Table of contents

The 3³P₁-3³P₂ interval in ⁴He is measured by the step-wise excitation level-crossing technique at 658.76+or-0.13 MHz, in fair agreement with the previous measurement by Wieder and Lamb (1957). The n³D fine-structure intervals are calculated by extending the theory of Van den Eynde et al. (1971), and fairly good overall agreement is obtained with recent experimental intervals, notably those of Astner et al. (1976). The singlet-triplet mixing coefficients of Van den Eynde et al. are recalculated using experimental singlet-triplet splittings, giving results in excellent agreement with the experimental values of Derouard et al. (1976).

Many-body perturbation theory is applied to calculate double electron photoionization for neutral argon. Results for the 3p² and 3s3p pair excitations are included in both the dipole-length and dipole-velocity formulations. Essential features of the cross section are in generally good agreement with recent experimental data.

A theory of laser-induced inelastic collisions is presented, containing atomic selection rules and specific intensity and velocity dependences. It is applied to the case of the process Sr(5p¹P⁰)+Ca(4s²¹S)+h(cross) omega (6408.6 AA) to Sr(5s²¹S)+Ca(4d¹D), for which the intensity and wavelength dependence of the cross section is evaluated.

New ultraviolet bands of MgO degraded to shorter wavelengths have been observed in emission in the region 2900-3000 AA. The value of Delta G"_0-1 was found to be 656 cm^-1 suggesting that these bands have the known A¹ Pi state for their final level. The partial rotational analysis of the 0-0 band has confirmed this observation and has shown that the bands represent an electronic transition E¹ Sigma -A¹ Pi .

Group-theoretical schemes of classification of the doubly excited states of the two-electron atom are based on the theory of addition of four angular momenta. A new scheme of classification of states with high total orbital momentum is proposed for the case when one electron is excited much more than another. The group-theoretical state vectors are compared with the results of the qualitative and numerical study of configuration interaction. Exceptions to the proposed classification scheme are discussed.

A method for the exact numerical solution of the problem of a hydrogen atom perturbed by a uniform electric field for arbitrary quantum states is presented. The authors present some data obtained for the energy of the levels and their width and compare their results with those of previous papers.

The photon-photon delayed coincidence technique has been used to measure the lifetimes of four excited states in argon II. Pressure-dependent effects have been avoided by using very low target gas pressures. A method of data analysis has been employed in which the instrumental response function is obtained directly and each time spectrum is analysed by a least-squares fit to the exponential decay convoluted with this known instrumental response function. The four levels, together with their measured lifetimes are as follows: 4p⁴P₅2/⁰(7.78+or-0.12)ns, 4p⁴P₃2/⁰(7.02+or-0.03)ns, 4p²D₅2/⁰(9.11+or-0.23)ns, 4p²D₃2/⁰(8.94+or-0.41)ns.

A saturated-amplification experiment has been used to determine the variation of the Lorentzian width of the 3.51 mu m line of xenon with pressure. Measurements show a nonlinear dependence for p<50 mTorr and a linear one for p>50 mTorr; the slope is then (30+or-3) MHz Torr^-1. The effect of strong elastic collisions is demonstrated by the superimposition of a broad background on the Lorentzian profile.

Presents a new method of calculating the dispersion force between two atoms. This force has usually been calculated by expanding the electric interaction into the interaction between the multipole moments; this method, however, has the defect of giving a divergent series for the dispersion force. The new method gives a convergent series which is expressed in terms of physical properties of the separated atoms and which approaches a well known inverse-power series as the nuclear distance goes to infinity. The method is applied to hydrogen atoms.

Relatively high-precision measurements on low-, medium- and high-Z atoms have been made in the 0.662-1.33 MeV photon-energy range for both the photon differential scattering cross section at all angles and the integral cross section. The measurements have been made through the adaptation of standard methods, with modification and use of a newly developed detection technique. The present measurements satisfy a need for improved quality and quantity of cross section data, because of the lack of and large errors in the old incoherent scattering data. The newly developed non-relativistic Hartree-Fock (HF) self-consistent-field model is used in view of its improvement over the earlier ones which had considerable uncertainty.

Electron-atom impact in the presence of an external electromagnetic field, with the frequency close to the transition frequency of the atom (the resonant situation), is discussed. The cross section for elastic and inelastic scattering of an electron in the presence of a resonant electromagnetic field are obtained.

The authors attempt to interpret the drastic modifications of the second-order term, especially the disappearance of Re f_B2, in the Glauber scattering amplitude as primarily due to the so-called straight-line approximation considered in the derivation of this amplitude. Within this view, a new approximate form for the scattering amplitude has been proposed, and new calculations have been performed for elastic scattering of electrons at intermediate energies by a hydrogen atom in its ground state. The results are found to agree very well with experimental data. The calculations may also be used to confirm that the eikonal-Born-series method is a good approximation for electron-hydrogen scattering at intermediate energies.

S-wave elastic collisions of positrons with helium have been studied using the method of models in an optical-potential formulation that gives lower bounds to the phase shifts within the accuracy of the helium-atom model. Four simple approximations to the helium ground-state wavefunction are used in a trial function with up to 84 correlation terms. The annihilation rate calculated from what is considered to be the best model treated (of analytic Hartree-Fock type with the correct polarizability) is in good agreement with the most recent experiment. The scattering results are compared with those of other recent calculations.

Measurements of electron scattering at low energy from Mg, Zn, Cd and Hg have been carried out using the electron transmission method. A large shape resonance is observed for each element which is identified with the (ns²np)²P ground state of the negative ion. The electron affinities in eV are found to be Mg(-0.15), Zn(-0.49), Cd(-0.33) and Hg(-0.63) with an error of +or-0.04 eV. The results are compared with the available theoretical predictions.

The ejected-electron spectrum of sodium vapour has been observed at angles up to 120 degrees C with respect to an incident electron beam of kinetic energy 35 to 400 eV. Comparisons are made with ultraviolet absorption data and ejected-electron spectra obtained by electron and proton impact excitation.

The probability of proton-induced Cu K-shell ionization has been investigated for small impact parameters by measuring coincidences between K X-rays and 0.5-20 MeV protons scattered through large angles. In such measurements, corrections for random coincidences are important, and it is shown that high-frequency time structure of the beam current can lead to large systematic errors. After elimination of these problems, the measured ionization probability for 2 MeV protons was nearly constant at large scattering angles, in agreement with predictions from semiclassical approximation (SCA) calculations with straight-line trajectories. At lower energies, however, a significant increase in the ionization probability has been found for the largest scattering angles. It is shown that these results may be explained in the SCA model when the deflection of the projectile is taken into account.

The nonlinear parameters in a transcorrelated calculation of the ground-state energy and wavefunction of the hydrogen molecule are optimized using the method of Boys and Handy (1969). The method gives quite accurate results in all cases and in some cases the results are highly accurate. This is the first time the method has been applied to the optimization of a term in the correlation function which depends linearly on the interelectronic distance.

In a previous paper (see J. Polym. Sci. (Polymer Physics Edn.), vol.10, p.2135 (1972)) it was shown that it is possible to obtain detailed information about the distribution of orientations of an assembly of Raman scatterers from suitable measurements of the intensity of the scattered radiation. The treatment was given in terms of Raman tensors referred to principal axes. The present paper gives a more general treatment in which tensors are referred to the symmetry axes of the scatterers and in which scattering from degenerate modes is explicitly considered. The distribution of orientations is assumed, as previously, to have three mutually perpendicular two-fold rotation axes and the information that can be obtained for such a distribution, and the measurements required, are discussed in detail for scatterers of all point groups and particular attention is given to helical molecules.

Discusses the ion-pair formation in collisional dissociation of H₂⁺ and follows earlier work on H₂ (Brouillard et al. (1975)). Cross sections for ion-pair formation and the energy distribution of the fragments have been measured for 6 keV H₂⁺ ions impinging on He, Kr, and Xe targets. In addition, cross sections for negative-ion production have also been determined. The results indicate that ion-pair formation from H₂⁺ in rare gases could be described as a two-step process involving electron capture into a predissociated Rydberg state. This contrasts with the case of H₂. Moreover, it is shown that ion-pair formation cannot account for the total yield of negative ions.

Calculations are presented of the gain for the transition between the n=2 and n=3 states in an expanding cylindrical plasma of fully stripped carbon ions. The results of these calculations are extended by means of a similarity solution to derive general scaling laws for recombination schemes of this type. The feasibility of such schemes is briefly examined in the light of these results.