Table of contents

Coupled Hartree-Fock perturbation theory is applied in extended ab initio calculations of molecular dipole moments, polarisabilities and first hyperpolarisabilities of first-row hydrides. Their dependence on internuclear separation is also analysed and discussed. Very strong hyperpolarisability gradients and large vibrational corrections are found. The relevance of the present results for interpreting the discrepancies between experimental and theoretical hyperpolarisabilities is pointed out.

An investigation of the beam-foil spectrum of beryllium has been undertaken which has revealed a number of spectral lines not reported earlier. Recently, Larsson et al. (1979) and Galan and Bunge (1980) have performed calculations for quartet states in Be II. The authors report experimental results which confirm the theoretical predictions.

Interaction energies for the X¹ Sigma _g⁺ ground state of Cs⁺+Cs⁺ and the ¹ Sigma _g⁺ doubly excited state of Cs⁺*+Cs⁺* have been calculated using the self-consistent-field method. Molecular orbital energies are presented and used to interpret the charge-changing mechanisms for Cs⁺+Cs⁺ collisions. The calculations indicate that a dominant low-energy mechanism in the charge-changing process is the formation of Cs⁺+Cs²⁺+e^-. The energy threshold for this process is low, approximately 160 eV, and the cross section may be as large as 6*10^-16 cm².

Thermal collisions between Xe(nf) Rydberg atoms and HF molecules have been investigated. The highly excited collision products were detected using selective field ionisation (SFI). Both n-changing and l-changing collisions were identified and the absolute rate constants for these processes, and for collisional ionisation, have been measured for n=26, 27. In addition, the first evidence of molecular rotational excitation through the transfer of energy of electronic excitation from a Rydberg atom is presented.

The velocity dependence of the full width at half maximum of projectile electron-loss cusps is investigated. For Si^q+, O^q+ and C^q+ projectiles traversing He, Ne and Ar targets in the velocity range 7-12.5 au, the FWHM is found to be independent of the ion velocity. These results disagree with theoretical predictions predating those of Day and restrict the velocity dependence of the anisotropy parameter beta in Day's theory (1980).

The charge exchange between a completely stripped lithium ion and a hydrogen atom is studied in the framework of the impact parameter molecular approximation for relative velocities lower than one atomic unit. The total cross section shows a strong increase as a function of the energy. The results are compared with theoretical and experimental data of other authors.

The cross sections 0 to 0, 0 to 1 and 1 to 2 for vibrational excitation of N₂ by electron impact at 5, 10, 20, 30 and 50 eV are calculated in a static-exchange-polarisation model. The calculations are based on the Hartree-Fock target wavefunctions of Cade, Sales and Wahl (1966) for the static potential and for the Hara (1967) free-electron-gas exchange potential. Target charge polarisation is included by the effective polarisation potential of Morrison and Collins (1978), scaled as a function of internuclear distance by the polarisabilities. The calculated cross sections are well converged with respect to channels. The results show clearly both the low-energy Pi _g resonance and the intermediate-energy Sigma _u resonance.

A set of quantum equations describing the superfluorescent emission by two groups of sources having different resonance frequencies is presented. Numerical solutions are derived which in particular show significant differences between the quantum and the semiclassical results.

The authors consider products of two first Born matrix elements linking an s state and the continuum. They show how such matrix elements may be averaged over the angular part of the continuum wavefunction, and give final expressions for two particular cases. The angular integration requires an extension of an identity due to Feynman.

For pt.II see ibid., vol.13, p.869 (1980). In previous papers by Safronova and Urnov this method was developed based on the relativistic expansion into perturbation theory series in powers of 1/Z (Z is the nuclear charge) suitable for the calculation of atomic characteristics for autoionisation states of many-electron ions. The calculations were carried out for states of the type 1s2sⁿ2p^m for ions with a number of electrons N=4-9 and spectroscopic symbol z=6-30 (z=Z-N+1). The comparison with results obtained in the framework of other approaches (Hartree-Fock and Thomas-Fermi) was also given. The authors present the results of calculations for the spectra of dielectronic satellites (DS) to the He-like ion resonance line excited in the ions of Be-, B-, C-, N- and O-like isoelectronic sequences in dense high-temperature plasmas; the case of low-density (coronal) plasma will be considered in a following paper.

The impact broadening of the potassium 4p²P_1/2-7s²S_1/2 (5782 AA) and 4p²P_3/2-7s²S_1/2 (5802 AA) lines by helium and argon foreign gases has been studied interferometrically in the emission spectrum of a low-current electrical discharge. Unlike the resonance lines and other transitions from low-lying states, the broadening and shift rates here are so large that very significant effects are observable with conventional spectroscopy at gas pressures as low as 10 Torr. The observed effects are interpreted with impact broadening calculations based on the pseudopotential interactions computed by Pascale and Vandeplanque (1974), and on the Fermi-type potential proposed by Roueff (1970). The pseudopotentials underestimate considerably the observed broadening and shift, while the Fermi potential gives an improved, although not entirely satisfactory, agreement. Potentials deduced for the 7s²S_1/2 state by fitting power-law interactions to the observed broadening and shift rates have a long-range attraction for argon and a long-range repulsion for helium. These fitted interaction potentials are about an order of magnitude stronger than the pseudopotential calculations for the internuclear separations of approximately 20 AA that contribute most significantly to the broadening and shift.

The carbon K-emission spectra of polycrystalline naphthalene and anthracene were measured. For single crystals of naphthalene the carbon K-emission spectrum was also measured as a function of the take-off angle. The angular-dependent intensity distribution of the spectra allows the contribution of the pi - and sigma -valence orbitals to be determined. As the size of the molecules increases the carbon K spectra approach the spectrum of graphite. The experimental results are compared with photoelectron spectra and are discussed on the basis of MO calculations for the free molecules.

It is proposed that the luminescence bands observed in high-pressure argon between 1600 and 2900 Å are due to transitions involving excited states, Ar₂⁺*, of the molecular ion, Ar₂⁺: the initial state is a bound state having a ²S-²S₀ ion-atom asymptote and the final states two of the first set of molecular ion states which dissociate to the ion-atom pair ²P-¹S₀. This assumption accounts for most known experimental data on this emission band. The competition between the radiative transitions and the quenching collisions between the excited ions and electrons in alpha -particle tracks has been studied experimentally and is discussed in terms of the present hypothesis. A comparison is also made between the neutralisation of excited and ground-state molecular ions.

The beam-foil technique is applied to study the spectra of As III-V in the wavelength range 300 to 2000 AA. Fifteen new transitions are assigned to As V, involving the classification of eight new levels. From the 4s, 5s, 6s and 4p, 5p, 6p levels the As V ionisation potential is found to be 62.76+or-0.06 eV. Lifetimes and multiplet f values are reported from transitions from the 4s²4d and 4s4p²²D terms of As III, the 4s5s and 4p²¹S, 4p²¹D, 4s4p¹P⁰ and 4s4d³D terms of As IV, and the 4p²P⁰, 4d²D and 5s²S terms of As V. Arbitrarily normalised decay curve (ANDC) analyses were used to overcome the problems associated with cascade repopulation in the cases of the 4s4p¹P⁰ (As IV) and 4p²P⁰ (As V) terms. The resulting multiplet f value for the As V resonance transition is in good agreement with theory, but that for the As IV resonance transition is about 20% below theory, possibly due to omission from the ANDC analysis of the cascade from 4s4d¹D, which is yet to be assigned.

A differential energy-loss study of electron detachment in Br^- and I^--inert-gas collisions in the 250 to 2000 eV laboratory energy range has been performed. Differential cross sections for elastic scattering, simple detachment (X+B+e^-)and detachment accompanied by excitation processes (X*+B+e^-) are reported. These measurements show the importance of dynamic effects in the simple detachment process. An interesting impact parameter dependence of the energy spectra of electrons detached in the simple detachment process is observed. The close analogy of the energy dependence of the elastic cross sections in these collisions and in H^--inert-gas collisions suggests that here also direct 'tunnelling' transitions into the continuum play an important role.

The charge state dependence of K and L X-ray emission has been studied for 1.4 MeV/N Xe^q+ (24<or=q<or=33) collisions with Ar, Kr and Xe gas targets. An increase in X-ray yield is observed for q>26. Dynamic couplings between molecular orbitals in the transiently formed quasimolecule are found to be responsible for this increase. Parameters characterising these couplings are estimated. The observed pressure dependence of K and L X-ray emission is found to be due to charge state changing effects.

An analysis is made of the existing data of Luyken et al. (1972) for the cross section for 2s subshell ionisation of neon by electron impact. These cross sections were obtained by detection of 2s2p⁶ to 2s²2p⁵ transitions in Ne⁺(2s2p⁶) at 46.1-46.2 nm as a function of electron impact energy. Their relative data have been normalised to the absolute cross section of van Raan et al. (1971) given at 300 eV impact energy. In the present article this absolute scale is compared with that derived by the application of the Bethe formula. In this formula the quantity M_i² for the 2s shell of neon is evaluated using existing experimental photoionisation data. The analysis shows that the absolute value of van Raan has to be reduced by a factor of 3.7_-0.8^+1.3.

The authors have calculated the collision strengths averaged over a Maxwellian distribution of electrons for all the transitions among the 12 lowest states of Si III. These are presented over a temperature range (0.50-25)*10⁴K. They used configuration interaction wavefunctions in the R-matrix calculations of these results. They compare their results with other work. Resonance effects play a dominant role in semi-forbidden transitions.

Fractions of long-lived excited species in noble-gas ion beams which were produced by various methods have been determined by virtue of slow ion impact induced electron emission from clean polycrystalline tungsten. Investigations with ion beams produced by single electron impact on atoms permitted the determination of cross sections for the production of singly charged metastable ions of the noble gases Ar, Kr and Xe. The cross sections obtained increase steeply from threshold up to their maximum values of about 10^-17 cm² within 1.5 time threshold energy. Processes responsible for production of these metastable ions are discussed and the method applied is compared with other techniques for metastable ion detection.

The dissociative recombination of the methane-derived group, CH⁺,...,CH₅⁺, has been measured over the centre-of-mass energy interval 0.03 to 2 eV. Assuming a Maxwellian distribution of the electron velocities, the derived rate coefficients for CH⁺,...,CH₅⁺ at 100K are, respectively, 5, 8.7, 12, 12.8 and 12.8*10^-7 cm³ s^-1. The recombination cross section for the diatomic ion CH⁺ is large. This has far reaching consequences for the theories of molecular formation in interstellar clouds. The cross sections for the polyatomics CH₂⁺,...,CH₅⁺ are also large and roughly equal in magnitude. Like CH⁺ they exhibit a close to E^-1 energy dependence below 0.1 eV. For CH₃⁺, CH₄⁺ and CH₅⁺ the slope of the cross sections increases above 0.1 eV.

Positron-hydrogen collision cross sections have been calculated for intermediate and high energies. Comparison with earlier theoretical papers has been made wherever possible. An expected broad peak is observed in the results for the total cross section in the intermediate energy range.