Table of contents

A phenomenological argument based on the results of Griffith et al. (1979-81) suggests that the anomalous isotope shifts observed in the spectrum of samarium are predominantly a mass effect rather than a nuclear field effect.

High-resolution ( approximately=0.003 AA) spectra of H₂S in the vacuum ultraviolet are reported. A prominent d Rydberg series can be followed up to n=26 and allows an accurate determination of the first ionisation potential of H₂S 84420.00+or-2 cm^-1. This d series exhibits an intensity anomaly at about 1200 AA which is assigned to an interchannel perturbation. Spectroscopic differences between H₂O and H₂S are enumerated and rationalised in terms of the natures of the core holes and the molecular ion geometries.

The relatively large probability of the fine-structure transitions (³P_0-1, ³P_0-2) which are observed in Ne*-Ne collisions at thermal energies (E>or=80 meV) cannot be accounted for by the spin-orbit (SO) interaction. The Ar₂* system is in some ways similar to Ne₂*, apart from a larger SO interaction. The probability of the ³P_{0 to 2} transition induced by the SO interaction is calculated for the Ar*-Ar collision at 326 meV. This probability is relatively weak ( approximately=2.6*10^-2). The effect of the rotational coupling (L^+or-) between Gamma states ³ Sigma _g⁺ and ³ Pi _g, which are degenerate at infinite R, is then examined in the framework of a simplified two-state (0_g^-, 1_g) model. In Ar₂, a larger transition probability ( approximately 0.1) is obtained for relatively large values of the angular momentum (l approximately=100-200). It seems however that this effect alone is not sufficient to explain the experimental data on Ne. In this latter case, an accidental degeneracy of ³ Sigma _g⁺ and ³ Pi _g potentials (shifted by the SO interaction) appears at finite distance ( approximately=6 au). It reinforces the effect of the rotational coupling and may give sufficiently large ³P_{0 to 1,2} transition probabilities at low energy and small l.

Exact numerical calculations for 1s-1s electron capture by protons on hydrogen show that the second Born approximation is characterised by a peak in the differential cross section at theta _cm=0.054 degrees when the projectile energy is above 5 MeV. Below 5 MeV this peak disappears. Between 3 and 1000 MeV the total non-relativistic cross section in the second-order Brinkman-Kramers (BK) approximation lies below the first-order BK total cross section.

The properties of an electron beam passing through an assembly of optically active molecules are considered. If the sample is a pure optical isomer, the spin polarisation of the emerging beam is expected to show phenomena which are analogous to the effects of circular dichroism and optical activity: (i) the attenuation of the electron beam depends on the longitudinal spin polarisation of the incident beam; (ii) an initially unpolarised beam emerges with longitudinal spin polarisation and (iii) in the passage through the sample the transverse polarisation of the incident beam undergoes a rotation about the beam axis. These electron optic effects, like the optical ones, rest on the lack of inverse symmetry of the target molecules. Comments are made on the possibility of detecting the described effects.

Differential elastic scattering of electrons at 7.5, 10, 12.5, 15, 17.5 and 20 eV from neon is reported. The s, p, d and f phaseshifts derived from the measured cross sections are also presented.

The polarisation of the 6³P₁-6¹S₀ line of Hg after excitation of the 6³P₁ state by electrons scattered in the forward direction is evaluated as a function of electron energy using cross sections based on the Born-Ochkur approximation. Good accordance is obtained with the recent experimental data of Hanne et al. In particular, the theory yields the value 7.1 eV for the incident electron energy where the polarisation vanishes, which is close to the value predicted by the experimental study. Moreover this energy should be independent of the isotope of Hg.

Milne's approach to the numerical solution of the Schrodinger equation via a non-linear differential equation an the quantisation of a quantum action is investigated in detail. An accurate and efficient computational method is presented which allows a rapid second-order convergence onto a desired eigenenergy E_n. Numerical sample calculations demonstrate the efficiency of the method, which has special advantages for accurate calculations of high quantum states. The present method can be easily extended to the calculation of quasi-bound levels at resonance (complex-values) energies.

Two natural polyisotopic potassium beams were illuminated respectively by a RF modulated and an unmodulated laser beam. The laser side band frequency is locked on an atomic frequency, while the unmodulated laser frequency is RF scanned. The RF accuracy of the frequency scale allows measurements to about 0.3 MHz of the hyperfine structures and the isotope shifts of the D₁ and D₂ lines of ^39,40,41K. The results obtained are reported and compared to the previous ones.

A two-step optical excitation, starting from a metastable level, has been used to populate Rydberg levels in a krypton atomic beam. The metastable level is populated in the beam 'oven' by means of a HF discharge and the Rydberg atoms are detected by the field ionisation method. Using two N₂-pumped pulsed dye lasers, the Rydberg spectra have been obtained with a resolution better than 0.1 cm^-1. The fine structure of the (4p⁵)²P_3/2 nd sub-configuration has been resolved and the wavenumber of the corresponding transition has been measured with an absolute accuracy of 0.03 cm^-1. The energies of 187 levels (n>or=24) belonging to seven different series, 4p⁵nd and 4p⁵ns, have been determined; 153 of these levels (J=2 and J=3) had never been observed before. The value of the first ionisation limit has been redetermined to be I=(112914.47+or-0.03) cm^-1. The critical ionisation fields and the lifetimes of some of the Rydberg levels have also been measured. Significant deviations from the hydrogenic model laws been found in both cases.

Extensive new data on the bound odd-parity spectrum of krypton have been provided recently by two-step laser spectroscopy experiments. Highly excited levels belonging to seven Rydberg series nd or ns converging to the ionisation limit have been observed, but some series are difficult to assign. Therefore, by means of multichannel quantum defect theory (MQDT) the authors have performed the analysis of all available energy data relevant to the J=0, 1, 2, 3 odd-parity bound spectra of krypton. These studies allow them to identify all the newly observed highly excited Rydberg series by connecting the new data with previous work relevant to the low-energy range. Perturbations of nd and ns principal Rydberg series by the lowest members ns' and nd' of Rydberg series converging to the second limit occur in the low-energy range; these perturbations are successfully interpreted. The energies of unknown levels are predicted and a new value for the ionisation limit is provided. Additional information on the fine structure of highly excited configurations is obtained by means of the Slater-Condon method. The Slater integrals of highly excited configurations are determined and the Jl coupling scheme of levels is discussed.

The authors report simultaneous measurements of both Na⁺ and Na₂⁺ ion yields when Na vapour in the 10¹²-10¹³ cm^-3 density range is irradiated by a pulsed laser tuned to one of the 3S_1/2-3P_1/2,3/2 resonances with intensities in the 10³-10⁶ W cm^-2 range. In the studied vapour density and laser intensity ranges, electron impact ionisation and multiphoton ionisation processes are negligible; the ion yields are hence interpreted in terms of atom-atom collisions. The influence of associative ionisation on the Na₂⁺ yield is analysed on the basis of the experimental data and a calculated potential curve for the ¹ Sigma _g molecular state correlated to Na(3p)-Na(3p). A critical discussion of possible laser-induced (laser-assisted and/or laser-modified) ionising processes is given. A good account of the data is obtained only if laser-induced associative ionisation and laser-induced Penning ionisation (as the most probably processes) are assumed to take place in the vapour. The distribution over the vibrational levels of the Na₂⁺ ions thus formed and the photodissociation of these levels are discussed as well as the existence of independent Na⁺ ion formation.

The authors report the observation and the measurement of an intenses atomic ion yield when Na vapour in the density range 10¹³-10¹⁵ cm^-3 is irradiated by a laser tuned to 3S_1/2 -3P_3/2 resonance with intensity 3*10⁵ W cm^-2. It is shown that super-elastically heated electrons are present in the vapour and play a determining role in the ionisation process. A numerical analysis following the LIBORS code of Measures et al. (1979) is given which emphasises the requirement of a large cross section for the electron seeding mechanism: 5*10^-15 cm² at the given intensity. Laser-induced collisional ionisation discussed in the preceding paper (Carre et al. 1981) is assumed to initiate further efficient ionisation.

Generally, the effective order of non-linearity k=( delta lg N)/( delta lg I) observed in multiphoton ionisation is equal to the number of photons k₀ absorbed except in the vicinity of a resonance, where k deviates strongly from k₀ (N is the ionisation probability, I is the field intensity). By taking into account the space dependence of intensity the authors show that k=k₀-1 is expected when (k₀-1)h(cross) omega is a little smaller than the ionisation energy of the initial state.

Laser temporal-coherence effects are investigated in four-photon ionisation of caesium atoms using a tunable wavelength Nd-glass laser pulse and tuning the frequency onto the resonant three-photon transition 6S to 6F. The laser can be operated in either single or multimode. In the latter case, the laser bandwidth can be varied between 3*10^-2 and 1.5 cm^-1. The resonance peaks obtained with incoherent laser pulses are enhanced, shifted and broadened with respect to those induced by coherent pulses with the same average intensity. The resonance shift induced with incoherent laser pulses with the same average intensity. The resonance shift induced with incoherent laser pulses with the same average intensity. The resonance shift induced with incoherent laser pulses is statistically enhanced by a factor of 1.8+or-0.2 compared with the shift induced by coherent pulses, in good agreement with calculations. The resonant multiphonon ionisation rate induced by multimode pulses depends both on intensity and bandwidth effects. It is enhanced by a factor of 2.5 for pulses with bandwidths less than 0.2 cm^-1, while it becomes smaller than that induced with a coherent pulse of the same intensity when the bandwidth becomes larger than 1 cm^-1. Finally, the resonance width depends both on intensity an bandwidth effects. At low intensity (2*10⁷ W cm^-2) it is governed only by the laser bandwidth, while at high intensity (10⁸ W cm^-2) it varies linearly with the intensity and is statistically enhanced.

Analytical expressions for the transition polarisabilities between any two s states for hydrogenic systems have been derived. Results are used to discuss the possibility of observing experimentally the spontaneous anti-Stokes Raman scattering from the metastable 2s state of hydrogen.

A multichannel distorted-wave approximation for atom-atom scattering based on an expansion of the total diatomic wavefunction in adiabatic electronic-rotational (AER) states is studied through a numerical example involving the ³P₁ to ³P₀ fine-structure transitions in excited Zn, Cd and Hg. The AER states are those which diagonalise the full electronic-rotational Hamiltonian continuously as a function of R. In the limit of weak interaction the coupled equations for the exact wavefunctions in the AER representation suggest a simple distorted-wave approximation. The scattering matrix element is evaluated using the single-channel radial wavefunctions obtained from the adiabatic potentials, and its magnitude depends on the R variation of the orthogonal matrix which diagonalises the complete multichannel interaction energy.

The model of Ciocchetti-Molinari (1965) and an alternative model of Blair et al. (1978) proposed as descriptions of collisions including nuclear-resonance scattering are discussed in the framework of the distorted-wave theory. The authors' study distinguishes two manifestations of the nuclear time delay in atomic collisions. The model they present is consistent with the original results of Blair et al. and does not predict the large variation in ionisation probability observed by Drinker et al. (1980).

Static exchange and static exchange plus polarisation calculations are carried out for elastic electron and positron scattering with Be, Mg and Ca. All three atoms are found to have a low-lying ²P resonance in the electron scattering case. Magnesium and calcium are also predicted to have higher lying ²D shape resonances near 2.0 and 0.7 eV, respectively. The ²D resonance in Mg is very weak and broad ( Gamma >3.0 eV) while that in Ca is much more pronounced and is much narrower ( Gamma approximately 0.6 eV). These findings are in agreement with recent experiments. In addition a very broad feature appears in e^--Be d-wave scattering near 5 eV. While maxima occur in the p-wave cross sections for positron scattering, these do not appear to be due to resonances. However, evidence is provided for exceptionally broad resonances in d-wave positron scattering and for a virtual (or antibound) state in s-wave scattering.

The ionisation asymmetry is measured in a crossed-beam experiment utilising a W/EuS field emitter as a source of polarised electrons and dual-frequency optical pumping as the polarising process for the lithium atomic beam. The antiparallel-parallel asymmetry of the total ionisation cross section is positive with a broad maximum of 0.35 near threshold and a gradual decrease towards higher energies. A comparison is made with results of binary encounter and Born approximations. At threshold no significant variation of the asymmetry with energy is observed within a resolution of 0.3 eV.

The electronic vibro-rotational excitation of molecules in collisions is considered within the framework of an adiabatic approximation. The Franck-Condon approximation an some of its consequences are discussed. A new method of investigating the deviations from the Franck-Condon approximation, based on the analysis of the relative electronic rotational cross sections, is proposed. It requires no information about the values of Franck-Condon factors for the corresponding electronic vibrational transitions. It is shown that all the cross sections for electronic vibro-rotational transitions can be expressed in terms of a finite number (usually small) of parameters which are to be derived from the experiment in a semiempirical approach or from calculations accounting for the collision dynamics. Thus a theoretical scheme is developed for the analysis of non Franck-Condon collisional transitions.

The fluorine K-shell (1s) excitation spectra of HF and F₂ have been studied by small-angle inelastic scattering of 2.5 keV electrons. The F₂ spectrum exhibits a dramatic 1s to sigma * shape resonance 14.5 eV below the F 1s ionisation threshold which is qualitatively different from the 1s to sigma * transitions observed in all other first-row diatomics. An empirical correlation between experimental term values for both sigma * and pi * K-shell shape resonances and the sum of the atomic numbers of the heavy atoms in a series of first-row molecules is demonstrated and tested with respect to the energy of the 1s to sigma * transition in F₂. The sigma * and pi * energies of a variety of molecules have been predicted using this relationship. In contrast to F₂, the HF spectrum is more atom-like, exhibiting a moderately intense 1s to sigma * transition followed by the np Rydberg series. The equivalent-core analogy is applied to the HF and F₂ results to predict excitation and ionisation energies of NeH and NeF.

The elastic S-wave scattering of positrons by hydrogen atom targets is calculated using a trial function which is a simple product of functions of the interparticle distances. Results are presented for incident positron momenta 0<or=ka₀<or=0.8, and compared to some pseudo-state and 'exact' calculations. The S-wave phase shifts (which are rigorous lower bounds to the exact values) lie in the 0.9 quality range, on a scale in which the static phaseshift are zero and exact results are one, except as k to 0 where they drop to 0.78. The effective number of annihilation electrons Z_eff is about 5.38 at zero energy, which is substantially below the 'exact' value of 8.83, but well above the Z_eff<or approximately=2 values obtained from pseudo-state calculations. A brief discussion of the generation of the present method to many-electron atoms is given.