Table of contents

The angular distribution parameter for the Xe ion state has been studied for the Xe autoionization resonances in the energy range between 20 and 24 eV using tunable synchrotron radiation from the Daresbury Laboratory SRS. A new magnetic angle-changing technique has been used and is reported for the first time in photoionization measurements. This technique allows the angular measurements to be made without any movement of the electron spectrometer. The present measurements have been made as continuous functions of both photon energy and photoelectron ejection angle which highlights the dramatic effect of those autoionizing resonances on the angular distribution parameter.

The two-dimensional R-matrix propagator is used to calculate total cross sections and resonance parameters for electron-impact excitation of the ground state into the states at collision energies up to the n = 4 threshold. For the 1s - 2s transition, the results are in excellent agreement with those of an algebraic variational method. Between the n = 2 threshold and the first resonances, the shape of the cross section differs from that obtained from a convergent close-coupling calculation, but is similar to that found by other methods. The long-standing discrepancy with the experimental cross sections remains.

In order to evaluate the influence of a plasma on the polarization state of radiation, we have solved the general radiative-transfer problem, the solution of which is not a scalar quantity but the Stokes vector, formed of four components. We have used a matrix formalism and, accordingly, the plasma properties (absorption, stimulated emission, etc) are represented by a matrix, i.e. the absorption matrix corrected for stimulated emission. We have calculated the Stokes parameters of the Ne-like Ge 2 - 1 lasing line (23.6 nm), for a situation where a linearly polarized wave, obtained from the output of a plasma called an injector, is amplified in a second plasma. The degree of polarization depends strongly on the intensity of the incident polarized beam and weakly on the length of the amplifier. The comparison with the experiment of Rus et al has shown good agreement.

An analytical solution for the off-resonant density matrix is presented in such a form that allows us to interpret at certain physical conditions the interaction of an ultracold atomic beam with inhomogeneous laser radiation as beams pass through a three-dimensional hologram. A new method for creating an atomic wave with intended amplitude and phase characteristic is offered on this basis.

The electron - electron coalescence h(0) and counterbalance d(0) densities are probability densities of finding any two electrons, respectively, at the same position and at the reflection points in the three-dimensional position space. The momentum-space counterparts and represent the probability densities for any pair of electrons to have exactly the same and opposite momenta, respectively. These electron-pair densities, as well as the average position and momentum one-electron densities, are computed systematically for the atoms He through Xe in their ground state by using the numerical Hartree - Fock method. The results are discussed in terms of the atomic number, electronic configuration and subshell-pair contributions with our particular interest in the electron - electron coalescence densities h(0) and .

The continuum molecular orbitals arising from calculations using special polynomial Gaussian functions are accurately tested to probe their capability in producing accurate photoemission cross sections. The molecular potential is represented by the local approximation, which permits a comparison with numerically integrated (exact) orbitals. Several strong anisotropic molecules are taken into account to check the flexibility of the bases in the presence of strong attractive potentials. This work shows that for adequate choices of Gaussian functions, phase shifts may be estimated with reasonable accuracy while the errors in the continuum transition probabilities are within 2 - 3% in the more unfavourable cases. Therefore, the proposed basis sets appear to be suitable for the description of the electronic continuum in molecular photoionization calculations.

The results of calculations investigating the effects of autodetaching resonances on the multiphoton detachment spectra of are presented. The R-matrix Floquet method is used, in which the coupling of the ion with the laser field is described non-perturbatively. The laser field is fixed at an intensity of , while frequency ranges are chosen such that the lowest autodetaching states of the ion are excited through a two- or three-photon transition from the ground state. Detachment rates are compared, where possible, to previous results obtained using perturbation theory. An illustration of how non-lowest-order processes, involving autodetaching states, can lead to light-induced continuum structures is also presented. Finally, it is demonstrated that by using a frequency connecting the 1s and 2s states, the probability of exciting the residual hydrogen atom is significantly enhanced.

The resonant Auger spectrum of CO has been measured at both the C and O K-edges. At the C resonance the decay spectrum was recorded selectively at the energies of the = 0, 1 and 2 vibrational states. Vibrational fine structure was not only resolved on the participator but also on the spectator lines. For the O resonance the photon energy bandwidth was sufficiently low that different, vibrationally distinct regions of the absorption profile could be selected. Ab initio calculations and their detailed analysis are presented for both the C and O decay spectra. At the C resonance the calculation explicitly accounts for the vibrational fine structure including vibrational-lifetime interference effects. The appearance of the spectator part of the spectrum is shown to be strongly influenced by the existence of unbound states and diabatic interactions. The nodal structure of the vibrational wavefunction of the core-excited state is reflected in the decay spectrum.

Measured L x-ray ionization cross sections for bombardment of Yb in the energy range 1.0 - 8.5 MeV are compared with the ECPSSR theory. Significant discrepancies between theory and experiment are observed at an incident beam energy below about 5 MeV. The discrepancies are greatly reduced by the inclusion of collision-induced intrashell coupling (ECPSSR-IS) and modifications of the binding factor in the theory in the united atom picture (ECPSSR-IS-UA). More specifically, in this case of Yb L-shell ionization by energetic ions, the contribution of L-subshell coupling towards better agreement with the theory is more significant than binding energy correction.

We show that an imperfection of velocity-selective coherent population trapping (VSCPT) in three-level atoms excited by standing light waves causes a rectified force on cooled atoms. The rectified force as well as the cooling force are calculated both analytically and numerically for and cascade three-level systems. Combination of these forces with the VSCPT mechanism can lead to localization of very cold atoms in potential wells created by the rectified force. This effect should be taken into account in experiments with VSCPT in standing waves, and can be used for realizing superlattices of cold atoms, in particular, cold Rydberg atoms.

The two-step triple photoionization (TPI) process is investigated in the framework of many-body perturbation theory for the first time. Results on TPI partial cross sections from the Kr ground state to the fluorescing states of the Kr IV configuration are obtained by numerical calculations in the energy range of Kr I resonances. Several mechanisms of two-step TPI are discussed, of which the low-energy double-Auger transitions are investigated in detail. The energy distribution functions of the Auger electrons for the different orbital momenta of the ejected two Auger electrons as well as the total transition probability are presented. Comparison of the calculated TPI cross section with experimental data obtained by photon-induced fluorescence spectroscopy demonstrates quite clearly the shortcomings of the two-step approximation for a quantitative description of the process, at least in the energy region under consideration. Nevertheless qualitative agreement has been achieved which explains in principle the new fluorescence transitions in Kr IV observed after excitation of the atom by photons of an energy from 90 to 100 eV.

We investigate the recently observed sidelobes in the angular distribution of high-order above-threshold ionization (ATI) by placing an atom in a bichromatic laser field of frequencies and where both components have a relative phase . Using an improved Keldysh - Faisal - Reiss (KFR) model, in which the Coulomb effects of the parent ions are taken into account, we show that even for a second field of very low intensity, drastic changes in the angular distributions occur as a function of the phase . Relations of our work to previous investigations are established.

We have studied multiphoton above-threshold dissociation (ATD) of and by a time-independent close-coupling (CC) method over a wide range of intensities for three initial bound states (v = 0, 1, 2 and J = 0) at a laser frequency of . Molecular rotation is taken into account by including 30 channels with J = 0 - 5 for and 20 channels with J = 0 - 7 for and number of absorbed photons n = 1 - 4. All the radiative couplings (including those due to the permanent dipole moment of ) have been considered in a truncated length-gauge form of interaction. At lower intensities the total dissociative decay is exponential, but at high intensity several dressed states with different linewidths contribute to the decay and no single decay rate can be defined. Use of the proper potential surfaces for , including intramolecular nonadiabatic coupling, enables us to obtain the branching ratios between the production of and fragments. The branching ratios for different numbers of photon absorption (n = 1, 2, 3) change dramatically with intensity. For , branching to channels, forbidden in by symmetry, may become dominant over a certain range of intensity. We have tried to interpret these intensity dependences in terms of the motion of the nuclei on the adiabatic field + molecule potential surfaces.

Phase control of the spatial asymmetry of the dissociative ionization channel, , of molecular hydrogen has been achieved by superposing interferometrically sub-100 fs, Ti:sapphire laser pulses at the fundamental and second harmonic frequency. The counterintuitive asymmetry previously observed with longer, less intense Nd:YLF laser pulses and HD molecules has been reproduced. However, phase modulation of total ionization and dissociation rates was not discernible. In order to further understand the process, a comprehensive study of appearance intensities, kinetic energy releases and angular distributions has been undertaken at each frequency.

We present measurements of the state-to-state differential scattering cross section (DCS) for with Xe using a new sub-Doppler spectroscopic method and compare results with those obtained using molecular beam techniques on the same system; the agreement is good. Criteria for comparison are based on the recent finding (McCaffery A J and Wilson R J 1996 Phys. Rev. Lett. 77 48) that simple quasiclassical vector relations determine the direction of the initial relative velocity vector and the scattering angle. The incident velocity direction is such that the perpendicular component just opens the inelastic channel and the parallel component is scattered. The relationship is Newtonian but is modified by the molecule's characteristic quantum structure. Angular distributions from a range of experiments are found to obey the vector relation, suggesting that atom - molecule collisions are controlled by a quantum-modified Newtonian mechanics, the modification arising as a consequence of the internal energy level structure of the molecule. We introduce the term quantum-constrained kinematics to describe the hybrid mechanics and suggest that this may also influence other collisional events.

L x-ray production in and was measured for bombardment in the energy range 1 - 5 MeV. Very thin target foils were used, and x-ray yields were measured simultaneously with elastically scattered ions. The L-shell and individual and production cross sections and their ratios were extracted. These cross sections are compared to the results of the ECPSSR theory (energy-loss and Coulomb deflection effects, perturbed stationary state approximation with the relativistic correction), its united-atom (UA) extension UA-ECPSSR and the UA-ECPSSR-MI, which also accounts for multiple ionization (MI). With a few exceptions, the standard ECPSSR appears to be better than its modifications when and ratios are analysed. and and total L x-ray production cross sections, however, are in the best overall agreement with the UA-ECPSSR-MI theory.

Electron impact excitation of the lowest nine excited levels ( and ) of Xe is studied theoretically. The cross sections, both integral and differential, are calculated up to 35 eV using the Breit - Pauli R-matrix method. At collision energies below 15 eV, all the integral cross sections show resonance structure. At 15 and 20 eV, the resulting differential cross sections are compared with experimental results. The overall agreement is satisfactory.

With an extended version of the classical trajectory Monte Carlo method to take into account the quasi-integrable initial state, we calculate the ionization cross section for singly charged ions colliding with Rydberg atoms in the presence of a magnetic field of 4 Tesla. Strong evidence for long-lived quasi-bound states, created and located around three centres (target nucleus, projectile nucleus and potential saddle between the two) is found which are caused by the trapping and multiple scattering of emitted electrons in the external fields.

The dissociation of ions produced in collisions of ions with CO has been studied by time-of-flight measurements. Both singles and coincidence time-of-flight techniques have been used to determine the kinetic energy release of the dissociating CO molecules. We describe the method to transform the singles and coincidence time-of-flight spectra into total kinetic energy distributions and discuss these distributions. They represent kinetic energy release distributions which clearly exhibit various contributions associated with different dissociation channels. In comparison with other ionization methods similarities but also clear differences are noted.

The dissociation of ions produced in collisions of ions with CO has been studied by time-of-flight methods. From the time-of-flight spectra the energy released in the dissociation process is determined. Our results for the kinetic energy release of and ions are compared with the results of other ionization methods. Clear differences are observed. We observe a counterintuitive dependence of the kinetic energy release on the velocity of the ions, i.e. the kinetic energy release increases with decreasing velocity.

Results from valence and core - valence multiconfiguration Dirac - Fock and relativistic configuration interaction calculations including the Breit interaction are presented for the and transitions (E1, M2) in the Mg isoelectronic sequence. In the calculations the orbital sets of the initial- and final-state wavefunctions were not restricted to be the same, but were optimized independently. The evaluation of the transition matrix elements was done with an efficient transformation technique. The calculated energy separations and transition probabilities are found to be in good agreement with experiment and compare favourably with other recent calculations. For completeness, results for forbidden transitions (E2, M1) within the fine-structure levels are also included.

We determined experimentally two critical points in elastic electron scattering by argon where the differential cross section (DCS) attains its smallest values. The points were found to be at and at . Special attention was given to improve the angular resolution in order to determine the exact positions of the minima. These minima are important because they are a sensitive test of the validity of experimental procedures, and are used to verify theoretical predictions of DCS shapes and magnitudes, and of the polarization of scattered electrons. Normalized DCS were determined by measuring the angular distributions of elastically scattered electrons at incident energies of 10, 15, 20, 25, 30, 40, 50, 60, 75, 80, 90 and 100 eV in the angular range . Results are compared with the available experimental and theoretical data. In addition, integral, momentum-transfer, and viscosity cross sections were determined by numerical integration of the measured DCS extrapolated to and to .

The convergent close-coupling method is extended to the calculation of electron scattering from atoms with two valence electrons above an inert Hartree - Fock core. The formalism is illustrated by calculation of electron scattering from beryllium. Integrated cross sections for scattering from the ground state up to n = 4 levels as well as the total ionization and total cross sections are presented in the energy range from 10 to 1000 eV. Good agreement is found with the available results of the RMPS calculations of Bartschat et al.

We have extended and further improved our previous R-matrix with pseudo-states (RMPS) calculation for electron collisions with beryllium (1996 J. Phys. B: At. Mol. Opt. Phys. L769 - 72). Results for transitions from the ground state to the lowest 11 target states with principal quantum number n = 2,3 are presented for incident energies from threshold to 100 eV. In the crucial `intermediate energy range' of 10 - 100 eV, our results are in good agreement with those obtained in the convergent close-coupling (CCC) calculation of Fursa and Bray, whereas significant discrepancies are found with predictions from simpler models. Consequently, a combination of RMPS and CCC predictions in this region, supplemented by our RMPS results in the low-energy resonance regime and further CCC results above 100 eV incident energy are believed to represent the most accurate currently available data for this collision problem.

We calculate the time dependence of the T-violating term in the stimulated Raman transition which was discussed for short times in earlier papers. We study a double -system and use numerical methods to find explicit results for caesium. The two stimulating fields are taken to be plane polarized with polarization vectors and . We investigate two terms , one of which is due to T-violation, the other one to the decay of the intermediate states. We discuss their different time dependence. In addition to these two terms there is a T-violating term which is proportional to the decay constant of the intermediate states and does not vanish even for unpolarized beams. We show its time dependence. Finally, we comment on the possible influence of T-violation on population trapping in a system possessing a dark state.

In this paper we suggest the many-body treatment for the inelastic scattering of fast electrons on metal clusters. The accurate many-body theory for this process is necessary because electrons in the cluster experience collective excitations during the collision. We perform our calculation in the random phase approximation with exchange using the wavefunctions of the Hartree - Fock jellium model and also in the plasmon resonance approximation. The latter approach is appropriate to treat collective excitations in clusters. We analyse the differential and total inelastic scattering cross sections and elucidate the relative importance of collective electron excitations and single-electron transitions in the formation of the inelastic scattering cross section on metal clusters. We perform this calculation for the magic sodium clusters and . Theoretical results are also compared with the experimental data available for electron collisions with the and clusters. We report some discrepancy between theoretical and experimental results.

Highly selective excitations, which occur in a three-level system under the conditions of zero- and nonzero-delayed lasers, are examined. The detuning ranges which reach half inversion and their dependences on the laser conditions are obtained numerically using the Bloch equation. It is found that in the optimal detuning method which uses the zero-delayed lasers, the range is independent of the Rabi frequency and the detuning, but in the adiabatic passage which uses the nonzero-delayed lasers, the range is dependent on the Rabi frequency and the time delay. The inversion methods show strong dependences on the pulse length. It turns out that the optimal detuning method which has the narrow range is more appropriate to obtain highly selective excitation than the adiabatic passage. The mechanisms of population inversion and the decay effects between the methods are compared.

Theoretical and experimental studies of nonlinear magneto-optical rotation in the principal series of calcium are reported. These are believed to be the first experiments on high magnetic field Faraday rotation (defined as achieving rotation angles of the order of or more) in which the nonlinear optical regime has been entered by using laser fields. The present theory is valid both in the linear and nonlinear domains and our theoretical predictions which take into account propagation effects agree very well with experimental results over the entire range of laser strengths.

An algebraic reformulation of the Bohr - Sommerfeld (BS) quantization rule is suggested and applied to the study of bound states in one-dimensional quantum wells. The energies obtained with the present quantization rule are compared to those obtained with the usual BS and WKB quantization rules and with the exact solution of the Schrödinger equation. We find that, in diverse cases of physical interest in molecular physics, the present quantization rule not only yields a good approximation to the exact solution of the Schrödinger equation, but yields more precise energies than those obtained with the usual BS and/or WKB quantization rules. Among the examples considered numerically are the Poeschl - Teller potential and several anharmonic oscillator potentials, which simulate molecular vibrational spectra and the problem of an isolated quantum well structure subject to an external electric field.