Journal of Physics B: Atomic and Molecular Physics

A new method for the determination of atomic screening parameters is considered. With the aid of this method the values of screening parameters are determined for the ground state and some excited states of He-Ne atoms.

A dressed-atom approach to resonance fluorescence in intense laser fields is presented. Simple and general results are derived which include the now well known predictions concerning two-level atoms but are not restricted to such simple cases. The positions of the various components of the fluorescence and absorption spectra are given by the allowed Bohr frequencies of the total system: atom+laser mode (dressed atom). The master equation, describing spontaneous emission from the dressed atom is solved in the limit of high intensities. Simple expressions, taking into account the effect of cascades, are derived for the widths of the components.

Values of the van der Waals interaction between the 2 ¹S and 2 ³S metastable states of helium and the systems Ne, Ar, Kr, Xe, H₂, N₂, O₂, CH₄, Li, Na, K, Rb, Cs and O are computed. The values are used to determine Penning ionization probabilities from the experimental data with results differing significantly from an earlier analysis. Estimates are made of Penning ionization cross sections for He (2 ³S) in the alkali metals, in atomic oxygen and in methane, for which no experimental data exist.

A formal solution is presented for an atom which is simultaneously in static and oscillatory electric fields. This is used to calculate the amplitudes of m-quantum resonances in two- and three-level atoms. These results are compared with experiment for the n=2 manifold in atomic hydrogen.

Calculations are reported on the electron impact excitation of the n¹P (n=2,3,4,5) states of He from the ground state. The incident energy range covered is from threshold to 500 eV, and the calculations are carried out in the distorted-wave polarized-orbital model. Total cross sections for all n<or=5, total differential cross sections differential cross sections for individual magnetic sub-levels, and orientation and alignment parameters for n=2,3 are presented and compared with other theoretical studies and with experiment. Excitation of He (2³P) is briefly discussed.

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.

The frequency dependent dipole polarizability of the ground ⁴S state of atomic nitrogen has been calculated using the R-matrix method. The effect on the static polarizability of improving the atomic state wavefunction through configuration interaction is studied. Excellent agreement is obtained with previous theoretical and experimental work, and indicates that the polarizability of the ⁴S state is now known to within 10%.

A multiconfiguration expansion in view of an improved angular representation of complex atoms is explored by employing two types of radial functions. When applied to C⁺² the calculated energy separation is much improved compared to the more traditional multiconfiguration method restricted to one complex. Resulting transition probabilities are closer to experimental values then those of older calculations.

It is shown that the Coulomb T matrix satisfies a dispersion relation off-shell. In the half on shell case (k²=k₁² not=k₂² or k²=k₂² not=k₁²) this provides an unambiguous definition for the imaginary part in terms of a generalized function. A subtractive procedure is introduced to extend these results to the real part.

The authors have performed molecular-state close-coupling calculations of charge transfer cross sections for collisions of the fully-stripped ions Li³⁺, Be⁴⁺ and B⁵⁺ with H(1s) atoms at low to intermediate projectile energies (1 to 15 keV amu^-1). Translation factor effects were included using molecular-state switching functions as in previous studies. Calculations with basis sets of different sizes were done for each system to provide a rough estimate of convergence. The results are compared with other theoretical calculations and with available experiments.

Non-relativistic rates for the decay of 2s hydrogen atoms to the ground state by single-photon and two-photon emission in the presence of a homogeneous magnetic field of arbitrary strength (0<or=B<or=4.7*10⁶ T) are calculated by variational procedures. Over the whole range of B, two-photon emission is the dominant process. As the magnetic field grows, the two-photon decay rate increases. It is found that the Markov approximation can be applied to the two-photon decay for magnetic fields of strength B>or=4.7*10³ T.

Differential cross sections for the one-photon emission free-free process are measured as a function of laser intensity when 10.55 eV electrons are scattered from argon atoms in the presence of a pulsed CO₂ laser. The cross sections are reported for both single longitudinal-mode and multimode laser pulses up to an intensity of 2*10⁷ W cm^-2. For intensities less than 2*10⁶ W cm^-2, the cross sections for both laser pulses are found to be linear with intensity, in agreement with perturbation theory. At higher intensities comparisons are made with predictions based on the low-frequency approximation and two laser models. The linear parts of the cross sections are used to obtain estimates of the spatial inhomogeneities in the electron-laser interaction region. Reasonable agreement is found between the experimental cross sections and those predicted by the two laser models.

Stark widths and shifts of the Ar I 696.5 nm and of the Ar II 480.6, 484.7 and 434.8 nm lines have been measured in the range of electron density and temperature 0.6-1.5*10¹⁸ cm^-3, 16200-18700 K. These high-density plasmas are created in linear flash-tubes. The plasma parameters are principally determined by measurements of the continuum radiation, from the intensity of optically thick lines in their centre and by the condition of local thermodynamic equilibrium of the plasma. The electron density and temperature radial profiles so deduced are found to be practically flat over more than half of the tube radius. In this quasi-stationary stage of the plasma, the experimental line profiles are recorded by an optical multichannel analyser coupled with a high-dispersion spectrometer. The profiles are analysed and fitted to a Lorentzian function. The Stark parameters, width and shift, show a non-linear dependence against the electron density.

The following applications of spectral line broadening theory to astrophysics are briefly reviewed: (i) understanding qualitative effects visible on spectrograms; (ii) quantitative understanding of hydrogen-line profiles for the determination of stellar atmospheric parameters; (iii) effects of line broadening on the determination of stellar chemical composition.