatively flat with two shallow minima in the case of Ag3. Hence for Ag3 there exist many thermally accessible geometries in a wide range of angles and bond lengths. For Au3, two distinct energy minima appear, being well-separated by a barrier of 180 meV. Already at room temperature, we find bond lengths changes of up to 5% for the studied trimers. Choosing Au3 as a case study for the changed reactivity of thermally excited modes, we find CO to bind up to 150 meV stronger to the excited cluster. Gold deposited on graphene and graphite was observed to form larger aggregates. In paper V, we study the electronic structures, high mobility, and substrate-mediated clustering processes of Au1-4 on graphene. Already in the 1970s is was speculated that dispersion forces, i.e. van der Waals forces, significantly contribute to the adsorption energies of gold atoms on graphite. We accounted for van der Waals interactions in our density functional theory calculations (paper VI) and investigated the influence of these dispersion forces on the binding of copper, silver, and gold adatoms on graphene. While copper and gold show a mixed adsorption mechanism, i.e. chemical binding plus attraction due to the van der Waals forces, silver is purely physisorbed on graphene. / Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 719

ν Octantis is a spectroscopic binary with a semi-major axis and period of 2.55 AU and 2.9 years, respectively. Ramm et al. (2009) discovered a 52 ms^(-1) radial-velocity (RV) perturbation with a period of 417 days in this system. All evidence, both photometric and spectroscopic, suggests the perturbation is the result of a 2.5 MJ planet orbiting the primary star. However, when assuming a “normal” prograde coplanar orbit, celestial mechanics predicts this orbit is unstable, contradicting the observed stability. Simulations by Eberle and Cuntz (2010) showed a retrograde orbit for the planet to be stable for at least 10^7 years. In this thesis, we performed a 10^8 -yr simulation of the retrograde orbit, and found it remained stable. Simulations over a range of planetary semi-major axes, eccentricities, and primary/secondary masses showed that stable retrograde orbits are not possible past a semi-major axis of 1.315 +/- 0.092 AU . Therefore, planetary retrograde orbits are most likely inherently more stable than prograde orbits owing to the absence of stability at known mean-motion resonances. Eccentricity simulations showed that the period of the planet's dominant eccentricity variation is related to the planet's semi-major axis by a second order exponential. However, retrograde orbits tend to have longer eccentricity periods than prograde orbits at the same semi-major axis. There is also evidence that this eccentricity period is connected to the orbital stability. By fitting a keplerian to both Ramm et al. (2009) and current radial velocities, the period of the ν Octantis binary was determined to be 1050.04 +/- 0.02 days with an eccentricity of 0.2359 +/- 0.001 . The planetary orbital solution for just the data reduced in this thesis gave a period of 416.9 +/- 2.1 days and an eccentricity of 0.099 +/- 0.015 , with an RMS scatter of 9.6 ms^(-1). Therefore, the orbital elements are within 1σ of the Ramm et al. (2009) elements. Assuming a retrograde coplanar orbit about the primary star then the planet has a mass of M_pl = 2.3 M_J and a semi-major axis of a_pl = 1.21 +/- 0.09 AU.

This work comprises two main parts. The first part outlines our efforts to expand on the recent work of Gütlich et.al. by synthesizing Co(II) based spin crossover systems within a dendritic framework. We wanted to investigate the possibility of synthesizing different first generation, triazole containing dendrimers using “click” type reactions and their coordination ability with Co(II) ions. To this end we have had limited success mainly due to the numerous challenges in synthesizing a pure dendrimer product. The second part details our efforts in the synthesis of a mononuclear Co(II) based single molecule magnet. This comes as an extension to recent reports by Chang and Long where they have successfully obtained mononuclear Fe(II) single molecule magnets by inducing structural distortions within the complexes to amplify the spin-orbit coupling. We postulated that the use of Co(II) in conjunction with a bulky ligand framework would lead to desirable magnetic properties. We chose the known bis(imino)pyridine ligand scaffold due to its rich chemistry and its interesting and unexpected coordination behaviour, as we have seen in previous research efforts by our lab. To this end we were successful in isolating and characterizing 4 compounds, and we have carried out detailed magnetic measurements on the two most magnetically interesting species.

Les carbures et oxydes des métaux de transition sont d'une importance capitale dans le domaine industriel voir catalytique. Le carbure de tungstène WC a été identifié comme un bon substituant des métaux nobles tel que le platine dans le domaine catalytique. Le but de ce travail de thèse est d'appliquer des méthodes de chimie quantique les plus poussées pour déterminer la structure électronique, la stabilité et la nature de liaison chimique des diatomiques WC et WC2+. Notre recherche préliminaire est motivée par les données spectroscopiques disponibles sur W, W+, W2+, WC et WC2+. La méthodologie adoptée, CASSCF/MRCI/MRCI+Q/aug-cc-pV5Z(-PP) implémentée dans le code MOLPRO, consiste à réaliser des calculs quantique tenant en compte des effets de corrélation et relativistes avec un traitement spécifique du couplage spin−orbite pour la recherche des courbes d'énergie potentielle de l'état fondamental et des états excités de plus basses énergies de WCn+ (n=0-2) tout en utilisant une base suffisamment étendue. Les résultats de ce travail sont en bon accord avec ceux disponibles dans la littérature. En outre, dans ce travail nous avons confirmé pour la première fois que le carbure diatomique dicationique WC2+ est thermodynamiquement stable / Metal carbides and oxides are more interesting in catalytic and industrial domains. Tungsten carbide WC has been detected as serious substituent of platinum Pt catalytic. The ultimate goal of this thesis is theoretical studies of electronic structure, stability and the bound nature on WC, WO and its cations. Our preliminary research were motiving by the available spectroscopic data on W, W+, W2+, WC et WC2+. We used the methodology (CASSCF/MRCI/MRCI+Q/aug-cc-pV5Z(-PP)) implemented on MOLPRO package to perform quantum calculations with high accuracy taking into account the correlation and relativistic effects with a specific treatment of spin orbit coupling for some low lying excited electronic states of WCn+, (n=0, 1 et 2). Our results are shown in good agreement with those available in the literature. Furthermore, in this work for the first time we demonstrated that a carbide dication (WC2+) is thermodynamically stable

Dans cette thèse, nous avons étudié la dynamique des parois de domaine sous courant dans des couches ferromagnétiques ultra-minces de type métal lourd/ métal ferromagnétique/ oxyde présentant un fort couplage spin-orbite. Dans ces systèmes, deux éléments liés au fort couplage spin-orbite et l'asymétrie structurelle d'inversion jouent un rôle clé sur la dynamique des parois : d'une part, l'amplitude des couples de spin-orbite (SOT) exercés sur la paroi lors de l'injection de courant; d'autre part, l'amplitude de l'interaction Dzyaloshinskii-Moriya qui stabilise la structure Néel interne de la paroi. L'objectif de ce travail a été de caractériser les couples agissant sur la paroi induits par le courant ainsi que l'amplitude de l'interaction DMI.Pour y parvenir, j'ai mis au point une nouvelle technique de mesure basée sur la mesure des déplacements nanométriques induits par le courant d'une paroi piégée dans un nanoplot découpé dans le matériau magnétique. Cette mesure quasi-statique permet de s'affranchir des difficultés liées à la modélisation de la dynamique des parois magnétique sous courant en présence de défauts.Par ailleurs, le dispositif a été conçu de façon à ce que le courant et le champ magnétique externe statique puissent être appliqués dans différentes directions orthogonales, ce qui permet séparer clairement les contributions des couples de transfert de spin (NA-STT) et de spin-orbite (DL-SOT).Les mesures ont permis de caractériser le couple exercé sur la paroi par le courant en fonction d'un champ magnétique planaire pour un empilement Pt/Co/AlOx et ceci pour quatre orientations champ/courant différentes. Les résultats permettent d'écarter l'hypothèse d'une structure de type Bloch bi-stable.Dans le cas où le courant est injecté au travers de la paroi, la comparaison des résultats avec le modèle aboutit à une valeur du couple NA-STT très faible. Nos mesures faites avec le champ magnétique planaire permettent de conclure à un champ SOT de 7,5+/-0,5 Oe pour 10 MA/m² en accord avec les résultats de couple publiés précédemment dans le cas d'une paroi de Néel. Si les deux configurations donnent de mesures de couple SOT similaire, elles ne permettent pas de conclure sur la valeur de DMI dans ce système. L'origine de ces valeurs contradictoires reste à expliquer. / In this thesis, we studied the current induced domain walls (DWs) dynamics in ultra-thin ferromagnetic films of heavy metal/ ferromagnetic metal/ oxide type with a high spin-orbit coupling. In these systems, two ingredients linked to the high spin-orbit coupling and the structural inversion asymmetry play a key role on the DWs dynamics: the amplitude of the spin-orbit torques (SOT) acting on the domain when a current is injected; and the amplitude of the Dzyaloshinskii-Moriya interaction (DMi) which stabilizes the Néel structure of the DW. The purpose of this work was to characterize the current induced torques acting on the DW and the amplitude of the DMi.For that purpose, I developed a new measurement technique relying on the measurement of current induced nanometer size motion of a DW, trapped inside a nanodot patterned in the magnetic material. This quasi-static measurement enables to avoid the difficulties related to the modelling of the DW dynamics in the presence of defects.Besides that, the device has been designed to enable different perpendicular directions for the current and the external magnetic field, which enable a clear measurement of spin transfer (NA-STT) and spin-orbit (DL-SOT) torques contributions.The measurements allowed the characterization of the torque exerted by the current on the DW with respect to a planar magnetic field for a Pt/Co/AlOx stack in 4 different couples of field/current directions. The results allow to exclude the hypothesis of a Bloch structure for the DW.In the case where the current is injected through the DW, the comparison between the results and the model leads to a very weak value for the NA-STT. Our measurements made with the planar magnetic field leads to a value of 7,5+/-0,5 Oe per 10 MA/m² for the DL-SOT, which is in agreement with previously published results in the case of a Néel DW. If both configurations lead to similar measurements for the SOT, they don't permit to conclude on the exact value of the DMi in this system. The origin of these contradictories values is still to be understood.