Table of contents

For pt.I see ibid., vol.12, p.L135 (1979). The method of scaling in second-order electron correlation calculations is extended to take account of the importance of triply excited and quadruply excited configurations in the perturbative expansion of the correlation energy with respect to a single-determinantal reference function for a closed-shell system. It is shown that the scaling parameter may be chosen so that these higher order components of the correlation energy may be neglected. The results of prototype calculations for the molecules CH₄, NH₃, OH₂, FH and N₂ are presented.

The Z+1 approximation is discussed in connection with the final-state distribution of the Rb 3d photoabsorption spectrum. A method of calculating correction terms to the Z+1 energies is discussed. Also the Sr 3d photoabsorption spectrum is analysed and from this the atomic Sr 3d_{3/2, 5/2} ionisation energies are extracted (144.14(10) eV and 142.41(10) eV, respectively). In the light of recently reported core ionisation energies for Rb and Sr previous assignments in the 3p absorption spectra are also discussed.

By comparing the numerical results of various authors, the authors have investigated magnetic field strengths up to which a standard perturbation calculation can produce reliable results. Energy levels and transition probabilities for 14 low-lying states of the H atom are determined for B=10³ T using perturbation theory and are discussed in relation to previously published numerical values. Discrepancies up to five orders of magnitude in earlier results for transition probabilities are cleared up by their approach.

In plasmas, atomic-line radiation is influenced by static and high-frequency fields. The authors discuss a simple method of calculating the Stark profiles of the Balmer alpha and beta lines for the case of one-dimensional fields. Using a Holtsmark field for the static component, the resulting profile of Balmer alpha shows a splitting of the satellites.

Electron autoionisation spectra resulting from Mg⁺-Ne collisions are reported in the electron energy range 19-30 eV, at Mg⁺ energies of 5.0, 2.0 and 0.70 keV. An energy shift in one autoionisation feature and a broadening of another are observed. Large absolute differential cross sections at theta =90 degrees (of the order 10^-17 cm² sr^-1) are measured by comparison of the Mg⁺-Ne electron signal to elastic scattering signal of electrons on Ne.

The He II alpha photoelectron spectra of atomic Ca and Sr have been recorded. The np^-1 ionisation observed is compared with the corresponding Rydberg absorption spectra, Auger spectra and configuration interaction calculations and reasonable agreement is obtained. Anomalous features observed in the spectra have been attributed to discharge contamination and autoionisation for Ca and Sr respectively. The analysis indicates that configuration interaction is more important for the heavier atom.

A brief discussion of the de-excitation probabilities associated with autoionisation, vibrational relaxation and predissociation channels suggests that the autoionisation route can dominate the others in high-n Rydberg molecular states. Experimental investigations at high resolution and energy verify this conclusion and demonstrate that the variety of profile shapes may be as rich as or even richer than that for atoms.

The angular distribution of photoelectrons with defined spin orientation is derived for unoriented diatomic molecules exposed to polarised or unpolarised light. Its general structure is the same as in atoms and is defined by five independent parameters. Expressions for these parameters in terms of dipole matrix elements and phaseshift differences are presented.

It has been experimentally verified for CH₃Br that spin-polarised photoelectrons can be ejected from unoriented molecules if the radiation is circularly polarised or even unpolarised. The polarisation values and the experimental data of the asymmetry parameter beta are discussed by use of Cherenkov's formulae (1981).

The foreign-gas collision broadening of the NaD₂ line by He at a temperature of 8K is measured. These data combined with those of other investigators provide an empirical scaling of the collision rates and cross sections over a wide range of temperatures and relative velocities, respectively.

It is demonstrated theoretically that one can use a high-Rydberg atom in a collision experiment as an experimental probe of the scattering of an extremely slow electron by a rare-gas atom by analysing a refined experiment on the collision between a high-Rydberg atom and a rare-gas atom.

The authors report transmission measurements made with an electron time-of-flight spectrometer. The gas pressure in the target cell was determined by means of a capacitance manometer. From 2 eV down to 0.07 eV the results agree extremely well with the predictions of Morrison et al. (1977) obtained from a coupled-channel theory.

A linear-algebraic approach to the solution of the Schrodinger equation for low-energy electron scattering from molecules is presented. The method is based on an integral equations formulation of the single-centre, body-frame (fixed-nuclei approximation) scattering equations. Results in the static exchange approximation for e-H₂, N₂, LiH and CO₂ are compared with those obtained with other methods.

The author discusses a probable explanation of the totally inconsistent nature of some previously published Feshbach resonance spectra of hydrogen fluoride which has been provided by the recent work of Brion and Hitchcock (1980). He has obtained new electron transmission spectra of HF in the energy range 12.8 to 16.0 eV, which confirm the earlier HF^- resonance data of Spence and Noguchi (1975) and which reveal several previously unobserved resonance structures. These new structures are energetically coincident with the higher lying members of a series of peaks recently found in the F^-/HF dissociative attachment cross section. He explains the anomalous scattering from the HF^-2 Pi ( nu =0) resonance level in terms of the mixing of the b ³ Pi and B ¹ Sigma parent Rydberg states which has been observed in optical absorption spectra. Finally, he points out a possible mechanism for a Lyman- alpha laser.

It is shown that the Langevin-Harper formula for the ter-molecular ionic recombination coefficient alpha remains valid when the ion density n_i is high (>10¹³ cm^-3), there being a logical defect in a seemingly straightforward argument which leads to the contrary conclusion that this formula then overestimates alpha owing to the effect of Debye-Huckel screening.

A two-electron model, similar to that which has been previously used to calculate energy levels for Be I, is applied to the bound states and resonance structure of Mg I. The method is based on electron scattering techniques and is equivalent to a frozen-cores approach. The two-electron Hamiltonian contains a core polarisation potential which includes a term that accounts for the valence-valence interaction by the distorted inner core; it is shown that this term is essential in order to obtain consistent results. The convergence of the close-coupling expansion is examined and the model is found to provide an accurate overall representation for the Mg⁺+e^- system. Calculated bound states and resonant states are presented for the ¹S, ³S, ¹P⁰, ³P⁰, ¹D, ³D, ³P, ¹D⁰, ³D⁰ spectral series of Mg I.

Electron correlation in position space is examined in HeH⁺, the simplest of heteronuclear systems, as a function of the internuclear separation R. Coulomb holes and various partial Coulomb holes are evaluated and compared for He, HeH⁺ and Li⁺. Several correlation coefficients tau are also determined. The results are discussed in the light of a recent correlation analysis of HeH⁺ in momentum space. At the theoretical bond length R_Th, the Coulomb hole for HeH⁺ in position space is found to be very similar to that obtained for the ground state of He. As in momentum space, each tau which assesses angular correlation shows an increase in magnitude on molecular formation. Of particular interest in position space is the dramatic increase in the importance of 'left-right' correlation along the bond axis when R approaches R_Th.

Finite-difference Dirac-Fock electric dipole polarisabilities alpha are reported for the (ns)¹ ground states of Li, K, Rb and Cs and for the (ns)² ground states of He, Be, Mg, Ca, Sr, Ba, Ra, Zn, Cd and Hg. The non-relativistic values increase from Ba to Ra and from Cd to Hg while the relativistic ones decrease. An alpha of about 240 au is predicted for Ra. Due to large correlation contributions, the present results are mainly of technical interest. The same numerical methods are used as in the previous Dirac-Fock one-centre expansion calculations. The principle of the program is explained here.

Redistribution and depolarisation of light scattered by atoms undergoing collisions outside the impact region of the line profile has been studied experimentally and theoretically. In a cell experiment intensity and linear degree of polarisation of the fluorescence components 3²S_1/2 to or from 3²P_1/2,3/2 of Na atoms induced by collisions with Ar buffer gas atoms were measured as functions of the laser frequency omega _L. The decrease of polarisation of the D₂ component observed when omega _L is shifted away from the D₂ toward D₁ resonance disagrees with calculations on the basis of the factorisation approximation of the theory of redistribution. Using Baylis (1975) potentials for the Na/Ar system, the effect is interpreted in terms of adiabatic coupling of the 3²P_3/2 electronic angular momentum to the collision axis during 'strong' collisions, giving rise to a greater than average depolarisation.

Using a CW dye laser and an original RF variant of the optogalvanic detection method, the authors have excited and detected xenon atoms in levels lying close below and above their first ionisation limit. Accurate positions are obtained for np(1/2)1 series (15<or=n<or=19) and for nf(3/2)1 series (11<or=n<or=43), as well as for autoionising levels of 5p⁵7p' and 5p⁵nf' (4<or=n<or=7) configurations.

The spectrum of the SbO⁺ molecule has been studied under high resolution for the first time. Analysis of the rotational structure in (1,2) and (1,3) bands leads to the following rotational constants: B'₁=0.325 13 cm^-1, D'₁=2.09*10^-7 cm¹; B"₂=0.359 64 cm^-1, B"₃=0.357 41 cm^-1, B"_e=0.365 13 cm^-1, a"_e=0.002 23 cm^-1, D"₂=1.57*10^-7 cm^-1, D"₃=1.47*10^-7 cm^-1, r"_e=1.8074 AA; nu _(1,2)=23 568.12 cm^-1, nu _(1,3)=22657.79 cm^-1.

In infrared-radiofrequency double resonance (IRRFDR), the IR dynamic Stark splittings is shown both theoretically and experimentally to be observable at the fixed frequency of the strong IR field itself. This is achieved by using a weakly saturating RF field and is revealed by a doublet in the IR absorption change versus the RF. As with the conventional Autler-Townes effect (1955), this phenomenon appears to be general and observable in other spectral regions.

A characteristic emission in the spectra of argon krypton mixtures can be seen at 640 nm. It reaches a maximum for krypton concentrations of about 30%. In argon xenon mixtures two emissions with the same origin are seen at 328 and 500 nm. The variation of their intensity with partial pressures is similar; a maximum is reached at a 10% Xe concentration. The shapes of the light pulses obtained at 640 nm in ArKR and at 328 nm in ArXe are described by an exponential decay relationship corresponding to the formation of the radiative state. In both cases the kinetic study shows that the emissions correspond to radiative transitions arising from the IV¹/₂ state of the heteronuclear ion which dissociates to the Ar(²P_1.2) state; the various two- and three-body reaction constants relative to the atomic ion are calculated.

Vibrational level population distributions of electrical-discharge-excited nitrogen were measured up to nu =7 by means of a Penning ionisation reaction involving He(2³S) metastables. Vibrational level populations of N₂(X, nu ) were obtained from the N₂⁺(B, nu ₁) level concentrations and the inverted matrix of the Franck-Condon factors. A vibrational temperature range of 2000-8000K was covered with all measured vibrational distributions exhibiting non-Boltzmann, near-Treanor quasi-steady-state behaviour.

An efficient computer code for calculating the impact parameter distribution of atomic ionisation probabilities caused by charge particle impact, has been developed. The program is based on the semiclassical approximation, and it allows the use of an arbitrary atomic central potential for deriving the one-electron orbitals that form the basis for the description of the atomic states. Extensive calculations are reported for proton induced K-shell ionisation in carbon and neon, covering energies in the range 0.1-10 MeV. Some calculations on proton-argon L-shell ionisation are also reported. Comparison between the results obtained using (screened) hydrogenic potentials and the recently reported energy optimised effective atomic central potentials demonstrates that wavefunction effects are generally important for inner-shell ionisation of light atoms. The agreement between theory and experiment in the K-shell case is improved for fast collisions upon using better wavefunctions.

Gryzinski's double binary encounter model (1965) using the correct expression for sigma _{Delta E} due to Vriens and the appropriate velocity distribution functions for the bound electrons has been used to calculate the proton impact double-ionisation cross sections of noble-gas atoms (Ne, Ar and Kr). The calculated cross sections are found to be in qualitative agreement with the experimental observations. The effect of the use of Hartree-Fock and hydrogenic velocity distributions for the target electron has also been discussed.

The branching ratios of the Penning excitation for the individual Cd II states by He(2³S) atoms were measured using Penning electron spectroscopy. Using the value of the total Penning excitation cross section measured by Schearer and Padovani (1970) the excitation cross sections for the upper and lower states of the Cd II 441.6 nm laser line are determined to be 0.54 and 0.14*10^-15 cm², respectively. Those of the Cd II 325.0 nm laser line are equal to the above values, respectively, within the limits of error of the determination. The excitation cross section for the CdII ground state is 2.8*10^-15 cm².

The continuous part of the electron spectra produced in Krⁿ⁺-Kr collisions has been measured. Electrons are measured in the energy interval between 80 and 1400 eV. Projectile energies between 50 and 800 keV, emission angles from 45 to 135 degrees and projectile charge states from 2+ to 5+ are used. The data are interpreted in terms of molecular autoionisation due to the vacancies present in the 4p pi MO of the Kr₂ quasimolecule. The authors' results confirm the assignment of Afrosimov et al. (1977) for the spectrum obtained at much lower projectile energies. From the spectra the experimental MO is determined and is compared with theoretical calculations. A simple model calculation is in good agreement with the data if decay rates as high as 10¹⁶ s^-1 are used.

For pt.I see ibid., vol.14, p.513 (1981). The continuum electron spectra produced in Neⁿ⁺-Ne (n=1, 2, 3, 4) collisions have been measured as functions of the angle and energy of the ejected electrons. Electrons were measured in the energy interval between 16 and 1100 eV. Projectile energies from 25 to 800 keV, projectile charge states from 1+ to 4+ and emission angles between 45 and 135 degrees have been used. Spectra for Krⁿ⁺-Kr (n=2, 3, 4, 5) were measured under the same conditions. For the Kr-Kr system only the exponential part of the spectrum was studied. For the Ne-Ne system the spectra have a pureexponential shape as a function of the electron energy. The cross sections decrease when the projectile charge is increased. The angular distributions are strongly anisotropic in the laboratory system but become almost isotropic in the centre-of-mass frame. It is found that the data can be described by a simple empirical relation. The authors' result indicate that radial and rotational couplings between highly promoted molecular orbitals and the continuum may be important.

Simultaneous measurements of the differential electron attachment cross section and the differential elastic scattering cross section have been measured for H(1s) atoms colliding with a caesium vapour target. To interpret the structures seen in all the measured cross sections the authors have calculated semiclassical differential cross sections using the lowest order stationary phase approximation, JWKB phaseshifts and the landau-Zener transition probability. For the inelastic channel leading to H^- production the measurements show two types of oscillating features. The Stueckelberg oscillations are due to the interference of scattering from different potentials inside the pseudo-crossing of the covalent and ionic potential curves. Interference due to scattering from the ionic interatomic potential produces a rainbow structure corresponding to a well depth D_e=1.90+or-0.04 eV at the equilibrium separation of the X¹ Sigma state. This well depth is consistent with the attenuated rainbow structure observed on the elastic channel.

For pt.I see ibid., vol.33, p.261 (1980). Relativistic effects in the elastic scattering of low-energy electrons by mercury atoms have been studied by employing a modified non-relativistic treatment. The direct relativistic effect is taken into account using the second-order Dirac potential as a perturbation to the interaction potential and a relativistic Hartree-Fock wavefunction is used to describe the target. There is good agreement between the present work and the relativistic calculations of Walker (1969) who has solved the Dirac equation explicitly and has also used a relativistic wavefunction to describe the atom. The calculation indicates that the scattering process can be described by a non-relativistic formulation which treats the direct relativistic effect as a correction to the interaction potential. The importance of the direct and indirect relativistic effects is investigated.

The three-level system interacting with two strong near-resonant monochromatic fields is treated. The relationship between the coherent dynamics of the system and the multi-component structure of the scattered light spectra is investigated. The authors show that a reduction of the component number in the scattered light spectrum is always accompanied by periodical oscillations of one or more level populations.

Using laser-produced plasmas of high temperature and density the authors have investigated the possible shifts due to local plasma polarisation on the H_alpha lines emitted by C VI and N VII. In contradiction with a number of theoretical treatments, they have found no evidence for shifts of the predicted magnitude. Whatever the plasma conditions the H_alpha lines have been observed to be unshifted to within +or-0.02 AA corresponding to the estimated accuracy of the measurements. Results are discussed in the light of the existing theoretical treatments and are compared with similar investigations in the isoelectronic sequences.