A subject of this thesis work is the excitation energy transfer and ionization. processes in optically excited alkali gases In this work ionization processes and radiative. resonance energy transfer in atomic vapours are theoretically investigated Several. theoretical methods are developed in order to describe complicated atom atom collision. processes photoionization photodissociation and associative ionization processes in. excited alkali gases, In atomic vapours when atoms are colliding usually a short lived quasi molecular. system is produced For exact quantum chemical approaches to the analysis of this quasi. molecular system already a precise calculation of a single potential surface of a system as. simple as triatomic is a challenge not to talk about processes in which several potential. surfaces are involved Such calculations require complicated computational procedures. which are extremely demanding in terms of computer resources and susceptible to errors. Therefore the chances to achieve reliable results using only the exact quantum chemical. approaches are questionable This is especially true when one of the partners of the. atomic encounter is in a highly excited state or when the collision complex appears in a. highly excited state during the encounter Under such conditions one has to deal with. multiple surface crossings and overlapping dynamic resonances They induce chaos like. instabilities in the evolution of trajectories of weakly bound electrons or atoms. eventually leading to ionisation or fragmentation Such chaotic processes can be. described by the stochastic theory One of the tasks of this thesis is theoretical. description of the stochastic phenomena in atomic collisions and assessment of the. applicability of the kinetic Fokker Planck like diffusion equations for the. description of the evolution of collision complexes within a dense spectrum of energy. states The obtained results demonstrate the complexity of ionization processes following. excitation of atomic Rydberg states and indicate the path along which ionization of more. complex systems involving molecules can be studied both experimentally and. theoretically The stochastic theory was developed in collaboration with prof N. Bezuglov from St Petersburg State University Russia It treats the ionization as. excitation of Rydberg electrons to the continuum by the electric dipole field generated by. exchange interaction within the quasi molecular ion. In the stochastic theory redistribution of the population prior to ionization over a range of. Rydberg states due to non adiabatic processes in overlapping multiple level crossings of. quasi molecular Rydberg states is described in terms of stochastic diffusion of electrons. This diffusion process in the Rydberg energy spectrum is analyzed using a Fokker Planck. type equation In this work we are making the first attempt to take into account the. stochastic motion of Rydberg electron and apply this model to describe several atomic. collision processes Collisions leading to the formation of atomic Penning ionization. PI or molecular ions associative ionization AI have been studied The stochastic. processes are expected to become increasingly important in low temperature systems with. reduced collision velocities and increased collision times like an ensemble of ultracold. atoms in a magneto optical trap MOT Other important motivation to develop stochastic. theory is the quest for active control of chemical processes. The second problem addressed in this thesis work is related to photoionization and. photo transition processes in sodium atom This part of thesis deals with obtaining atomic. data that are needed for various scientific studies Of particular interest was the prediction. of radiative lifetimes of excited atomic energy states Ocscillator strengths and transition. probabilities to discrete atomic levels and to continuum states are often needed in. contemporary atomic physics experiments This was the main motivation of this part of. the thesis work Studies of literature showed that the existing transition probability data. of alkali atoms is limited to relatively low excited states whereas for Rydberg states they. were usually extracted by using in part oversimplified estimates The second task of the. present work was to calculate phototransition probabilities of Na 3p3 2 atoms to the. high Rydberg states and to the ionization continuum To achieve this task we used the. model potential method which has been already successfully exploited by other authors. in seventies and eighties of the last century The parameters of the potentials used in our. work were adjusted to reproduce the experimental energy eigenvalues The effects of. further small interaction terms on the valence electron were also taken into account. The third task of the thesis work is the investigation of the radiation trapping. in optically excited gases in complicated gas cell geometries and calculation of. radiation trapping factors in elliptical gas cells For this purpose we construct. analytical solution of Biberman Holstein radiation trapping equation using a new. semiclassical approach for multi dimensional semiclassical quantization laws called. geometrical quantization technique GQT If the absorbing medium is sufficiently. optically thick the radiation in it will undergo several reabsorption and reemission events. before escaping from the medium Radiation trapping in an atomic vapour can influence. many spectroscopic experiments For example radiation trapping plays an important role. in photodissociation experiment in supersonic Na Na2 beam analyzed in this thesis. Radiation trapping influences alo velocity distribution of photodissociation fragments and. because of that also the rate constants of the reactions In 1947 Holstein and Biberman. independently proposed a Boltzmann type integro differential equation describing this. phenomenon and this equation remains the starting point of most of the radiation. trapping models In our study we interpret the basic integro differential trapping equation. as a generalized wave equation for an associated hypothetical quasiparticle Some. modifications of the multi dimensional semiclassical quantization laws GQT allowed us. to determine analytically the trapping factors for all practically occurring line shapes all. opacities and all modes in a large variety of practically important types of 1D 2D and. 3D vapour cell geometries for which the separation of spatial variables is possible An. important point is that there are no geometrical and topological differences between. GQT for the Biberman Holstein trapping equation and conventional semiclassical. billiard like theories because of the similarity in the trajectories straight lines within the. cell connecting reflection points on the walls, The next fourth task of the promotion work is related to the investigation. of a photodissociation of sodium molecules Na2 X1 g in supersonic Na Na2. beam and calculations of cross sections for photodissociation of sodium molecules. Na2 X1 g by PD 458 nm radiation from an Ar ion laser required for the. interpretation of an imaging experiment with this photodissociation process It was. assumed that the electronic wavefunction can instantaneously adjust as the molecular. bonds stretch break and finally form the free toms This implicit assumption known as. the electronically adiabatic or the Born Oppenheimer approximation was used by us in a. simple two state model Exact knowledge of the photodissociation cross sections allows. calibration and testing of experimental set up Such testing method can be used also for. other spectroscopic experiments, Results are presented in the publications publ1 publ7 constituting the. publ1 N N Bezuglov V M Borodin V Grushevskyi A N Klyucharev K Michulis. F Fuso and M Allegrini Diffusion ionization of the Rydberg diatomic. quasimolecular complex formed upon collisions of rubidium atoms Opt. Spectrosc 95 515 524 2003, publ2 I I Ryabtsev D B Tretyakov I I Beterov N N Bezuglov K Miculis and A. Ekers Collisional and thermal ionization of sodium Rydberg atoms I. Experiment for n S and nD atoms with n 8 20 J Phys B 38 S17 S35 2005. publ3 K Miculis N N Bezuglov I I Beterov I I Ryabtsev D B Tretyakov A. Ekers and A N Klucharev Collisional and thermal ionization of sodium. Rydberg atoms II Theory for n S nP and nD states with n 5 25 J Phys B 38. 1811 1831 2005, publ4 I I Beterov D B Tretyakov I I Ryabtsev N N Bezuglov K Miculis A Ekers. A N Klucharev Collisional and thermal ionization of sodium Rydberg atoms III. Experiment and theory for nS and nD states with n 8 20 in a crossed beam. geometry J Phys B 38 4349 4361 2005, publ5 K Miculis W Meyer Phototransition of Na 3p3 2 into high Rydberg states and. the ionization continuum J Phys B 38 2097 2108 2005. publ6 N N Bezuglov A K Kazanskii A N Klyucharev K Michulis F Fuzo M. Allegrini On accounting for the effect of particles of a condensed dispersed. phase on radiant energy transfer in gaseous media Opt Spectrosc 95 631 637. publ7 O Kaufmann A Ekers K Bergmann N Bezuglov K Miculis M Auzinsh. and W Meyer Velocity redistribution of excited atoms by radiative excitation. transfer I Experimental demonstration by photodissociation of Na2 and field free. imaging J Chem Phys 119 3174 3186 2003, Introduction 7. 1 Ionization of Rydberg diatomic quasimolecular complex formed by collisions of. alkali atoms, s rmu met lu atomu sadursm s, 1 1 Introduction 11. 1 2 Non linear DSMJ model 14, 1 3 Collision velocity distributions 18. 1 4 The theory of Rydberg electron migration 21, 1 4 1 Diffusion equation 22. 1 4 2 Boundary conditions 24, 1 4 3 Effective collision and diffusion times 25. 1 5 Associative rate constants for Na nS P D states 27. 1 6 Conclusion 29, 2 Photoexcitation of Na 3p3 2 to high Rydberg states and photoionization. 2 1 Introduction 30, 2 2 Model potential 32, 2 3 Transition probabilities and transition dipole moments 34. 2 4 Photoionization cross sections 36, 2 5 Conclusion 38. 3 Semiclassical treatment of radiation trapping in spatially nonuniform media. 3 1 Introduction 40, 3 2 Biberman Holstein equation 41. 3 3 Geometric Quantization Technique 43, 3 4 Quantisation Rules 45. 3 5 Boundary phase jumps of the quasiparticle 46, 3 6 Parallelepiped with sides of length Hx Hy Hz 49. 3 7 Examples of elliptical geometries 49, 3 8 Conclusion 52. 4 Photodissociation of Na2 molecules, 4 1 Introduction 53. 4 2 Experiment of Na2 X g photodissociation process 54. 4 3 Theory of Na2 X g photodissociation process 55. 4 4 Conclusion 57, 5 Summary 58, References 61, Introduction. The alkali metal vapour plays an important role in the modern spectroscopy as a. test media for new experimental methods and theoretical approaches At the same time an. alkali atom vapour serves also as a media which allows to implement advanced. technological solutions For example the most sensitive magnetometers currently are. using rubidium vapour as a main ingredient, When an interaction of alkali vapours with laser radiation is analyzed one can not. neglect ionization processes radiation transfer and stochastic dynamics in optically. excited alkali gases, Collisional processes involving Rydberg atoms and ions are of interest both as a. fundamental problem of modern atomic physics and theory of atomic collisions and also. for numerous applications to spectroscopy kinetics of high and low temperature plasmas. and gases and astrophysics Atom in Rydberg state consist of a single excited electron. Rydberg electron far removed from atomic core while the remaining electrons screen. the nuclear charge Thus the Rydberg electron moves essentially in a Coulomb field. created by the core An electron in highly excited orbit is very weakly bound to the core. The energy gap between the adjacent excited states varies as n 3 and for atoms in Rydberg. states is very small Therefore such atoms are influenced extremely strongly even by a. weak external perturbation Under the action of electromagnetic fields or interaction with. other atomic particles Rydberg electron may easily make a transition to other highly. excited states or may be separated from the core resulting to the ionisation of the Rydberg. atom Numerous studies 29 79 of the time evolution of a Rydberg electron moving in a. Coulomb potential under the action of a variable field using quasi classical description. were made These studies revealed that Rydberg electron motion perturbed by this field. in the region of highly excited bound states exhibits transition from regular behaviour to. chaos as the external fields are increased As a result of this sensitivity to the external. field the observed behaviour of physical system that exhibit chaos appears to be random. even though the model of the system is deterministic in the sense that it is well defined. and contains no random parameters The highly excited hydrogen atom in a. monochromatic field is one of the simplest real non linear quantum system with. stochastic behaviour The theory of stochastic drift of a Coulomb electron in a microwave. field was developed in the works 29 30 31 In this thesis several methods borrowed. from these works were adopted to describe stochastic behaviour of the Rydberg atom in. the collisions with the ground state atom in an associative ionization process. optically thick the radiation in it will undergo several reabsorption and reemission events before escaping from the medium Radiation trapping in an atomic vapour can influence many spectroscopic experiments For example radiation trapping plays an important role in photodissociation experiment in supersonic Na Na2 beam analyzed in this thesis

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