Table of contents

A diatomics-in-molecules calculation of the potential energy surface for O⁺+H₂ to OH⁺+H (linear approach) yields a 22 kcal mole^-1 activation barrier. This is in contrast to a previous ab initio calculation by Chambaud et al. (1974) in which the intermediate species (O . . . H . . . H)⁺ is found to be bound with respect to reactants and products.

Continuum states in photoionisation may be dressed by laser coupling to bound states. The authors show that the photoexcitation of such dressed continuum states can lead to the trapping of part of the population in a coherent superposition state and the diminution of total ion yields.

Reports the distribution of final Rydberg states which are populated through charge transfer in collisions of Na⁺ ions at 1000 eV with Na Rydberg atoms laser excited to the 25 s state. The projectile speed is equal to the RMS orbital speed of the 25 s electron. The crossed-beam experiment uses field ionisation for the detection and analysis of the neutralised 1000 eV projectiles which emerge from the collision region. Charge transfer carries the active electron into a band of final states which is centred at n=27 and has Delta n=21 full width at half maximum. The final-state distribution represents a range of binding energy from 8 to 34 meV. Near-resonant enhancement of charge transfer cross sections is thus confirmed in the realm of highly excited atoms.

The lambda and chi parameters for He(3¹P) excitation by electron impact at 81.2 eV have been measured for scattering angles in the range from 60 to 120 degrees . The values of lambda show two deep minima and the values for chi show a broad maximum value, similar to previous 2¹P measurements. The results are not in agreement with values calculated from the first Born approximation, a distorted-wave polarised-orbital method, a multichannel eikonal method and a first-order, many-body approximation.

Vibrational excitation of BF₃ has been studied in an experiment with crossed electron and molecular beams from 1 to 20 eV. The differential inelastic cross sections are dominated by broad maxima which are attributed to shape resonances with a₂" and e' symmetries and which have their counterparts in the boron and fluorine K-shell photo-absorption spectra of the molecule.

As the radius of the hard-sphere core of the Langevin interaction (1905) is increased from zero the mobility rises slightly to a maximum and the momentum transfer rate coefficient falls slightly to a minimum. These unexpected characteristics are explained in simple terms. Support is provided for the common assumption that it is quite a good approximation to take the momentum transfer cross section to be half the cross section for orbiting collisions.

Asymptotically precise calculations have been carried out with n to infinity , for the Rydberg states 4f¹³6sns of the Tm atom, the 4f¹⁴6sns, 4f¹⁴6snp states of the Yb atom and the Lu⁺ ion, the 4f¹⁴ns, 4f¹⁴np, 4f¹⁴nd states of the Yb⁺ and Lu²⁺ ions and the 4f¹⁴6s²ns, 4f¹⁴6s²np, 4f¹⁴6s²nd states of the Lu atom. The Lu atom and Yb⁺ and Lu²⁺ ions are considered as a system with one electron above the core of the closed shells 4f¹⁴6s² or 4f¹⁴. The Yb atom and Lu⁺ ion are regarded as a system with two electrons above the core. The Tm atom is considered as a system with 4f^-1 vacancy and the 6s, ns electrons above the 4f¹⁴ core. In calculating energies with one electron above the core the energy is extrapolated as a function of the principal quantum number n by the model potential technique. The potential constructed at this stage is used as a zero-order approximation in calculating states with two and three quasiparticles (vacancies and electrons) above the core. The corrections for the energies of the states of two- and three-quasiparticle systems are calculated with the use of the perturbation theory. The first order has been allowed for completely. In the higher orders effective allowances have been made for the corrections which with n to infinity make contributions of approximately 1/n³. Consideration is given to the non-Coulomb features of Rydberg spectra not vanishing within n to infinity .

Isotope shifts between the stable isotopes of calcium have been measured in the transition 4s²¹S₀-4s5s ¹S₀ by the method of two-photon laser spectroscopy. The relative positions of the isotopes are (in MHz): ⁴⁰Ca, 0.00(27); ⁴²Ca, 606.33(7); ⁴³Ca, 906.32(5); ⁴⁴Ca, 1169.99(4); ⁴⁶Ca, 1704.33(70); ⁴⁸Ca, 2193.49(5).

Recently published measurements of isotope shifts in samarium have shown unusual effects in lines connecting to two closely-spaced levels, giving evidence that there is appreciable mixing in these levels caused by the operators responsible for isotope shifts. In the present paper a theoretical treatment of this effect is given, based on diagonalisation of the Hamiltonian matrix describing the two levels. Application of the theory to the experimental data leads to values of the off-diagonal matrix elements and shows that the nuclear field effect is primarily responsible for the mixing. Measurements for an additional even-even isotope pair would test the validity of the analysis. A second-order treatment of the effect is briefly discussed.

High-resolution spectra of chromium in the wavelength interval from 2.18 to 2.24 AA have been obtained from TFR plasmas using a bent-crystal spectrometer. The authors observe the characteristic resonance, forbidden and intercombination lines of the helium-like Cr XXIII ion together with 1s2s^q2p^m-1s²2s^q2p^m-1 transitions from Cr XXII down to Cr XIX. All observed features are identified as the most intense lines obtained from ab initio calculations. Electron temperatures and ion abundances are derived from relative line intensities.

The argon 2s photoelectron satellite spectrum has been calculated for high incident photon energy using correlated final ion state wavefunctions determined with a configuration interaction technique. The contributions to the lower kinetic energy satellite structure, which are predicted to amount to 10.3% of the main line intensity, correspond to the population of (2s3p)np and (2s3s)ns final states. Satellites with a total intensity of 1.4% relative to the main line are also predicted to occur at higher kinetic energy, corresponding to the population of mainly (2p3p)nd and (2p3s)np final states. The theoretical findings are compared with the available experimental data.

For pt.I see ibid., vol.14, p.4171 (1981). Semiclassical and quantal time-dependent results are presented for a model of N identical two-level atoms occupying the same site (the Dicke model) and driven by a CW off-resonance laser field. The exact semiclassical time-dependent solutions are derived from an analysis of the Fokker-Planck equation for the system in the atomic coherent states representation. Steady-state behaviour predicted by these solutions is in agreement with that obtained from the exact quantal steady-state analysis in the thermodynamic limit N to infinity . The semiclassical theory predicts (in the steady state) one type of fluorescent spectrum, which is a single sharp line ( delta function) centered at the driving field frequency. This is like the case at exact resonance and below threshold. Quantum mechanically, analytical results are obtained in the strong-field limit and within the secular approximation of the master equation: the fluorescence spectrum is the usual dynamical Stark triplet, apart from a cooperative factor N². The absorption spectrum and the second-order intensity autocorrelation function are also calculated. Numerical results are also presented for finite N(N<or approximately=5). The effect of detuning on the additional sidebands (occurring at higher harmonics) in both the fluorescent and adsorption spectrum are examined.

The authors report a measurement of the photoelectron angular distribution in non-resonant multiphoton ionisation of atomic sodium. Linearly polarised Nd:YAG laser radiation was used to ionise ground (3²S₁2/) state sodium atoms in a non-resonant five-photon process. The angular distribution observed contains significant contributions of trigonometric functions of the type cos(2mx) up to m=5. The absolute ionisation cross section is compared with previous measurements.

Semiempirical pseudopotentials which are determined by fitting to experimental term values of Mg⁺, Ca⁺, Sr⁺, Ba⁺ and Ar⁷⁺, are tested for ground and various excited states of the corresponding neutral atoms, using a spin-density functional approximation for valence correlation energies. Experimental triplet excitation energies are reproduced with an average error of 0.1 eV. The method is then applied to study adiabatic level crossing between ground and excited states in X-Ar complexes (X=Mg, Ca, Sr, Ba). Estimates of deactivation energies are given and are compared with results from deactivation cross section measurements.

A perturbative treatment in the adiabatic representation, which considers the Coriolis transition, and connects to the impact parameter Born theory at large impact parameters, is formulated and applied to the rotational excitation of the HDO molecule by ion collisions. By this method, the collision dynamics at large impact parameters are described much better than by the impact parameter Born theory. At very low energies, however, the collision dynamics are better described by neglecting the Coriolic transition.

High-charge low-velocity neon recoil ions have been produced by impact of 1.4 MeV amu^-1 uranium ions. Electron capture from surrounding neutral background atoms in subsequent collisions has been studied by observing the quenching of K alpha X-ray lines from metastable states. Capture rate coefficients have been determined for the systems Ne⁸⁺ on He, Ne, Ar, CH₄ and Xe.

Excitation cross sections of Si⁹⁺ by electron impact have been computed, for comparison, in different approximations and in different target descriptions using a computer package developed at University College London. Collision strengths have been calculated for fine-structure transitions between the ground configuration term 2s²2p ²P⁰ and the excited configurations 2s2p²⁴P, ²D, ²S, ²P and 2p³⁴S degrees , ²D degrees , ²P degrees of Si⁹⁺ using the eight-state close-coupling approximation and distorted-wave approximation for the energy range 6.0 to 70.0 Ryd. Configuration interaction and spin-orbit interaction within the configurations 2s2p², 2p³, 2s2p3d, 2p²3d, 2s3d² and 2s²3d have been taken into account. Wavefunctions for the target states and hence the oscillator strengths are calculated by using the multiconfiguration Hartree-Fock approximation with the four orbitals 1s, 2s, 2p, 3d and the above mentioned three odd and four even configurations. Collision strengths have also been calculated by a five-state close-coupling approximation between the configurations 2s²2P ²P degrees and 2s2p²⁴P, ²D, ²S, ²P allowing configuration interaction between 2s²P ²P degrees , and 2p³²P degrees configuration and spin-orbit interaction within the 2s2p² configuration. The authors' results indicate that the use of an elaborate target description and a more accurate treatment of the collision problem may change some cross sections by more than 25%. The results are compared with earlier work. This problem has important applications in plasma diagnostics and in the interpretation of the extreme ultraviolet solar emission spectrum.

Energy levels of the krypton atom in the range 23-28 eV have been observed using low-energy electron impact excitation and ejected-electron spectroscopy. The levels observed belong to the configurations 4s4p⁶ns, 4s4p⁶np, 4s4p⁶nd and 4s²4p⁴nln'l'. Those of the former kind have been identified but an analysis of the doubly excited levels has not been possible. The ¹S and ³S terms of the 4s4p⁶5s configuration have been studied in detail in order to determine their energies, their widths and the behaviour of their shape parameters as a function of incident energy, angle of ejection and final ion state.

The scattering amplitude for the scattering of e^- by the mu ^-p ground state is analysed by carrying out an expansion in powers of m_e/m_mu . The analysis includes corrections due to the finite mass of the proton. The first two terms in the series provide the bounds -1.718*10^-13 cm<or=a<or=-1.677*10^-13 cm for the scattering length, with corrections of order m_e/m_mu . The dependence of the differential cross section on the scattering angle, though small, is not insignificant.

An integral equation approach to the second-order potential method is used to study the elastic scattering of positrons by hydrogen and helium atoms. The contributions from the principal value parts of the kernels, although small for incident energy values above 100 eV, are found to be important at lower energy. The cross sections for positron-hydrogen and positron-helium scattering for incident energy ranges of 20-200 and 50-300 eV, respectively, are calculated and compared with those available from other methods. The present values are found to be in general agreement with the results of close-coupling calculations.

The continuous emission of a wall-stabilised argon arc at atmospheric pressure was investigated at the wavelengths 255-700 nm. The electron density (7*10²²-1.7*10²³ m^-3) was obtained from laser interferometric measurements of the plasma refractivities at 632.8 and 1152.3 nm. Continuum xi factors were determined with a probable error of +or-8.5% on average. Agreement to within 15% was found up to 460 nm between the present experimental continuum data and published theoretical and experimental results. At longer wavelengths the experimental xi factors are noticeably dependent on temperature and are, in general, larger than theoretical values.