Table of contents

With a new laser method the authors have directly observed highly excited hydrogen atoms with large values of the magnetic quantum number mod m mod . They emphasise the importance of studying substate production in atomic collision processes and outline a procedure for measuring ratios of substate production cross sections with their method.

The authors report the first measurements of photodetachment cross sections for He^- (formed through charge exchange of He⁺ with Ca vapour) over the photon energy range from 1.77 to 2.75 eV. Uncertainties associated with the electronic configuration of the He^- ion are removed by determining the energy of the autodetached electrons. The measured autodetached electron energy agrees with the known (1s2s2p)⁴P⁰ He^- energy level.

A new theoretical method outlined here for an important inelastic mechanism based on A-B⁺ encounters in A-B(n) thermal collisions indicates a substantial contribution to l-changing transitions which therefore cannot be viewed as proceeding via (e-A) encounters alone. Recent descriptions of l-changing thermal collisions which are based on an (e-A) impulse treatment appear defective in that they disregard certain impulse validity criteria and yield results which greatly exceed the upper limit specified by the basic impulse expression, and do not recognise the above important inelastic mechanism associated with the non-inertiality of the core.

A succession of peaking approximations is used to study the structure of the impulse approximation in its impact parameter form. The results of numerical calculations are compared with experimental results.

The triple differential cross section for the ionisation of atomic hydrogen by fast electrons is analysed in a coplanar, asymmetric geometry by using the second Born approximation. The authors find that the binary peak is shifted to larger angles relative to the first Born result and is reduced in magnitude. The recoil peak exhibits a major enhancement and is also shifted towards large angles.

It is observed that unusually strong desorption effects and chemical reactivity can produce totally spurious results in experiments using hydrogen fluoride gas. It is shown, for example, in the case of electron energy-loss spectra that completely erroneous results are obtained unless adequate sample inlet conditioning is carried out. Comparison of the present experimental work with some literature examples illustrates the need for extreme caution in the interpretation of spectral results when studying hydrogen fluoride. In particular, it is shown that the only published high-resolution electron energy-loss spectrum attributed to HF is in fact due to molecular hydrogen. The high-resolution (0.05 eV FWHM) electron energy-loss spectrum of HF has been measured in the range 12 to 16.5 eV. It is suggested that effects due to desorption by HF are reasonable for the anomalies in published spectra for electron scattering resonances for hydrogen fluoride.

The formalism of the frozen-core variant of the Hartree-Fock approximation for atoms has been extended to some states which are inadequately described by means of a single electronic configuration. Different possible choices of the core orbitals have been considered, and calculations performed in the particular case of the boron isoelectronic sequence. A large number of s-p and p-d dipole oscillator strengths have been computed, with good results in most cases.

It is shown that the usual dispersion formula used in all calculations of the real part of the anomalous scattering factors, f', is only valid within the dipole approximation. A new correction term which is positive and of the order of E_tot/mc², with E_tot the total binding energy of the electrons in the atom, is needed. Based on experimental data, the K-shell contribution to the new correction for f' is found to be larger than 0.013 for Si and 0.073 for Ge. Formulae expressing a total K-shell photoelectric cross sections in terms of the dipole approximation values are given. It is concluded that at present no theoretical calculations give f' better than to within an accuracy of the order -E_tot/mc².

The possibility of subnatural linewidth laser absorption spectroscopy is discussed and several examples using pulsed and CW excitation are proposed.

The photoionisation of xenon is completely described by the knowledge of either three or five parameters, corresponding respectively to the ²P_1/2 and ²P_3/2 states of the ion, and their dependence on photon energy. In the photon energy range from 12 to 41 eV two of these have been experimentally determined from measurements of the spin polarisation of photoelectrons emitted by unpolarised and circularly polarised VUV radiation. After a description of the apparatus used the measured results are presented and compared with different theoretical results.

For pt.I see ibid., vol.13, p.4353 (1980). Using the results on spin polarisation of photoelectrons reported in the previous paper the bound-free dipole matrix elements and the phase differences of the continuum wavefunctions have been determined. The comparison of these data with results obtained by other authors in the discrete spectral range of xenon shows excellent agreement at the photoionisation threshold. This agreement is experimental evidence for the validity of the multichannel quantum defect theory.

The generation of energetic electrons is studied through a process involving S extra photon absorptions above the N-photon ionisation threshold. It is shown that this mechanism can produce significant amounts of fast electrons for high-order ionisation processes.

It is shown that the van der Waals interaction between two molecules in relative motion depends on the relative velocity. The velocity-dependent corrections depend on powers of a parameter (v/R omega ₀) where omega ₀ is a characteristic absorption frequency of the molecules and R their separation distance. The implication of this velocity-dependent force on the frictional drag on a molecule moving near a slab is discussed.

The problem of finding the scattering amplitude from the experimental elastic differential cross section for fixed energy is reduced to finding the solution of a non-linear system of equations. The method is based on a new expression which shows that the phase of the scattering amplitude is uniquely determined from a knowledge of the still unknown phaseshifts and well defined integrals of the differential cross section which are functions of the cosine of the scattering angle.

Total inelastic cross sections for atomic collisions are usually obtained by computing the complex phaseshifts of the incident wavefunction often using perturbation techniques. Such calculations are generally based on a theoretical formalism in which a local complex potential, V(R)-i¹/₂ Gamma (R), is employed to describe the entrance channel. However, the assumption that the imaginary component is local breaks down when there are only a few final states energetically open. The characteristics of the effects associated with this breakdown are discussed through a numerical study of the associative detachment process H+H^-(¹ Sigma _g⁺) to H₂(² Sigma _u⁺)+e^- at incident energies of 0.0129, 0.129 and 0.646 eV. The cross sections to individual vibrational-rotational levels of H₂ are also reported. It is shown that the breakdown in the expression often used for a local complex potential can produce substantial errors in computed cross sections. Methods of detecting and analysing the divergence of calculated cross sections from true ones are presented, along with means of estimating actual values within uncertainty brackets.

A theoretical study of the detachment process in H^--He collisions is presented for the 1-10³ eV collision energy range. The outer electron of the colliding system is described in the zero-range potential approximation, the relevant parameters of which are extracted from an ab initio calculation of the (HHe)^- molecular ion. This enables an exact treatment of the bound-state-continuum transitions induced by the nuclear motion. A very efficient approximation is also proposed to solve the collision problem. A detailed picture of the final state of the system is thus obtained (differential cross sections, total cross sections, spectra of ejected electrons, isotope effect) and compared with experimental data when available.

The authors present an investigation of collisional excitations in systems characterised by the presence of two valence electrons localised outside closed shells in the energy range 1-300 keV. Two types of systems have been studied: (i) systems such as Mg-rare gases with both valence electrons localised on the projectile, and (ii) systems such as Li, K-Na with one valence electron localised on the projectile and one on the target. Experimentally, the total emission cross sections and polarisations have been measured for the Mg I 3¹S-3¹P, 3³P-4³S, s³P-3³D, 3³P-3p²³P, Li I 2²S-2²P, Na I 3²S-3²P and K I 4²S-4²P transitions. The content metastable Mg(3s3p³P) atoms in the applied neutral Mg beams has been determined by means of an optical method based on the Wigner spin conservation rule in low-energy atomic collisions. The Mg(3s²)-He collision has been examined theoretically using a Hartree-Fock frozen-core potential to describe the interaction between the Mg valence electrons and the He atom.

An algebraic variational close-coupling code is used to evaluate electron impact excitation cross sections between the n=1, 2 and 3 levels of atomic hydrogen at energies up to the n=4 threshold. Results are obtained in a six-state basis and with a much larger basis (14 to 18 states). Comparisons are made with earlier six-state calculations. The results from the best basis are significantly lower than the previous six-state calculations for the 2l-3l' cross sections: in particular for 2p-3d they are more than a factor of two lower close to the n=3 threshold. The total n=2 to n=3 cross section is 76% lower than that of Burke and co-workers (1967) at the lowest energy. The positions and widths of the Feshbach resonances for L=0,1, in general, agree well with those of Ho (1979). There is a broad shape resonance (³S) just above the n=3 threshold.

Using distorted waves for the incident, scattered and ejected electrons, and including polarisation effects triple differential cross sections are calculated for the ionisation of helium by the impact of electrons in the range 80 to 105 eV and compared with experimental data. The shapes obtained for the ejected-electron angular distribution agree well with one previous theoretical calculation, but disagree with another. The results are not particularly sensitive to the description used for the incident electron and target, nor to the inclusion of polarisation and static exchange potentials when calculating the scattered electron wavefunction.

Electron impact excitation of the configurations 3s3p⁶4s, 4p, 3d and 5s in argon has been observed in the energy-loss mode over a range of scattering angles and impact energies. The energy-loss spectra have been analysed to obtain the variation with angle and energy of the line shape parameters and intensities for individual terms of these configurations. A number of accurate values of energy and width for terms of the 3s3p⁶4s, 4p, 3d, 5s, 4d and 6s configurations have also been derived.

A theoretical treatment is presented of ionising collisions between a charged particle and a hydrogenic atom taking place in the presence of an intense laser radiation field. It consists of a reformulation, appropriate to the case, of the well known Coulomb wave Born (CWB) approximation. Two sets of calculations are presented for a coplanar geometry. In the first set (asymmetric case), the fast electron is taken at the fixed scattering angle theta ₁=20 degrees and the triple differential cross section (TDC) is calculated for different values of theta ₂, the angle of the slow electron ( theta ₂=40, 60, 140 and 260 degrees ). The energies of the incoming electron are epsilon (K_i)=250, 300 and 400 eV, while those of the ejected electron are epsilon (q)=50, 75 and 100 eV. In the second set (symmetric case) the TDC is calculated for theta ₁= theta ₂=30, 45, 60 and 75 degrees and for equal energies of the outgoing electrons. The energies of the incoming electron are epsilon (K_i)=300, 400 and 500 eV. For both cases the electric field of the laser is E_L approximately=5*10⁷ V cm^-1 parallel to the direction of the incoming electron, and of frequency h(cross) omega =1.17 eV. Among other results, it is found that a sum rule, of the same kind as that known for laser-assisted free-free transitions, holds and exhibits a larger validity than expected. It is suggested that multiphoton effects predicted in ionising collisions could be directly observed by analysing the energy spectrum of the outgoing electrons.

Positronium formation in the ground state and in a number of excited s states including the asymptotic n to infinity has been studied for the reaction e⁺+H₂ to Ps(ns)+H₂⁺ in the molecular Jackson-Schiff approximation (1953) in the energy range 50-1000 eV. It is found that the molecular effects play a significant role in the case of Ps formation and a hydrogen molecule cannot be considered as a pair of independent hydrogen atoms at any energy. The excited-state formation cross sections satisfy an n^-3 law approximately. It is also found that Ps formation involving the gerade transition of H₂⁺⁺ is the dominant process, although depending on the choice of molecular wavefunction there is a 10-28% probability of Ps formation involving the ungerade transition.