Table of contents

The charge-state distribution of slow recoil Ar ions has been measured by 1.05 MeV amu^-1 Ar^q+ (4<or=q<or=14) ion impact. The ionisation probabilities of target Ar atoms deduced from the measured relative distributions of the charge state are found to be nearly constant for q<or=8, while they increase drastically for q>or=9 where the projectile Ar ions have L-shell vacancies. This enhancement is believed to be due to the direct transfer of target L-shell electrons into the projectile L-shell vacancies.

Total electron capture cross sections by C⁴⁺(1s²), N⁵⁺(1s²) and O⁶⁺(1s²) from atomic hydrogen are calculated in the molecular approximation. The problem is treated in a one-electron approximation by using a model potential approach. Electronic momentum transfer is accounted for through the use of the common electron translation factor proposed by Errea et al. (1982). Comparison is made with experiment in the energy range 0.25-25 keV amu^-1.

The matrix elements of the operator d/dR between the first three ¹ Sigma ⁺ singlets of HeNe²⁺ have been calculated by the rigorous finite differences technique and by Levy's projection method, based on the determination of the effective Hamiltonian in a quasi-diabatic basis. A numerical diabatisation procedure based on the knowledge of the d/dR matrix elements has also been applied. The results of the two methods for determining non-adiabatic couplings and quasidiabatic states have been compared. The nature of the avoided crossing between the A and B ¹ Sigma ⁺ states of HeNe²⁺ is discussed.

In order to assess the relative importance of the relativistic effects, core polarisation and relaxation in ionisation potentials for Rb through Mo⁵⁺ and Cs through Pr⁴⁺ the single-configuration relativistic Hartree-Fock ionisation potentials have been computed in the 'frozen-core' and 'relaxed-core' approximations with and without allowance for core polarisation. The relativistic effects are studied by comparing the non-relativistic and relativistic Hartree-Fock data, and polarisation of the core by the valence electron is included by introducing a polarisation potential in the one-electron Hamiltonian of the valence electron. The core polarisation potential depends on two parameters, the static dipole polarisability of the core alpha and the cut-off radius r₀, which are chosen independently of the ionisation potential data. Both relativistic and core polarisation effects have essential influence on the ionisation potentials of s, d and f states, whereas the core relaxation is important for the d and f states only. By including relativistic effects, core polarisation and relaxation in the single-configuration Hartree-Fock ionisation potentials it is possible to achieve an average agreement with experiment of around 1-2%.

The hyperfine structure of caesium and francium levels has been calculated using the relativistic Hartree-Fock (RHF) method and with the correlations being taken into account by means of the many-body perturbation theory. The hyperfine interaction has been included in the Hartree-Fock equations. The effect of the finite size of the nucleus has been considered. For the s and p_1/2 states, the agreement of the authors' calculations with experiment is not worse than 3%. Using the experimental data on hyperfine structure the magnetic moments of isotopes ^208-213Fr are obtained.

The autoionisation widths of the core-excited 1s(2s2p³P)²P degrees and the 1s2p²²D states in Be II have been measured by means of optical emission spectroscopy, yielding widths of 4.58+or-0.13 and 30.3+or-1.1 meV, respectively. The results are compared with theoretical values and with the recently published experimental values for the isoelectronic states in Li I.

The photoionisation cross section for a hydrogen atom placed in a uniform electric field is calculated using separation of the variables in parabolic coordinates and the semiclassical approximation with account for the tunnelling and reflection above the top of the potential barrier. The corrections to the semiclassical approximation are established accounting for the peculiarities of the effective potential barrier form. The approximate separation of the centrifugal terms contribution in the phase barrier form. The approximate separation of the centrifugal terms contribution in the phase integrals allows the latter to be expressed via hypergeometric functions. The structure in the energy dependence of the cross section is associated with its complex poles. The motion of the poles under variation of the photon energy is investigated. The equations defining the resonance positions and widths are obtained and analysed for energies below and above the potential barrier. The boundary between these two regions is described. The analytical expressions for the parameters of resonances lying above the barrier are obtained for the first time. In the vicinity of the resonance the cross section can be well parameterised by Fano's formula. The approximate expressions for the profile index are deduced. The analytical results are compared with numerical calculation data. The recent experiments on the photoionisation of rubidium and sodium atoms are discussed.

For pt.I see ibid., vol.17, no.10, p.1981 (1984). The photoionisation cross section for a hydrogen atom placed in a uniform electric field is investigated as a function of the light frequency, Watson transformation allows one to represent the partial cross sections sum (i.e. the total cross section) as a sum of contributions of two series of complex cross section poles and the background term. Analytical formulae are obtained describing the cross section structure in various regions of photon energy and fields strength. The contribution of one of the series of poles is related qualitatively with the population of resonance states in the superposition of Coulomb and uniform fields (these states exist even for photon energies exceeding the zero-field atomic ionisation potential). The Fano parametrisation of resonance peaks is generalised for the overlapping resonance case. The second series of poles generates cross section oscillations corresponding to interference of photoelectrons ejected from the atom along the field axis with electrons moving in the same direction after reflection from the potential barrier bounding the classically accessible region. The interference structure proves to be suppressed compared with the short-range potential case where this mechanism of structure formation is unique. If the photon energy is less than 4 pi ²* (ionisation potential) then the resonance structure is significantly larger than the interference one in amplitude and differs in phase. When the photon energy is close to the ionisation potential the resonance peaks are strongly asymmetrical. A comparison is made with experiments where the structure is observed in the photoionisation of sodium and rubidium atoms.

Photoionisation cross sections of the ground state and oscillator strengths for the 3s-np transitions of several numbers of the sodium isoelectronic sequence are presented. The need for careful consideration of the effects of polarisation in these ions is demonstrated by the use of two different approximations. In particular, semiempirical corrections to the dipole matrix element are shown to be unreliable.

The paper is concerned with the results of measurements of three- and five-photon one-electron ionisation probabilities of the barium atom in a wide range of variation of the radiation frequency. Multiphoton ionisation cross sections alpha ₃ approximately=10^-75 and alpha ₅ approximately=10^-141 are measured in the inter-resonance intervals with linear polarisation of the radiation. The variation of the ionisation probability with the radiation polarisation is discussed.

Argon under pressures ranging from 0.5-8 bar was excited by an intense relativistic electron beam. The fluorescence of the Ar₂* excimer radiation at 126 nm and of the Ar*(4p) to Ar*(4s) transitions in the visible as well as the transient absorption between 200 and 600 nm have been studied. The analysis of the results leads to a kinetic model which accounts for all observed features. For high excitation densities Ar₂*(1_u, O_mu ^-) is determined to be the main absorber in the near UV. Cross sections for photoabsorption in Ar₂*(1_u,O_u^-) are given. The so-called 'background' absorption is identified to be caused mainly by photoionisation of Ar*(4p) and photodissociation of vibrationally excited Ar₂⁺.

The fourfold differential cross section of electron-electron bremsstrahlung from bound electrons is calculated by means of the impulse approximation. The atomic electrons are described by Hartree-Fock wavefunctions. Numerical results are given for parameters used in coincidence experiments.

The cross section for the ionisation process e+Fe⁺ to 2e+Fe²⁺ has been measured from below threshold up to an electron energy of 750 eV and at energies greater than 25 eV the overall accuracy is estimated to be +or-4.3%. The crossed electron-ion beams technique was used and there is evidence that the target ion beam contained a substantial component of metastable ⁴D and other long-lived excited Fe⁺ ions in addition to the ground-state ⁶D ions, but the ionisation data are likely to be directly relevant to Fe⁺ ion impurities sputtered from the walls of fusion devices. However, it is probable that the measured cross section is insensitive to the presence of these excited ions, at least at electron energies above about 30 eV. Comparison with theoretically predicted cross sections shows that the McGuire approach (1977) is in fair agreement with the experiment but that the parameters proposed by Lotz (1969) for his formulation do not yield a satisfactory fit to the measured data.

The design of an electron impact spectrometer for use in inner-shell studies of atoms and molecules is described. The use of the spectrometer to investigate inner-shell excited states of CO is reported. These states include the lowest lying valence states, (1s sigma _C)^-1(2p pi )¹ Pi and ³ Pi , Rydberg states below the 1s_C ionisation potential and doubly excited states above the ionisation potential.

For pt.I see Chem. Phys. Lett. vol.96, p.464 (1983). The non-linear multiphoton excitation dynamics of two-level systems under the influence of two strong linearly polarised monochromatic fields is studied exactly for the first time along the semiclassical many-mode Floquet theory. The present approach relaxes the restrictions of the conventional generalised rotating-wave approximation (GRWA) and allows correct treatment of various non-linear features such as power broadening, resonance shift, asymmetry in absorption lineshape, etc. By extending the nearly degenerate perturbative method of Salwen (1955), approximate analytical expressions for transition probabilities are obtained for multiphoton resonance processes for which (n+1) photons of one field are absorbed, whereas n photons of the other field are emitted, where n is an arbitrary integer. Detailed comparison of the analytical, the GRWA and the numerically exact results is given. Also presented is a case study of the time evolution of spin ¹/₂ systems in multi-quantum NMR transitions driven by pulsed bichromatic radiofrequency fields.

The author formulates the problem of the steady state for a laser-cooled trapped ion using a generalisation of the method of moments. The problem is solved using a perturbation expansion and he obtains a steady-state solution which is exact in the Lamb-Dicke limit. The stead-state energy is compared with earlier results. The populations of the occupation number eigenstates of the harmonic trap are also obtained.

The authors obtain values of chi ⁽³⁾ up to 5*10^-7 esu for non-degenerate four-wave mixing near the band edge in InSb using CW, low-power, CO laser sources. The results of a new calculation of the resonant bound-electron contribution to chi ⁽³⁾( omega ₁, - omega ₂, omega ₁) are presented. Both the magnitude and frequency dependence are in agreement with experiment, implying the dominance of the bound-electron contribution under the present conditions. This is in direct contrast with the implications of non-resonant mixing at CO₂ frequencies.

Polarisation spectroscopy (PS) experiments were performed on 1,3'-diethyl 1-2,2-quinolythiacarbocyanine iodide (DQCI) to study the contribution of the thermal grating effect to the PS signal. This contribution can be significantly reduced.