Table of contents

A quantum-mechanical master equation approach to the spontaneous emission from a two-level atom in the presence of a strong pump field is reported. Access is available to both a one-photon treatment and a treatment incorporating photon cascades. A peak height ratio of 3:1 for the complete many photon approach is reduced to 2:1 in a one-photon approximation. Broadening of the sidebands is also lost through this approximation.

The excitation of (3p)⁵4s and (3p)⁵4p levels of Ar is investigated by use of the Born approximation and a four-state calculation including ¹S₀(3p)⁶, ¹S₀(3p)⁵4p and P₁(3p)⁵4s (M=0,1). HF-type wavefunctions are used. The spin-orbit coupling is treated in a rather semiempirical way.

A general formula for the cross section for potential scattering in the presence of a strong electromagnetic field is derived using the eikonal approximation for a force centre.

For pt.VI see abstr. A3208 of 1975. A revised phase shift analysis of elastic electron-mercury scattering in the energy range below the inelastic threshold, using new polarization and differential cross section measurements of Duweke et al. (1974) with recently recalibrated energies, is reported. It also includes momentum transfer cross section data of Rockwood (1973). A good fit was found to all the data, with the exception of total cross sections which appear to be in error. The computed phase shifts remain in reasonable agreement with the relativistic, polarized orbital calculation of Walker (1970), at 3.5 eV, and confirm the suggestion of broad p-wave resonances around 0.7 eV.

Measurements have been made of low energy elastic scattering from magnesium using the electron transmission method. A large shape resonance has been observed at 0.15 eV which the authors identify with the (3s²3p)²P state of Mg^-. The results are compared with two close coupling calculations of the scattering cross section and a model potential calculation for the shape resonance.

Differential and integral electron impact cross sections for elastic scattering and for excitation of the first four electronic states of bismuth have been determined at 40 eV impact energy. The cross sections were normalized to the absolute scale by using the optical f-value of the 6p²7s⁴P_1/2 transition. The integral cross sections for elastic scattering (6p³⁴S_3/2) and for the excitation of the 6p³²D_3/2, ²D_5/2, ²P_1/2 and 6p²7s⁴P_1/2 states are: 45.0, 0.29, 0.46, 0.17 and 3.8*10^-16 cm², respectively. Inelastic scattering from Bi₂ molecules was also detected.

Eleven new bound electronic states within 12 eV of the ground state of N₂⁺ have been identified from detailed configuration interaction calculations. Certain of the new states offer explanations for the recent photoabsorption experiments in N₂.

Previously unsuspected pitfalls in the application of a variational procedure first used by Hylleraas (1930) are analysed. Numerical results for some model problems suggest that very strict convergence criteria must be imposed on the variational functional if spurious results are to be avoided.

The cross section for multiphoton ionization of hydrogen is computed using the momentum translation method (MTA) in the cases of linearly and circularly polarized light, alternatively. Extended numerical comparison is made with well-established results obtained from the Nth-order time-dependent perturbation theory in the weak intensity limit. The MTA does not reproduce the values given by perturbation theory and considerable differences are observed. Higher order iterations than the lower one are introduced and an analysis of the leading terms of the development is made: at weak intensities the zeroth-order term that defines the MTA cancels with higher order terms and the final result obtained after making (N-1 iterations is none other than the well-known transition amplitude deduced from standard perturbation theory, defined by the remaining terms of the last iteration after mutual cancellation with lower-order ones. According to the results obtained in this work, it is not possible to affirm that the MTA is adequate to calculate atomic multiphoton ionization cross sections.

A Z-expansion method is used to calculate the energies (including first-order relativistic corrections) and probabilities of 1s²2s²S_1/2-1s2s2p⁴P_5/2 and 1s²2s²¹S₀-1s²2s2p³P₂ magnetic quadrupole transitions in the lithium and beryllium sequences respectively. It is found that, compared to spin-spin autoionization to the 1s²kf²F_5/2 continuum level, the magnetic quadrupole decay of 1s2s2p⁴P_5/2 to the ground state becomes increasingly significant as the nuclear charge increases. Probabilities for the ¹S₀-³P₂ transition are found to be small.

An experiment was performed to measure the shape of the sodium resonance doublet ( lambda 5890, lambda 5896) in a high-temperature helium atmosphere. A piston compressor was used to produce temperatures in the range 3000K to 5500K. Absorption line widths and shifts are reported and the widths agree well with calculations.

The atomic form factor and the incoherent scattering function of Ar calculated from the Bethe-Goldstone method are presented. The results include electron correlation effects in the M shell as well as those between the L and M shells. The form factor calculated from the Hartree-Fock wavefunction agree within 2% with the result from the Bethe-Goldstone method for a wide range of momentum transfer. However, the correlation effects reduce the incoherent scattering function for small momentum transfers (<or approximately=1 au) by about 20% from the Hartree-Fock values.

Nutational motion of aligned mercury atoms in the metastable 6³P₂ state has been observed by use of a RF pulse technique. The comparison with theoretical calculations gave good agreement.

The most important experimental results concerning the 2s²¹S, 2p²¹D and 2s2p¹P resonances of the helium atom, as observed in energy spectra of ejected electrons, are given. Characteristic parameters are obtained numerically with Shore's formula (1969). The shape of the resonance is studied with Shore's parameter a, in the angular range 16.5 degrees to 160 degrees and for 33 to 150 keV proton energies; additional results concerning only the parameter q have been obtained at 10 degrees ejection angle, down to 26 keV proton energy. The intensity of the resonances is also measured in the same angular and energy ranges, and is characterized by Shore's parameter b. The results are compared with other experimental or theoretical works by means of the annulment diagram for the parameter a. This method permits the authors to synthesize all the experimental data actually available up to 500 keV.

It is argued that, in a process involving ejection of an inner shell electron by an unpolarized beam of photons and subsequent atomic reorganization via Auger ejection, the two ejected electrons can be strongly correlated in direction. In general, the detailed form of the correlation will depend on both photoionization and Auger transition amplitudes. However, for the important special case where the Auger electron can be represented by a single partial wave, the form of the correlation depends only on parameters governing the photoprocess. For such a case, Auger asymmetry parameters beta ( theta ), where theta denotes the polar angle of the photoejected electron, are predicted using theoretical data of Burgess (1964) and McGuire (1970). It is shown that beta ( theta ) may attain large values and be sensitive to the theoretical parameters. Hence a coincidence experiment designed to measure beta ( theta ) is proposed.

This paper reports the computation of total and differential cross sections for the reaction in the range E=50-250 eV_cm. It is found that the dominant reaction is production of He(2¹P). The total cross sections for production of He(2¹S) are, at energies above 100 eV_cm, approximately 10% the magnitude of those for He(2¹P). The calculated differential cross sections compare well with the experimental data of Morgenstern et al. (1973). The calculations were carried out by generating ab initio wavefunctions and potential curves for the x¹ Sigma _g⁺, B¹ Pi _g and C¹ Sigma _g⁺ molecular states of He₂, computing the L_y (rotational) coupling matrix elements between this set of molecular states and solving the resulting three-state quantum close-coupling problem.

Polarization measurements have been performed on the K(4²P_3/2 to 4²S_1/2) line emitted after collisions between K and He, Ne, Ar, Kr and Xe. In all cases polarization is found in the studied lab energy range from 50-1500 eV; at low energies the polarization fraction is strongly dependent on collision energy. It is shown that the shape of the polarization function depends mainly on the relative population of the molecular states A² Pi and B² Sigma correlating with K(4²P_3/2)+rare gas (¹S₀).

Some formulations of the theory of coincidence experiments have been reformulated in terms of density matrices, using the helicity representation, and applied to recent experiments.

A local, central potential is used to describe elastic scattering of charged particles by atoms. This accounts for second-order direct and first-order exchange effects, the only parameter involved being the target dipole polarizability. Detailed values of differential, integrated elastic, and total cross sections are presented for electron-hydrogen scattering (50<or=E<or=200 eV) and electron-helium scattering (100 eV<or=E<or=1 keV). Values of integrated elastic and total cross sections for positron scattering by hydrogen and helium are also given.

A consistent and efficient variational procedure is described and applied to the calculation of s-wave, e^--H elastic scattering phase shifts.

The theoretical investigation carried out by Arthurs and Moiseiwitsch (1958) has been extended to higher energies and to atoms with greater nuclear charge Z. Allowance for the difference between the theoretical and experimental ionization energies arising from the potential field of the electrons in the outer shells is made using a similar procedure to that due to Perlman (1960). The authors have calculated K-shell ionization cross sections as functions of the electron impact energy for C, Al, Ni, Cu, Ag, Sn, Au and compared their theoretical curves with the available experimental data, the accordance being somewhat variable.

For the singlet levels of (2p)⁵3s and (2p)⁵3p of the Ne atom wavefunctions of Hartree-Fock type are constructed. The Born calculations yield the new Bethe parameter b_n=0.050 for the (2p)⁵3p states. A four-state impact parameter calculation is carried out including the states ¹S(2p)⁶, ¹S(2p)⁵3p and ¹P(2p)⁵3s (M=0,1). With this relatively simple model all known experimental data for the excitation of (2p)⁵3s and (2p)⁵3p by proton impact can be explained satisfactorily.

Complete valence shell CI calculations have been carried out for a large number of non-Rydberg states of N₂ molecule and related ions. Full potential curves are presented for these states, and are used for a calculation of spectroscopic properties. Comparison is made with experimental data where such are available, and several new states and their properties are predicted. Some of these are of great interest in connection with the observation of unidentified states in recent experiments.

The vacuum ultraviolet fluorescence from fragments of N₂O has been studied and the fluorescence production cross sections have been measured. The fragments are produced by extreme ultraviolet excitation using synchrotron radiation in the wavelength region lambda lambda 175-750 AA. Structure appearing in the lambda lambda 620-680 AA region has been identified as the absorption Rydberg series VI and VII. Two broad fluorescence bands were observed with peak cross sections of 0.6 Mb at lambda 370 AA and 0.4 Mb at lambda 230 AA. The photodissociation and dissociative ionization processes that are probably responsible for the observed fluorescence are discussed.

The contribution to the two-body dispersive energy from third-order perturbation terms can be expressed as a multipole series in inverse powers of the interatomic separation R. Using the Unsold (1927) approximation, a general expression for the coefficients of this series is derived for the case of the interaction between two spherical atoms each of whose wavefunctions can be expressed as a single determinant of one-electron functions. The leading term (which is proportional to R^-11) is evaluated for pairs of rare gas atoms and is found to be small in comparison with the second-order energy.

It has been noted that line radiation-for example, the first member of the Lyman series of C⁵⁺-is optically thick at the surface of a laser-irradiated carbon target. Optical depths of the order of 10⁴ can be anticipated with appropriate focusing geometry. Using such an optically thick and broadened line source it is calculated that population inversion and a measurable super-radiant gain can be achieved by resonance pumping in neighbouring ion species.

Measurements have been made, at gas temperatures of 300K of electron temperature, T_e, relaxation rates (2000K>or approximately=T_e>or approximately=350K) in pulsed flowing afterglow plasmas of pure argon and argon-nitrogen and argon-oxygen mixtures. Momentum transfer cross sections were derived, which are in good agreement with previous data. Values of G, the fraction of energy transferred per collision, were deduced using previously published results for momentum transfer cross sections. These G values show a marked variation with T_e as expected and are interpreted in terms of energy transfer to rotational states of the molecules. The G values over the complete T_e range are less than theoretically predicted in the case of nitrogen, but greater than predicted in the case of oxygen.