Table of contents

Electronic transitions of ScI observed in high resolution spectra of the fluorescence induced by laser line excitations are analysed. Preliminary values of molecular constants are obtained for states of the lowest electronic configuration, , , , , , and for the laser-excited state.

Non-dipolar corrections to the angular distributions of Auger electrons produced by high-energy photons in photoionization are considered. Higher-order corrections change the anisotropy of Auger electron emission. For linearly polarized light the angular distribution becomes axially asymmetric with respect to the photon polarization vector. However, in contrast to photoelectron angular distributions, there is no forward/backward asymmetry with respect to the photon beam.

Recent calculations on the intrinsic spin polarization parameter of Auger emission have reported a constant value of for most of the Ar Auger transitions. This is of physical significance since two partial waves are emitted for these transitions. A full analytical analysis is given on this effect. It is shown that the result is unique to at least 50% of the occurring Auger lines of any closed sub-shell element provided the intermediate inner shell hole is created in a state. An example is given for the Kr Auger transitions to illustrate the general result.

We have used interferometric techniques to show unambiguously that the fragment ion energies produced in the multielectron dissociative ionization (MEDI) of iodine vary with laser pulse rise time. A field ionization, Coulomb explosion model is found to reproduce MEDI data on remarkably well. The lack of features associated with post-dissociative ionization in the MEDI spectra of is explained in terms of low dissociation rates for the singly (and possibly doubly) charged parent ions. These results remove the need for the much discussed, but highly speculative, process of `laser-induced stabilization'.

New spectroscopic data have been used to extend the Rydberg - Klein - Rees potential energy curve of the A state of the KH molecule from v = 27 to 38.

The scaling behaviour of cross sections for electron capture from circular Rydberg states by protons is studied for a wide range of principal quantum numbers n = 4 to 100 at intermediate collision speeds. We find the convergence to -scaling to be slow and dependent on the initial orientation. The origin of the slow convergence is traced to the capture mechanisms that probe different parts of the electron's phase space distribution.

We study the ultrafast dynamics of clusters immediately after the photodetachment of by performing molecular dynamics simulations based on a microscopic electronic theory. We determine the time scale for the change from the linear (initial situation) to a triangular structure (potential minimum) of the ground state of . Our results are in good agreement with experiment. We show how the time dependence of the internal degrees of freedom is reflected in the `pump and probe' signal. Furthermore, we discuss the application of our results to magnetic systems and chemical reactions.

A simple process based on direct manipulations of recurrence relations and taking into account the asymptotic behaviour of their linearly independent solutions is proposed to solve the problem of monoelectronic diatomic ions in the continuum by extending a method from Killingbeck associated with Miller's algorithm. As an illustrative example, phaseshifts are presented for various states of ion.

Relative dipole oscillator strengths for the transitions in cadmium have been measured using magneto-optical rotation spectroscopy in the VUV. The uncertainty in the relative oscillator strengths is estimated to be 3%. Absolute oscillator strengths were determined on the basis of the known oscillator strength of the transition. Our experimental results agree with quantum defect orbital calculations which we have carried out for transitions in the principal series in cadmium up to . A revised value of Mb for the total photoionization cross section at the ionization threshold is proposed.

Numerical Hartree - Fock calculations are reported for all the finite moments, with , of the electron momentum densities for the ground states of the atoms from hydrogen to lawrencium. We also report the first three coefficients of the MacLaurin expansion and the leading coefficient of the asymptotic expansion of the electron momentum densities for these atoms. These results supersede and extend numerous tabulations based upon Roothaan - Hartree - Fock calculations.

Relativistic multiconfiguration Dirac - Fock (MCDF) transition energies and oscillator strengths are determined in an `optimal level' scheme for both the spin-allowed - and spin-forbidden 3 - transitions in the magnesium isoelectronic sequence through . The modest relativistic configuration mixing to represent the intravalence correlation is combined in MCDF calculations with a polarization model to account for valence - core electron correlation. For the first few members of the sequence (Mg I to Cl VI) rather extensive relativistic configuration interaction Dirac - Fock (CIDF) calculations (with and without core polarization) are also performed to improve the representation of the intravalence electron correlation which is important for the neutral end of the sequence. The influence of the intravalence correlation, as well as the valence - core electron correlation on oscillator strengths, excitation energies and ionization energies is investigated. The proper representation of the intravalence electron correlation is found to be crucial for oscillator strength calculations near the neutral end of the magnesium isoelectronic sequence. For some elements of the sequence the present data are the only ones available.

First values of the absolute transition probabilities for 10 In II lines have been determined by measurement of emission line intensities from an optically thin laser produced plasma of In in Ar atmosphere. The experimental values are , , and for the more intense lines , , and , respectively. Theoretical values are also obtained for comparison in the Hartree - Fock approximation including relativistic corrections. The present results and those concerning known lifetimes are interpreted on the basis of intermediate coupling calculations. Analysis of the interaction between levels from and 5s5d configurations reveals a 77% contribution from 5s5d configuration for the lowest one demonstrating the inadequacy of the standard assign for that level. Regularity in the small departures of In II 5s nl levels from LS coupling is also observed.

The total cross section for photodetachment of the metastable state of the negative beryllium ion is calculated from threshold to a photon energy of 0.34 Ryd. The calculation employs a multichannel theory based on the R-matrix method for electron - atom collisions. Excellent agreement is found with the recent theoretical results of Sinanis and co-workers and with the single recent experimental datum of Pegg and co-workers.

Measurements of the photodetachment cross section of have been performed in the vicinity of the threshold, with photon energies ranging from 5.23 - 6.04 eV. The experimental cross section is nearly constant throughout this energy region, in marked contrast to recent theoretical predictions of strong resonance structures. The present data indicate that the quasibound state may be located at higher energies than so far predicted.

It is demonstrated quantitatively for the first time that for a well defined case of inner-shell photoionization and subsequent Auger decay, photoionization and - Auger transition in xenon, the conventional two-step model breaks down for equal kinetic energies of the emitted electrons. This is in accordance with the theoretical predictions of Végh and Macek; within the one-step formulation, the effect is traced back to the interference of two amplitudes where the indistinguishability of the two electrons allows that either one of the electrons can be associated with the photoprocess, the other electron being associated with the Auger decay.

- MM Auger spectra of argon have been measured with very high resolution using excitation by monochromatic synchrotron radiation. The photon energy was selected slightly above the 2p ionization threshold for creation of the single 2p core hole state only. The Auger spectra are thus free from any satellite lines originating from two-hole initial states and therefore well suited to study the configuration interaction in the final ionic state of the decay. Electron correlation is investigated by calculating the energies and intensities of the transitions using the multiconfiguration Dirac - Fock method and by comparing them with the experimental results.

We have measured the total collisional loss rate for ultracold sodium atoms held in a magneto-optical trap operating on the D1 line. The collisional rate is extracted from the decay of the fluorescence from trapped atoms. We observe an intensity dependence corresponding to radiative escape and hyperfine changing collisions in the trap. The results obtained are compared with previous measurements for traps operating on the D2 line.

The R-matrix Floquet approach is applied to study the negative and ions in a light field. Detachment rates are obtained for detachment processes involving up to three photons. The results obtained in the present approach are compared to other experimental and theoretical results. For two- and three-photon processes reasonable agreement with other calculations has been found, while for two-photon detachment the results agree with the experimental cross sections. The three-photon results are in less good agreement with experiment although the larger error bars make accurate comparisons more difficult. The changes in the detachment behaviour for these ions are compared to each other as well as to the detachment behaviour of .

Absolute VUV optical absorption cross sections for ozone have been measured between 325 and 110 nm (3.0 - 11.3 eV) using a synchrotron radiation source. Vibrational fine structure is resolved in Rydberg bands and comparison of this with the limiting bands in the photoelectron spectrum confirms that the order (increasing ionization energy) of the three lowest ionization bands is . Near-threshold electron energy-loss spectra have also been recorded. In these, in addition to the known triplet states between 1 and 2 eV, a low-lying triplet state has been located around 3.4 eV and several others between 6 and 9 eV. Characterization of the valence states (both optically allowed and forbidden) are discussed in relation to the results of early theoretical computations which seem to give a good account of the ozone spectrum.

The valence shell photoelectron spectrum of butadiene has been studied using He I, He II and synchrotron radiation. In addition to the main bands associated with the single-hole states, complex satellite structure due to many-electron effects has been observed in the inner valence region. Two variations of the many-body Green function method have been employed to evaluate the ionization energies and pole strengths of all valence states and the results have facilitated an interpretation of the experimental spectra. Photoelectron angular distributions and branching ratios have been measured using synchrotron radiation in the 12 - 120 eV range. Vibrational structure has been observed in several of the photoelectron bands excited with He I radiation and analyses have been performed for the , and ionic states.

We present a theoretical study of resonant two-photon dissociation (TPD) of the molecule in the presence of pulsed two-colour laser fields using the density matrix formalism. The study includes the calculation of dissociation probabilities and decay rates as functions of time and the variation of total dissociation probabilities with detuning when both fields are assumed to have the same temporal shape and duration. Results are reported for two simultaneous laser pulses as well as for two pulses with a time delay. The investigations were done with different values of peak intensities , for the initial and intermediate levels as v = 0, J = 0 and v'=17, J'=1, respectively, of the ground electronic state and the final continuum energy reached by photons was taken to be . The time variation and delay between pulses lead to interesting features in the time-dependent decay rates and spectral shapes of total dissociation probabilities.

Time-dependent wavepacket calculations have been carried out to elucidate the quantum dynamical evolution of coherent molecular states undergoing case predissociation. An ultrashort optical pulse acting in the perturbative limit is invoked to create a non-stationary superposition state which propagates with time over model potential energy curves. At energies below the diabatic crossing point, coherent product states are formed by quantum tunnelling through the barrier formed on the lower adiabatic potential and, for ultrashort pulses, adiabatic passage over the barrier summit. Above the crossing point the dissociation yield is mediated by interference between oscillating wavepackets trapped in the bound diabatic and excited adiabatic potentials, leading to formation of products by repeated curve-crossing between the diabats. Coherent asymptotic states can still be produced with very fast optical pulses, though at longer times or with broader pulse widths rapid dephasing of the quasi-bound wavepackets gives an almost time-independent product yield.

Single and multiple ionization of carbon disulphide by intense picosecond laser fields is the subject of this paper. Mass spectra were measured at five wavelengths from the infrared to the ultraviolet. In terms of the Keldysh adiabaticity parameter, we cover both the multiphoton and the tunnelling regimes. The dynamics of the dissociative ionization process is shown to be dependent upon the regime in which the laser - molecule interaction occurs. Resonances, which may be possible and which could access electronically excited states of the molecule, appear to play little part in the dynamics. Ion abundances have been measured as a function of laser intensity in the tunnelling regime; no correlation is found between measured values of saturation intensity and zero-field molecular properties such as dissociation or ionization thresholds and ionization energies. In addition, the covariance mapping technique is applied to study the dissociation dynamics of multiply charged ions at 1064 nm. The measured values of kinetic energy release accompanying formation of fragment ion-pairs are very much less than those measured in single-photon and electron-impact experiments. It is postulated that this reduction may be a manifestation of the extent to which potential energy surfaces of ions are `flattened' by the action of the intense, linearly polarized laser radiation, akin to the bond-softening process that has been observed in the case of diatomic molecules. Our observations indicate that distortion of molecular potential energy surfaces may be the dominating feature in intense laser - molecule interactions.

We have applied many-body theory methods to study the interaction of low-energy positrons with noble-gas atoms. The positron - atom correlation potential includes explicitly the contribution from the target polarization by the positron and that from the virtual positronium (Ps) formation. It is demonstrated that the correlations and Ps formation (or tunnelling of electrons from the atom to the positron) create virtual levels in the positron - atom system. The existence of the virtual levels strongly influences the scattering and increases the positron annihilation rate by up to 400 times. The calculated elastic cross sections are in good agreement with experimental data. The inclusion of virtual Ps formation greatly improves the agreement with experimental annihilation rates with respect to calculations taking only the polarization into account. Our calculations have highlighted the difference between the calculation of positron - atom scattering and the calculation of corresponding annihilation rates. The annihilation rate is very sensitive to the behaviour of the wavefunction at small positron - electron separations. A simple approximate formula based on the Sommerfeld factor is suggested to account for the effect of electron - positron Coulomb attraction on the annihilation rates.

Absolute differential ( - ) cross sections for low energy (1 - 10 eV) electron scattering from argon have been measured on two separate and different spectrometers using the relative flow technique. The data have also been analysed using phaseshift techniques. Comparisons are made between the present cross sections and phaseshifts and similar data from previous experiments and theory. The results are particularly encouraging, with recent experiments and scattering theories being in good general agreement.

Absolute cross sections for elastic scattering and rovibrational excitation have been measured with a crossed beam spectrometer and calculated, utilizing the R-matrix formalism, for incident electron energies between 1.0 and 30 eV. Particular emphasis is placed on the comparison between experiment and theory in the region below 5 eV where the cross sections are enhanced by a strong shape resonance.

A systematic study of the electron impact excitation of lithium, sodium and potassium from their ground states is considered in distorted-wave approximation theory. Results for differential and integrated cross sections, alignment, orientation and spin asymmetry parameters in the energy range 10 - 100 eV are calculated and presented. Our integrated cross section results show good agreement when compared with other theories and available experiments. Present differential cross section results for sodium are also compared with the only available recent differential cross section measurements. For the sake of further meaningful comparison and discussions, the first Born calculations for the cross sections are also performed and compared.

A pulsed electron beam and ion extraction system has been employed to measure cross sections for formation of C, , , C, and by electron impact on acetylene in the energy range varying from threshold to 800 eV. The relative flow technique has been used to obtain normalized values of cross sections. Appearance potentials (APs) for ionic species are reported. Present values of APs are compared with previously published data.

The question of the shape of the curve that describes the yield of zero-kinetic-energy electrons near the threshold for ionization of an inner shell is considered. The theoretical situation appears to be ambiguous as to whether or not the theoretical curves should have an additional Lorentzian broadening. Comparison with experimental data for argon ionization shows a significantly better fit to the data with curves that have been broadened than with those that have not.

We apply the method introduced recently by Korol for the approximate calculation of the compound two-photon non-dipole free - free matrix element in a central field which characterizes the amplitude of the two-photon bremsstrahlung (2BrS) process. The method is based on the accurate treatment of the diagonal singularities in the one-photon matrix elements, which appear in the compound matrix element.

Results of calculations are compared with predictions of simpler theories and with the experimental data on 2BrS.

The acceleration of the electron in a two-level atom driven by a short laser pulse of intermediate strength is calculated and therefore analysed through two wavelet transforms in order to obtain the temporal evolution of the spectrum of the emitted radiation. We show (i) the full spectrum at different instants and (ii) the time profile of some representative harmonics. The growth of the harmonics as well as the appearance and erratic behaviour of the hyper-Raman can be followed with the laser intensity.

We present an experimental study of harmonic generation in molecular gases, using the fundamental (800 nm) and the second harmonic (400 nm) of a 150 fs titanium - sapphire laser at an intensity of . We compare the conversion efficiency and the maximum energy obtained in different species: rare gases (Ar, Xe), diatomic molecules (, , , CO) and polyatomic molecules (, O, , , ). The harmonic spectra from molecular gases are very similar to those obtained in the atomic gases, with a plateau and a cutoff whose location is strongly correlated to the value of the ionization potential. The conversion efficiency is not higher than that of the rare gases.