Table of contents

Lifetimes of 23 atomic levels of yttrium I and scandium I have been determined using selective pulsed laser excitation of the atoms in an atomic beam and applying the delayed coincidence method for registration.

The high-resolution spectrum of methyl bromide in the wavelength range 1100 to 1250 AA has been studied by using synchrotron radiation as a background source. Discrete structure due to the excitation of the 4p pi lone pair electron localised on a bromine site has been observed. As a result, the ns, np and nd Rydberg series converging to the ²E_3/2 and to the ²E_1/2 levels of CH₃Br⁺ have been considerably extended to higher principal quantum numbers, yielding more accurate values of the corresponding ionisation potentials. An MQDT analysis is also presented. The new observations are discussed in relation to the 4p spectrum of krypton and potential barrier effects in the p to d channel.

The authors present the results of close-coupling calculations of cross sections for rovibrational excitation of para-H₂ by He at a collision energy midway between the first and second excited vibrational excitation thresholds. Green's MOLSCAT computer program was employed. The authors' results are found to be in excellent agreement with previous computations of partial-wave cross sections by Easts and Secrest (1972), given the potential and basis set, but large discrepancies are found with the more recent calculations of Lin and Secrest (1970). Structures in the energy variation of pure rotational excitation cross sections, attributed by these authors to the opening of the first vibrational threshold, are absent in the authors' calculations.

Recent differential measurements of H⁺-H charge transfer for incident proton energies of 25 and 60 keV are used to test the classical-trajectory Monte-Carlo (CTMC) method for atomic scattering problems. For projectile scattering angles theta _cm of 0.5-3 mrad the CTMC results agree roughly with the experimental data, whereas for theta _cm<0.5 mrad the theory underestimates them. The relation between impact parameter and projectile scattering angle is examined within the framework of the classical-trajectory collision model.

The semiclassical approach recently proposed by Popov and Benayoun (1981) to account for the electron-electron repulsion in the electron impact ionisation of atoms is analysed.

Positron impact cross sections for the excitation of the n=2 levels of hydrogen have been calculated using the close-coupling approximation for energies up to 100 eV. Various target bases were employed and the effect of the inclusion of pseudostates is discussed.

Results of two five-state close-coupling (5CC) calculations of positron excitation of helium (1¹S to 2¹S,2¹P) at energies from 40 to 200 eV are presented and compared with an earlier 3CC calculation. In contrast to the e^- case, inclusion of k¹P and 4¹S pseudostates increases the cross sections (by 40% at 60 eV). For the 2¹P excitation the effect is almost entirely due to the k¹P pseudostate. The e⁺ cross sections are substantially larger than those for e^-. Total cross sections are given, but are poor below 100 eV due to neglect of positronium formation.

For pt.I see ibid., vol.14, p.4213 (1980). The combination of Milne's theory for calculating bound-state energies and wavefunctions with the complex rotation method yields an appealingly simple and powerful tool for the computation of complex-valued resonance (Siegert) energies and wavefunctions. The method provides an unambiguous assignment of a quantum number n=0,1,... to a resonance state and permits a unified picture of bound-state and resonance properties. The numerical technique is sufficiently fast and stable to enable reliable calculations of higher resonances. This is illustrated for some model potentials. Furthermore the complex energy WKB quantisation is discussed and tested numerically.

A pseudopotential method describing the alkali-core and the configuration-interaction method are used to obtain the lowest ^1,3 Sigma _g,u⁺ states of Na₂ and K₂. For the ground states of Na₂ the authors find reasonable agreement with experimental estimates of the dissociation energies. Wells followed by bumps were predicted for the ³ Sigma _u excited states dissociating to Na(3s)+Na(3p) and K(4s)+K(4p). Double minima for the ¹ Sigma _u states dissociating to Na(3s)+Na(4s) and K(4s)+K(5s) are reported.

The K Auger spectrum of palladium was studied on an iron double-focusing beta spectrometer. ¹⁰⁶Ag served as a source of Auger electrons and was obtained by irradiating a palladium target enriched up to 80% with ¹⁰⁸Pd with 30 MeV protons. Relative intensities of KLL, KLM, KLN, KMM, KMN and KNN lines and groups of lines were determined. Experimental results were compared with recent theoretical calculations; possible reasons for the discrepancies between theory and experiment were also discussed.

The lead K alpha X-ray spectrum is recorded by a Ge detector and fitted with an analytical representation that convolutes detector response with the Lorentzian profile. This procedure permits measurement of the intensities of the forbidden K alpha ₃ line and of radiative Auger bands. The K alpha ₃/K alpha ₁ ratio of (1.2+or-0.15)*10^-3 and the KL₂M/K alpha ₁ ratio of (5+or-2)*10^-4 agree with HFS predictions.

It is shown that when a monochromatic laser couples a single atomic ground level to two closely spaced excited levels the system can be driven into a state in which quantum interference prevents any fluorescence from the excited levels, regardless of the intensity of the exciting field. This steady-state interference occurs only at a particular excitation frequency which depends on the separation of the excited states and the relative size of the two transition dipole matrix elements. The results are derived from the density matrix equations of motion. It is shown that a correct description of the effect requires the inclusion of generalised Einstein A coefficients which are usually neglected in phenomenological damping theories. A dressed-state analysis is introduced to simplify the generalisation to atoms having more complex manifolds of excited states. Analogous interferences in multiphoton absorption and ionisation are also discussed briefly.

Radiative lifetimes have been determined for the z²D degrees _3/2,5/2, z²F degrees _5/2,7/2, y²D degrees _{3/2, 5/2}, y²P degrees _3/2 and y²F degrees _{5/2, 7/2} levels of neutral yttrium using the newly established technique of laser excited fluorescence from sputtered metal vapours. The results differ significantly from previously reported values obtained by the Hanle and level-crossing techniques (1970, 1971) and by beam-foil and beam-sputtering techniques, but they are consistent with relative lifetime values derived from the recent hook data of Cardon and co-workers (1980). The new lifetime results are used to determine a revised normalisation factor for the conversion of Cardon's relative f values into absolute values.

Decay branching ratios have been measured for all the observed transitions from nine levels in neutral platinum (5d⁸6s6p and 5d⁹6p configurations) excited in a hollow cathode discharge, in the 200-800 nm spectral range. Using the lifetimes given in the literature for three of the levels, a set of absolute transition probabilities is given for 15 lines connecting these levels.

Continued fractions of matrices solve the spin J-RF-field interaction completely. However, the general method requires (2J+1)²*(2J+1)² matrices. In this work the authors develop a treatment for the interaction of an aligned system with arbitrary spin J and an oscillating RF field requiring 2*2 matrices only. A numerical evaluation of the Bloch-Siegert shift and a comparison with recent experimental results are presented.

Photoionisation mass measurements on acetylene have been made between 1100 and 400 AA by using synchrotron radiation as a light source. Photoionisation efficiency curves are obtained for the molecular ion C₂H₂⁺ and also for fragment ions C₂H⁺, C₂⁺, CH₂⁺ and CH⁺. To assign the structures of C₂H₂⁺, ab initio calculations have been performed for the excitation from the second (3 sigma _g) and third (2 sigma _u) occupied molecular orbitals. Two broad maxima at 930 and 810 AA between the first and second ionisation thresholds are assigned to the autoionisation from valence-type excited states. New Rydberg series (np pi _u) converging to the second ionisation threshold were found. The appearance potentials of fragment ions by photoexcitation are given and discussed in terms of the heats of formation of molecular ions. For each of C₂⁺ and CH⁺, two values of the appearance potential are obtained from a double-step structure. The maximum heat of formation of C₂⁺ is estimated from the appearance potential to be 1980 kJ mole^-1.

This paper is concerned with the spectra, such as the oscillator strength distribution for transitions of linear, symmetric-top or asymmetric-top molecules, induced by any external perturbation. The sum over a rotational structure in the spectrum for a fixed initial rotational state Gamma is expressible in a simple form independent of Gamma . This sum rule is a general form of a theorem proved recently by the present author for the mean-energy-loss cross section. The theorem is a geometrical statement, and is valid regardless of the physical mechanism of the transition. A proof different from that given previously shows that the geometrical meaning of the theorem is the second cosine formula J'²=J²+J_t²+2J.J_t for the addition of the initial rotational angular momentum J and the angular momentum J'. As examples, sum rules for the dipole and generalised oscillator strengths for both discrete and continuum transitions are discussed. Also, sum rules for the rate constant and for the integral and differential cross sections for the mean energy loss hold for the collision of a molecule with any particle having or not having an internal structure, if the adiabatic-rotation approximation is valid.

A critical analysis is presented for three different methods which introduce translation factors in the molecular approach to atomic collisions. Unlike other contributions in this field, approximate one- and many-electron wavefunctions are explicitly considered from the start. The authors' conclusions are illustrated with the calculation of the charge exchange cross section for He²⁺+H(1s) to He⁺(2s,2p)+H⁺, which exhibits a strong origin dependence in the standard perturbed stationary states (PSS) approach. The authors' main (and new) conclusions are: (i) that the Bates-McCarroll method, and the method of projection on atomic components, are not necessarily convergent, and are exceedingly time consuming for approximate wavefunctions and (ii) that the common translation factor method is the most promising approach at present. However, before it can be used systematically in molecular calculations, more research is needed regarding its sensitivity to the analytical form of the common factor-otherwise some of the defects of the PSS approach reappear (in disguise) using this method.

A growth curve technique has been used within the energy range 6.7-800 keV to determine cross sections for electron capture by He²⁺ and He⁺ ions in lithium vapour. Cross sections sigma ₂₀ for two-electron capture by He²⁺ ions, which are relevant to a recently proposed technique for alpha particle diagnostics in fusion devices, have been measured and considered in relation to cross sections for one-electron capture by He²⁺ and He⁺ ions.

For pt.II see ibid., vol.14, p.559 (1981). The elastic scattering of electrons by krypton, xenon and radon atoms in the low-energy region up to 30 eV has been investigated. Direct and indirect relativistic effects are taken into account. The increase in the contribution of the direct effect with the nuclear charge of the target has been studied. Polarisation is included using a procedure based on the Pople-Schofield approximation. Partial-wave phaseshifts with l=0-8 are presented. The calculation predicts results which are in quite good agreement with measurements on total, momentum-transfer and elastic differential cross sections for krypton and xenon. There is poor agreement between theory and experiment on spin polarisation for xenon at energies of 25 and 30 eV. The present work indicates that radon exhibits the Ramsauer-Townsend effect also.

Calculations have been carried out on the elastic scattering of slow positrons by krypton and xenon atoms. The polarisation potential has been obtained using a procedure based on the Pople-Schofield approximation. The authors present two sets of results calculated with normalised and un-normalised polarisation potentials. It is shown that the results are very sensitive to the polarisation potential used. For krypton a Ramsauer minimum is predicted when the normalised polarisation potential is used whereas no structure in the total elastic cross section is obtained with the un-normalised polarisation potential. The correlation between electron and positron scattering is discussed.

The working out of an empirical expression for the cross section for production of atomic K-shell vacancies by electrons is described. The basis is a critical analysis of the existing experimental data. The ranges for which the function is evaluated are 6<or=Z<or=79 and 1<or=U<or=20 (U being the ratio between the incident electron energy and the binding energy of electrons in the K shell). The possible meanings of the various factors present in the expression are examined by comparison with the asymptotic Bethe expression. The results obtained by the empirical function can also be applied to both Z and U intervals even wider than those first considered.

The authors present an approach for dealing with particle scattering by a large number of fluctuating plasma electrons when a laser field is present. Using first-order time-dependent perturbation theory to treat the particle-plasma interaction, and taking account of the fluctuations of the plasma potential, the scattering probability and the particle energy losses are derived and expressed in terms of the spectral density of the charge-density fluctuations of a plasma embedded in a laser field. The particle energy losses are considered in detail in the limit of small momentum transfers (classical limit) and weak fields, omega ₀ approximately= omega _p and omega ₀>> omega _p ( omega ₀ and omega _p being, respectively, the laser field and the plasma frequency). For omega ₀ approximately= omega _p the particle losses are larger than in the field-free case, while for omega ₀>> omega _p the losses may be smaller or larger, depending on the allowed amount of the particle energy transferred during the interaction. Within the present approach both close and distant collisions are accounted for, and also the field-free results are recovered when the laser field is zero, or when the scattered particle is an electron.