Table of contents

The transformation called the space-translation method upon the non-relativistic molecule-radiation Hamiltonian is extended to higher multipole orders and to the situation where the radiation field is part of the dynamical system. Comparison is made with other interaction Hamiltonians for one-photon processes on and off the energy shell.

The matrix element for two-photon absorption by atoms and molecules is obtained from different forms of the interaction Hamiltonian. The use of the minimal coupling, multipolar and space-translated interaction energies are compared and terms arising from higher multipoles are discussed.

A simple Green's function transformation is used to semi-classically describe the atomic wavefunction in very strong electromagnetic fields. It is concluded that a series of 'quasi-stationary' states are formed, which only ionize slowly.

Differential cross sections for multiphoton ionization of atomic hydrogen are calculated for the range of two photons to eight photons. Some possible applications of such information are suggested.

Using 'exact' intermediate-coupling wavefunctions, transition probabilities for 4s²4p⁴5p-4s²4p⁴5s transition in Kr II have been calculated. The radiative lifetimes of the 5p states are obtained and compared with the available experimental data.

Positronium formation fractions have been determined from lifetime experiments for all the noble gases. Taken in conjunction with the positron beam total cross section measurements of Canter et al. (see abstr. A27547 of 1974), it is possible to estimate values for the positronium formation and positron excitation cross sections for helium and, with reduced accuracy, for neon and argon in the energy range from the positronium threshold E₀ to the ionization potential E₁. It is shown that these cross sections are approximately 0.2 pi a₀² at E₁ for helium and somewhat larger for neon and argon.

Differential cross sections for the scattering of fast K atoms from Ar have been measured for CM collision energies between 50 and 200 eV in the angular range 0-9 degrees . Pointwise inversion of this data yielded a potential for the ground state of this system between 0.2 and 5.3 eV.

The use of a two-centre prolate spheroidal coordinate system in ab initio calculations of scattering of slow electrons by N₂ molecules is studied. The speedier convergence of the resulting multipole expansion of the static potential near the nuclei, compared with that of its one-centre form, is demonstrated. Eigenphase sums calculated using two-centre coordinates also converge more rapidly.

The crossed-beam technique has been used to measure differential cross sections for vibrational excitation in e-HCl collisions from threshold (E_th=0.357 eV for v=1) up to 8 eV. The excitation function for the v=1 level shows a sharp peak close to threshold, a second maximum of larger width around 2.5-3 eV and a monotonic decrease above 3 eV. The angular distribution measurements give nearly isotropic scattering for this process indicating the dominant role of a resonance mechanism via the HCl^-2 Sigma ⁺ state in this energy range. The absolute cross section in the threshold peak, integrated over all angles, is estimated to 1.3 * 10^-15 cm² +or- 50%.

A simple perturbation procedure is described, using two non-orthogonal functions chosen so as to automatically diagonalize the operator (H₀-E₀). The method yields good results for the second-order energy contributions E₂(l) to the 1s² ground state energy.

On the basis of the field form of perturbation theory energy levels of atomic systems with numbers of electrons up to 10 are calculated. Configurations 1s²2sⁿ¹2pⁿ² (n₁=0-2, n₂=0-6) are considered. The coefficients Delta E₁, Delta E₂ in the energy expansion by powers of 1/Z E=E₀Z²+ Delta E₁Z+ Delta E₂+ Delta E_3Z/¹... are calculated. Hartree-Fock energies are also represented in the form of a Z-expansion, and the coefficients Delta E₁^HF, Delta E₂^HF, Delta E₃^HF of this expansion are computed. These calculations allow us to represent the correlation energy in the form of a Z-expansion.

Semi-empirical calculations of continuum properties of Li^- and Na^- are presented based upon the construction of an approximate Green's function for each system from a finite set of square-integrable functions. The results for photodetachment from the ground state of Li^- and Na^-, and for radiative decay from the ³P^e state of Na^- are in harmony with calculations employing the Stieltjes imaging procedure and the photodetachment profiles are also in accord with close-coupling studies. The P-wave phase shifts for electron-alkali elastic scattering show good agreement with values from other theoretical calculations.

The results of a level-crossing experiment are reported. The hyperfine constants of the 5 ²P_3/2 state of ⁸⁵Rb are a=24.99(1) and b=25.88(3). These values are compared with those from a previous optical double resonance experiment. When radiative shifts are carefully taken into account in the ODR measurements, very good agreement is found.

Measurements are reported of two-photon ionization probabilities in atomic Cs at the nine wavelengths of the Ar ion laser (5145-4545 AA), the first step of the two-photon process occurring to intermediate levels in the vicinity of the 7 ²P_1/2,3/2 levels. Additional ionization due to molecular Cs was monitored by ion time-of-flight analysis and found to be negligible when choosing proper oven temperatures. An absolute scale for the two-photon rates is provided by normalization on a one-photon UV cross section. Comparison of our data with calculations based on quantum defect Coulomb wavefunctions shows discrepancies of up to four orders of magnitude.

Elastic and 2s, 2p excitation cross sections for electron-hydrogen collisions have been investigated by an approximate form of the Faddeev equations, taking into account the coupling of the 1s, 2s and 2p states of the target atom. Results with and without the inclusion of the exchange effect have been presented and compared with other theoretical findings. Present results neglecting exchange are the same as those obtained by Burke and Seaton (1961). In the case of elastic scattering, the effect of exchange is not negligible even at the incident energy 500 eV. The results for the 1s-2s and 1s-2p excitation cross sections including exchange are in very good agreement with the experimental findings.

Absolute differential cross sections for electrons losing 10.2 eV in exciting the n=2 states of atomic hydrogen have been measured over the angular range 20 to 140 degrees for incident electron energies of 54 to 680 eV. The experimental apparatus, method and techniques are based upon those normally used in crossed electron and modulated atom beams experiments which study electron energy-loss spectra with good angular (2 degrees ) and energy (0.03 eV) resolution of the incident and scattered beams. Absolute cross section values have been determined by a method based on (i) a phase shift analysis of the elastic differential scattering of electrons from resonant states the (1s2s²)²S state in helium and the (3s²3p⁵4s²)²P_3/2 and ²P_1/2 states in argon and (ii) measurement of beam source pressures.

Absolute angular differential cross sections for the elastic scattering of electrons from argon atoms have been measured for incident electron energies from 20 to 400 eV over the angular range 20 degrees to 150 degrees . The experimental method is based on the crossed electron and modulated atom beams techniques with an electron energy resolution of 0.095 eV and an angular resolution of 2 degrees for both the incident and scattered electron beams. The experimental cross section values have been calibrated absolutely by a phaseshift analysis over the (3s²3p⁵4s²) ²P_1/2, ²P_3/2 resonance doublet in argon at 11.0 and 11.27 eV. At 11.18 eV, the measured phaseshifts are delta ₀=5.063+or-0.050 rad, delta ₁=2.533+or-0.020 rad, delta ₂=1.092+or-0.020 rad, delta ₃=0.112+or-0.018 rad and higher phaseshifts were calculated from an effective range expansion. These values are in good agreement with the measured values of Andrick and Bitsch (1975), but the s and d wave phaseshifts of Weingartshofer et al. (1974) are outside the experimental limits.

For pt.I see ibid., vol.8, no.10, p.1641 (1975). Absolute angular differential cross sections for the elastic scattering of electrons from atomic hydrogen have been measured for incident energies from 0.5 to 8.7 eV and for scattering angles between 10 and 150 degrees. The experimental techniques and method were those normally used with crossed electron and modulated atom beams. The angular behaviour of the cross section is consistent with a partial-wave explanation; that is at the lowest energy of 0.5 eV, the angular distribution is peaked in the backward direction because of dominant p-wave exchange scattering; at 3.4 eV there is an angular minimum around 70 degrees because of interference between s, p and increasing d waves and at 8.7 eV the distribution is peaked in the forward direction because of the increasing effect of higher partial waves.

It is shown that the third-order second Born approximation, in which the second Born cross section is evaluated correct to only third order in the interaction potential, gives poor results for electron impact excitation of the 1¹S to 2¹S transition. The full second Born approximation, in which cross sections are calculated by squaring the modulus of the full second Born amplitude, is better but still not quite satisfactory. A corrected simplified second Born approximation is proposed in which couplings between the 1¹S and 2¹S states are given special treatment. The new approximation is in good agreement with experiment except at low energies and large angles.

A ten-channel eikonal treatment of the n=2 and 3 collisional excitations of helium by incident electrons with energy E (eV) in the range 40<or=E<or=500 is performed. Differential and integral inelastic cross sections are obtained, together with theoretical predictions of the ( lambda , chi ) parameters which provide, as functions of scattering angle theta and E, the orientation and alignment vectors and the circular polarization fraction of the radiation emitted from the n¹P levels. The results are in satisfactory agreement with recent measurements.

Lifetime spectra for positrons annihilating in helium gas have been measured at room temperature for gas densities up to 60 amagat. The equilibrium annihilation rate for the free positron component is proportional to the gas density D, in amagats, given by lambda ₁=(0.793+or-0.004 D mu s^-1 which is equivalent to Z_eff=3.94+or-0.02 and is in good agreement with recent calculations. The variation of the equilibrium decay rate with gas density for the ortho-positron component is also linear, given by lambda ₂=7.24+(0.1006+or-0.0006) D mu s^-, which is consistent with pick-off quenching. No other long-lived component has been observed. For neon at room temperature, a shoulder region of width 1700+or-200 ns amagat has been established. Some new measurements for argon, krypton and xenon are also reported. A comparison is made with other recent measurements.

The cross section for K-shell ionization in nickel, silver and gold by electrons of energy between 3 and 30 MeV has been measured by counting the K X-rays produced. The results are compared with a virtual-photon calculation using two different forms for the X-ray photoeffect cross section and also with the theory of Kolbenstvedt (1967).

For the calculation of generalized oscillator strengths of atoms, the use of a one-particle central potential which treats the Coulomb interaction correctly and has the exchange effect as a small correction is advocated. This potential is used to calculate the generalized oscillator strengths for 2s and 3s excitation of the helium atom, and the results are found to be superior to Herman-Skillman calculations (1963).

A model potential method is applied to the calculation of the eigenfunctions and interaction potentials of several valence states of the molecules MgHe⁺ and MgHe. The interaction potentials are used in the impact approximation to compare the line broadening caused by collisions with helium of the ionized and neutral magnesium systems and in the elastic approximation to estimate the cross sections for collision-induced transitions between the fine structure levels.

For pt.III see abstr. A24136 of 1974. The energy distributions of the N² ion produced in the electron impact dissociative ionization of N₂ have been measured for incident electron energies up to 300 eV. Pronounced structure, which was not observed in a previous measurement using 125 eV electrons, is seen in the distributions. From this structure, it is obvious that the N² ions are produced by at least three different mechanisms. The angular distributions of the ions show little anisotropy, but the effect of momentum transfer to the molecule by the incident electron is observed.

Following the work of Bates and McKibbin (1974) the quasi-equilibrium statistical method is applied to ionic recombination in an ambient gas taking specific account of the angular momenta of the ion-ion pairs instead of assuming as usual, that the micro-canonical distribution is preserved. It is found that the elaboration leads to the calculated value of the recombination coefficient being reduced by only around 2%.

Using the light absorption technique, the decay constant for the density of metastable xenon atoms Xe*(³P₂) has been measured in a pure xenon afterglow at pressures between 4*10^-2 and 8 Torr. At a gas temperature at 300K the diffusion coefficients D₀ for the Xe*(³P₂) metastable atoms in the parent gas has been found to be 19+or-2 cm² Torr s^-1, the relaxation cross section sigma of the Xe*(³P₂) atoms in a two-body collision with a xenon atom has been determined to be (7.4+or-1)*10^-20 cm² and the coefficient of a three-body collision of the Xe*(³P₂) atoms with two xenon atoms A₂ was found to be (7.6+or-1)*10^-32 cm⁶ atom^-2 s^-1. At pressures above 4 Torr the authors have observed a repopulation of the metastable level resulting from the electron-Xe₂* molecular ion recombination.

The electron-ion recombination mechanisms of Xe⁺ and Xe₂⁺ ions were studied in pure xenon by means of the stationary afterglow method. The time dependence of the electron density was determined by measuring the shift of the resonant frequency of the cavity containing the plasma. The time variation of the number of atoms created by recombination mechanisms in the various excited levels of xenon was measured by observation of the intensity of the xenon lines emitted in the afterglow. The simultaneous study of the electron density and the 6872 AA line intensity in the afterglow shows that when the electron density is lower than 10⁹ cm^-3, the electron-Xe⁺ recombination is a two-body recombination involving only one electron. An upper limit of 10^-9 cm³ s^-1 has been determined for the recombination coefficient for this type of recombination.