Table of contents

The n¹D-n³D intervals in helium have been measured for n=12 to n=20 using the anticrossing technique. The results are in good agreement with quantum defect calculations and disagree with values derived from earlier optical measurements.

The Glauber impact-parameter approximation for particle-atom collisions has been modified by including a finite mean excitation energy of the atom in its intermediate states.

The cross section ratio for the symmetric (e,2e) reaction on helium leading to the n=2 and ground states of the helium ion has been calculated as a function of the ion recoil momentum q, using a correlated helium wavefunction, and compared with the results of a 1200 eV non-coplanar experiment and some previous results at 800 eV. The calculation agrees well with the measured (e,2e) cross section ratios and at high q with ratios measured in photoelectron spectroscopy experiments.

A generalized Hylleraas trial function, properly antisymmetrized in the electron coordinates, is used to investigate positron attachment to ³S helium. A state lying below the ground state of the ³S system (-4.35 Ryd) and which would also be stable against dissociation into He⁺ and Ps in their ground states (-4.5 Ryd) is searched for (unsuccessfully). When the mass of the positron (and hence the energy of ground-state Ps) is allowed to vary, however, such a stable state is obtained for m<or=0.94; this critical mass is so close to the physical value that further calculations with improved trial functions are indicated.

An approximate correction formula for the Coulomb K-shell ionization cross sections of atoms by heavy charged particles is found. It represents an improvement over an earlier formula for projectile energies of current experimental interest. A correction formula for the ionization probability as a function of impact parameter is also given.

The a³ Pi _r( Omega =1)-x¹ Sigma ⁺ transition of SiS has been obtained in emission. A v" progression (with v' assumed to be 0) of five bands has been observed in this system. The analysis of the 0-6 band yields the rotational constant for the v=0 level of the a³ Pi _r state. The position of the a³ Pi _r( Omega =1) state relative to the ground state ( nu ₀₀) has also been calculated.

The description begins with an operator master equation for the atom plus incident field. Reduced atomic matrix elements are derived for arbitrary field strengths. First- and second-order correlation functions in the scattered field are also obtained and discussed in relation to the scattered spectrum and intensity-fluctuation measurements. This formalism has the appealing feature that all information is readily available from the one set of four coupled equations. The deficiencies in both the one-photon approximation and the semiclassical perspective are established in a natural and transparent fashion.

For pt.I see ibid., vol.8, p.1795 (1975) where the theory of Doppler-free two-photon transitions was discussed for the case of one single laser frequency. Here, the effects of two different frequencies are considered. When the residual Doppler width of the two-photon transition is less than the natural linewidth, the previous results are essentially regained. For large residual Doppler shifts, the absorption profile is a power-broadened Lorentzian if the frequency of the probe field is larger than the frequency of the strong field. In the opposite case, the absorption peak splits because of the AC Stark effect. The high-intensity distortions are minimized when equal frequencies can be used. Power-dependent shifts do not ordinarily occur with a resonantly tuned intermediate level. Some modifications of the spectra are introduced by phase-disturbing collisions.

Splittings in the n²D sequence were measured for n=4-9. An accuracy of about 0.1% was obtained in level-crossing measurements on an atomic beam. The D states were populated by step-wise excitation, using an RF lamp and a CW dye laser. A method for measuring the sign of the spin-orbit coupling constant is demonstrated. With the present measurements, the highly anomalous n²D sequence in sodium has now been studied for n=3-16.

The photon-photon delayed coincidence technique is used to measure the lifetime; the value (6.21+or-0.10) ns is found. The measurements are obtained under conditions where pressure-dependent effects, as well as other known systematic errors, are negligible. A procedure for data analysis which has not previously been used with the present technique is employed. In this procedure, the apparatus response function is obtained directly and the measured time spectrum is deconvoluted using this response function to yield the lifetime.

General expressions are given for the transition amplitudes from which total generalized cross sections, angular distributions and photoelectron spin polarization may be obtained. The effect of light polarization (linear or circular) is taken into account. Numerical results for selected cases of three-photon ionization of three of the alkali atoms, sodium, potassium and caesium are given and compared with available experimental data.

The infinite summations over the complete set of unperturbed atomic states appearing in the Nth-order time-dependent perturbation theory are performed using the Green's function formalism. In order to test the method extended calculations of the cross section for two- and three-photon ionization for atomic hydrogen are made and numerical comparison is shown with the values obtained by different methods. The resonant character of the process is made evident since the poles of the radial part of the Green's function represent the actual energy spectrum of the atom. The relevant transition matrix elements are written in closed form and then computed exactly without averaging over the intermediate states. The cross sections vanish for circularly polarized light near some energies which explains the form of the cross sections for circularly and linearly polarized light.

Cross sections for s, p and d have been computed in the framework of a single-electron model (non-relativistic) by using a parametric central potential. The evolution of the non-hydrogenic behaviour of photoionization cross sections near threshold is studied along Rydberg series; non-hydrogenic characters are particularly large for all the ns series and for the np series of K. The validity of the method is discussed and it appears that sigma sigma _np and sigma _nd values are more reliable than sigma _ns. The results are compared to experimental data and other theoretical results, mainly provided by the quantum-defect theory. The agreement between the various results is generally very good for sigma _np and sigma _nd; the agreement is satisfactory even for the most unfavourable cases provided by sigma _ns of Na and K.

Calculations of Hutton (1972) in the classical-path T- 242T-matrix (CPTM) approximation are compared with the absolute measurements of Williams and Willis (1975) at 100 and 680 eV. Comparison with other theoretical predictions is also presented. For the CPTM model lambda = sigma ₀/ sigma =cos² theta , in contrast with the sin²( theta /2) dependence of the Born approximation. While the CPTM approximation appears to be quite reasonable for high angles, reservations are expressed concerning its consistency.

A number of local potentials have been suggested to represent the non-local exchange kernel in electron-atom scattering. A comparative numerical study has been made in which phase shifts for the elastic scattering of electrons by H, He⁺ and He using these potentials are tested against those calculated from the exchange integro-differential equations. The influence of additional long range polarization potentials is also investigated. It is shown that, except at the lowest energies, exchange effects can be represented accurately by the Furness-McCarthy and related potentials.

The ratio of generalized oscillator strengths for single and double ionization of He, Ne and Ar has been remeasured in the energy-loss range from threshold up to 200 eV, using 8 keV electron impact and time-of-flight ion analysis in coincidence with electrons scattered through zero degrees. The results are in excellent agreement with the recent photoionization (synchrotron) data of Schmidt et al. (1976) and appear to remove the discrepancy which previously existed between photon and electron impact work.

A three-state (4s-4p-3d) close-coupling approximation is applied to the scattering of electrons with energies of up to 5 eV by potassium atoms. Amplitudes for elastic scattering and 4s to 4p excitation have been computed. Good agreement with the behaviour of the elastic differential cross section at fixed angle observed by Eyb and Hofmann (1975) in the immediate vicinity of the first excitation threshold is obtained. The threshold structure is analysed in detail. For elastic scattering below the excitation threshold good agreement is obtained with absolute differential measurements. Above the excitation threshold and in its vicinity however, some differences between the calculations are apparent. Good agreement is obtained except for the case of elastic scattering at 3 eV.

For pt.II see ibid., vol.8, no.11, p.1909 (1975). A law is derived which allows universal curves to be given for excitation via 2p sigma -2p pi coupling in ion-atom collisions. The scaling law is tested by comparison with ab initio calculations using both Hartree-Fock and hydrogenic molecular orbitals, and with experiment.

Emission cross sections and polarizations have been measured for the 2²S-2²P Be II multiplet in collisions between 0.3-70 keV Be⁺ ions and He or Ne, and the 3²S-3²P, 3²P-3²D and 3²P-4²S Mg II multiplet components in collisions between 1-150 keV Mg⁺ ions and the rare gases. The excitation of the projectile resonance multiplet is found to be the dominating inelastic process for all collision systems. The ratio between the cross sections for the fine-structure components in magnesium is constant and equal to the statistical weight ratio 2:1. Cascade effects are shown to be important at higher energies. The results are compared with related experiments by Kempter et al. (1974) and Ovchinnikov et al. (1974). Some characteristic features are in agreement with the predictions of the one-electron model calculations of Nielsen and Dahler (1976).

Cross sections and polarization of emission have been calculated for the excitation of ground-state ions in collision with rare gas atom targets in the energy range E_CM=1-100 keV. Each ion-atom pair has been treated as a quasi-one-electron system, the interaction of the valence electron of the ion with the closed-shell ion core and with the rare gas atom being represented by semi-empirical model potentials. Close-coupling solutions to the impact-parameter equations have been obtained. The calculations for Be⁺ excitation are based upon the three states of the ion valence shell, 2s, 2p₀ and 2p₁, and those for Mg⁺ excitation upon the six states of the ion valence shell, 3s, 3p₀, 3p₁, 3d₀, 3d₁ and 3d₂, and the energetically similar 4s state. Characteristic features of the predicted excitation cross sections and the polarization of emission are in agreement with the experimental results of Andersen et al. (1976).

Results of a detailed study of the He²⁺/Hg system in the collision energy range from 2 eV to 1 keV are reported. In this energy range the ionization leads to formation of He⁺ and Hg²⁺ in its ground state and four lowest excited states arising from the 5d⁹6s configuration. The ionization cross section has a maximum at about 25 eV and drops by about an order of magnitude between the maximum and 2eV, and also between the maximum and 400 eV. An analysis of the electron and ion energy spectra leads to the conclusion that the ionization mechanism is molecular: at large separations the system undergoes charge exchange, He²⁺+Hg to He⁺*(n=2)+Hg⁺(²S₁2/), and at separation of about 5 a₀ the system autoionizes from the three potential curves arising at smaller distances from the configuration He⁺*(n=2)+Hg⁺(²S₁2/). A potential curve model is proposed.

The equations of the adiabatic representation for a collision problem are rederived in a new and compact way, and the kinematic nature of the long-range matrix elements which arise in this representation is shown explicitly. The matrix transformation of the adiabatic representation is introduced, which greatly simplifies its asymptotic form; the asymptotic values of matrix elements are effectively subtracted. The relation of this transformation with the translational factor operator is demonstrated. In the case of a one-electron diatomic system, the explicit form of the matrices introducing the asymptotically adapted adiabatic representation is found.

Following the semi-empirical approach of Khare (1969 and 1970) and Khare and Padalia (1970), the ionization cross section per unit energy range and the total ionization cross sections of CO₂, CO, H₂O, CH₄ and NH₃ due to electron impact ionization for energies varying from the ionization threshold up to 10 keV are calculated. The results are in fair agreement with the experimental data and other theoretical investigations.

The reactions of He⁺ ions with N₂, O₂, CO₂, and CH₄, and of C⁺ and N⁺ ions with O₂ are presented. The measurements were carried out in the SIFT apparatus (selected ion flow tube), which involves the injection of a mass-selected positive-ion beam into a flowing neutral gas into which a reactant gas is introduced at a position downstream in the flow. Data acquisition and analysis are made by the flowing afterglow technique. The results for the He⁺+N₂ reaction agree with the previously established branching ratio. For the other reactions, considerable differences are apparent between the product distributions obtained and those previously reported and an explanation is attempted. A brief discussion of the reaction mechanism is presented for each reaction in the light of the determined product distributions, and the anticipated development of the SIFT technique for ion-molecule reaction mechanisms is outlined.