Table of contents

Calculations of the two-photon ionisation probability rates of atomic caesium are reported for incident photon energies between 19400 and 22000cm^-1. In this work the authors have taken into account the electric-dipole plus quadrupole approximation. The discrepancy between the theoretical results and experimental measurements is not explained in spite of the introduction of the quadrupole contribution as was suggested elsewhere.

Diabatic states of the H⁺-Xe collisional system have been calculated using the projected-valence-bond (PVB) method. The two lowest diabatic ¹ Sigma ⁺ PVB states are found to cross at an internuclear distance R=4.7a₀. For collision energies E<or approximately=1 keV no transition takes place at this crossing since it is strongly avoided in the corresponding adiabatic energy curves. On the other hand it is shown that Demkov-type transitions occurring at larger distances (R approximately=6.7a₀) are responsible for the near-resonant charge-exchange processes in H⁺+Xe collisions. The PVB results, in conjunction with diabatic states for spin-orbit coupling, are used in quantum calculations of differential cross sections. The results are compared with previous theoretical and experimental data.

The photoabsorption Schumann-Runge continuum in O₂ has been decomposed into the contribution from states of both valence and Rydberg character. The decomposition of the experimental photoabsorption (or high-energy, zero-angle, electron energy-loss) data contains contributions from three valence states, four Rydberg states, and one state of unknown character. Information on the perturbed valence-state potential-energy curves in the Franck-Condon region obtained by this analysis substantiate theoretical conclusions of strong valence-Rydberg perturbations in the 8-12 eV excitation region. There is presently no unambiguous identification for the state at 8.93+or-0.04 eV, found in the optical and all the electron energy-loss data.

Extensive high-resolution measurements have been made of the emission spectrum of P₂. A vibrational scheme based on band origins is given for the visible system (³ Pi _u-³ pi _g) and relates the lower state to the upper state of the infra-red system. Rotational and vibrational constants are presented. Perturbations in the c ³ Pi _u state are identified as being caused by the c ¹ Sigma _u⁺ state and enable the c ³ Pi _u state and hence the other triplet states of P₂ to be located on the energy-level diagram. The energies obtained are: a ³ Sigma _u⁺, 18686.7+or-0.5 cm^-1; b ³ Pi ₁, 28128.9+or-0.5 cm^-1; c ³ Pi ₁, 46965.3+or-0.5 cm^-1.

The convergence of single-channel scattering phaseshifts determined from the consistent variational method is examined. It is shown that if certain conditions are satisfied, then the variationally determined phaseshift will approach the true phaseshift monotonically from below. The possibility of pseudo-resonances or anti-resonances is discussed and illustrative calculations are presented.

Using the zero-field quantum-beat technique after beam-foil excitation the authors determine fine-structure and hyperfine-structure parameters of doubly excited Li I 1s2P ²⁴P at 60.8 eV above the ground state from the multiplet at 3714 AA. The data interpretation uses simultaneously experimental results from both stable Li isotopes, ⁶Li and ⁷Li. The values for the fine-structure splittings are E₅2/-E₃2/=(-57+or-8) GHz E₅2/-E₁2/=(1.36+or-0.07) GHz. The one-electron parameters of the hyperfine structure are a_s(⁶Li)=5.58+or-0.09) GHz a₀(⁶Li)=(0.023+or-0.015) GHz.

The absorption spectrum of B I has been studied for 900 AA<or= lambda <or=2000 AA by the flash pyrolysis technique. Transitions were observed from both the 2s²2p ²P ground state and the 2s2p²⁴P metastable state. Five new quartet terms have been identified, with improved values obtained for two previously identified terms. A method of focusing has been developed, allowing the BRV source to be used as a source of single-shot continuum in the lower wavelengths (300 AA<or= lambda <or=1200 AA) of the normal incidence region.

The integro-differential equations for electron-ion scattering processes are solved for bound-state energies of the compound system, a method called frozen-cores approximation when introduced by Wilson and Seaton (1972). The solutions are used to calculate oscillator strengths. The core states chosen in the present paper are such that the results should be comparable with the recent multiconfiguration Hartree-Fock calculations for Mg by Froese Fischer (1975).

A method is described for the calculation of doubly excited energy levels of rare-gas atoms. The calculation makes use of frozen-core Hartree-Fock orbitals, optimized orthogonalized Slater-type functions to describe excited orbitals and virtual orbitals appearing in a superposition of configurations, and an empirical correction for core correlation. The method is found to be accurate to 0.1 eV for the 1s²2s²2p⁴(³P,¹D,¹S)3snp(¹P^o) levels of neon (n=3,4), as well as the singly excited 1s²2s2p⁶np(¹P^o) levels (n=3-6). Modifications are described for the case of two or more states of the same symmetry, formed from the same core state and the same excited orbitals (e.g. 1s²2s²2p⁴(³P)3s(^2,4P)3p(³P^o)).

By means of a high-resolution electron spectrometer, energy-loss spectra have been measured in Kr I for various values of impact energy (15 to 400 eV) and scattering angle (0 to 90 degrees ). The intensities of the peaks in these spectra have been used to determine the relative values of the effective and of the generalized oscillator strengths of the strongest transitions. Furthermore the absolute values of the optical oscillator strengths have been determined for most of the observed transitions by normalizing relative data with the help of absolute values found by other authors.

The response of a two-level atom driven by a strong resonant amplitude-modulated field is calculated in the Bloch model. The population difference and dipole moment show a resonant behaviour when the Rabi frequency is a multiple of the modulation frequency. This phenomenon is related to the resonant AC Stark effect and is the optical counterpart of the coherence resonances known in radiofrequency magnetic resonance.

K-shell X-ray emission cross sections are determined for gas targets (methane, nitrogen, neon, argon) ionized by H⁺, He⁺ and He²⁺ impact (energies from 100 to 2400 keV). The fluorescence yield for carbon, nitrogen and neon is calculated using known Auger cross sections; the absolute uncertainty is 20-30%, the relative values are determined within 5-10%. The impact-energy dependences of the fluorescence yields and of the detection efficiencies are measured. Both increase with decreasing impact energy. Values of the ionization cross sections are compared with BEA and PWBA calculations. The latter, corrected for binding, polarization and Coulomb deflection effects, are in reasonable agreement with experiment, especially for the projectile ratios (ratios of cross sections for He⁺ compared with H⁺ impact).

The energy spectra of 279.2 keV photons scattered incoherently by copper, tin and lead have been measured for the scattering angle 135 degrees using a Ge(Li) detector. The experimental method and the evaluation of the data are described. Experimental cross section profiles d² sigma /d Omega dE as a function of the energy of the scattered photon are compared with the form-factor approximation. A disagreement by a factor of 1.5 to 4 occurs at the high-energy ends of the profiles.

Recent experimental results are used to test the dispersion relation for electron scattering by hydrogen, helium and neon. Calculations of the H^- residue and of the Born exchange term for helium are reported. The sum rule appears to fail for electron, but not for positron, scattering. The nature of the singularities in the exchange amplitude is discussed.

The four-state coupled-channel second-order-potential model of Bransden and Coleman (1972) is employed to calculate differential cross sections for the elastic scattering of electrons by atomic hydrogen and for the excitation of the n=2 levels, allowing comparison with recent absolute measurements by Lloyd et al. (1974), and by Williams and Willis (1975), and with other theoretical models.

Elastic and inelastic, direct and exchange differential cross sections have been calculated for proton-hydrogen scattering in the energy range 250 eV<or=E_lab<or=2000 eV. The results are compared with recent experimental data and with other theoretical predictions. The authors applied the impact-parameter approximation and expanded the electronic wavefunction into adiabatic electronic states. Both straight-line and Coulomb trajectories have been used to integrate the set of coupled equations. Furthermore the influence of the size of different molecular bases on the cross sections is studied and qualitatively explained in terms of the rotational couplings between the various Sigma and Pi states. Finally, integral cross sections for the 2p_+or-1, 2s, 2p₀ and 3p_+or-1 excitation are calculated in the range of 1 keV<or=E_lab<or=10 keV.

Collisions between H⁺ ions and He(1¹S) atoms are studied theoretically. The collision spectroscopy of large-angle scattering in slow and fast collisions is analysed using the concept of perturbed symmetric resonance. The single-electron capture probability is calculated as a function of theta T, the product of the scattering angle and the impact energy in deg keV. Excellent agreement with the measurements of Helbig and Everhart (see Phys. Rev. A, vol.136, p.674 (1964) is obtained.

The primitive causality condition-namely that an effect cannot precede its cause-yields analytic properties and dispersion relations for the polarizability when the cause is an external electric field acting at the centre of an isolated molecule and the effect is the induced electric dipole moment. For a chiral molecule the lowest approximation that allows for the optical activity involves also the spatial derivatives of the electric field at the molecular centre, and a more stringent condition must be imposed in order to show the analyticity of the optical rotatory parameter as well as that of the polarizability. This condition is illustrated by the classical model of an optically active molecule consisting of two dissymmetrically coupled damped linear oscillators. The various approaches that can be used to derive dispersion relations for optically active isotropic media are briefly reviewed.

Using double phase-locked source-modulated microwave spectrometers operating at frequencies up to 110 GHz and at gas pressures to 30 mTorr, pressure-induced widths and shifts have been measured for lines in the spectra of OCS CH₃Cl, CH₃OH, CH₃CN and CH₃I. Results are given for foreign-gas broadening and shifting as well as self-perturbation for the first four gases. The results differ between the two classes of lines observed, those due to transitions between rotational energy levels and those between levels created by hindered motion.

Differential scattering experiments with the crossed-beam method have been performed to study vibrational excitation of HF and HCl by electron impact at very low collision energies. The excitation functions for nu =1 and nu =2 have been measured from threshold to some eV above. In each case, an unusually strong and sharp threshold peak is observed with a broad resonance region following thereafter. The absolute cross sections reach values of the order of 10^-15 cm² for the threshold peaks in the nu =1 channel. The angular distributions are isotropic in the whole resonance region. The threshold resonances are interpreted as quasi-bound ² Sigma ⁺ states in the dipole field possibly correlated with H*+X^-. The broad resonance region is attributed to a very short-lived resonance state which may be the HX^-2 Sigma ⁺ state correlated with ground-state fragments H+X^-.

For pt.I see ibid., vol.9, no.14, p.2521 (1976). Differential scattering experiments with the crossed-beam method have been performed to study vibrational excitation of H₂O by electron impact for collision energies from threshold to 10 eV. Differential and integral cross sections for the excitation of the (100, 001) and 010 modes are given in absolute units. Two distinct resonance regions are observed which are superimposed on a background of direct excitation: strong and sharp threshold resonances and a broad resonance region centred around 6-8 eV. The broad enhancement of the cross sections around 6 eV is attributed to a very short-lived ²A₁ state of H₂O^-. The threshold resonances are interpreted as quasi-bound states in the dipole field of H₂O. They are expected to be of general importance in electron-polar-molecule interactions.

The ion speed distribution previously considered by Woo, Wong and Hong (see J. Chem. Phys., vol.55, p.3531 (1971) and J. Chem. Phys., vol.57, p.2593 (1972)) is modified to agree substantially with the Monte Carlo calculations of Skullerud (see ibid., vol.6, p.728 (1973)). Application to the interpretation of ion-molecule reaction cross sections is discussed.