Table of contents

The authors report an accurate remeasurement of the oscillator strength of the Be I 2s²¹S-2s2p ¹P resonance line (2348.6 AA). The result, f=1.34+or-0.05 is based on beam-foil experiments at several ion energies and detailed studies of cascade effects.

The angular distributions of photoelectrons ejected from the outermost p-shells of argon and krypton have been measured for electron energies up to 20 eV using polarized synchrotron radiation. Agreement between theory and experiment is found to be good.

The photoelectron angular distribution parameter has been measured in the energy region 0-34 eV above threshold for argon 3p electrons, using a synchrotron light source. The cross section of the argon 3s subshell has also been measured in the 0-20 eV region above its threshold. Both sets of data are compared with recent theoretical calculations.

Ne K-shell Auger electron production cross sections, sigma _A, have been measured for Ne bombarded by 1.5 MeV/amu H, O, and sigma _A is found to increase with increasing projectile charge state which is a clear indication that the K-shell vacancy production cross section, sigma , is increasing with increasing projectile charge state since the fluorescence yield, omega , is increasing with increasing charge state and since sigma = sigma _A/(1- omega ). The cross sections are compared to Coulomb ionization predictions.

It is shown that the generalized random phase approximation of elastic scattering can be formulated as a coupled channel problem. Its relationship to the many-channel scheme, the close-coupled equations and the first-order many-body theory for inelastic scattering is also discussed.

The second Born amplitude for elastic electron scattering at energy 1 Ryd on the ground state of the hydrogen atom is given.

The equivalence of two theoretical approaches (see abstr. A12513 of 1974, and abstr. A41415 of 1972) is shown and the advantages of a common formulation are discussed.

Exploratory calculations are presented which indicate that the azimuthal asymmetry in the cross section for scattering of unpolarized electrons by polarized s-state atoms in the presence of spin-orbit interaction, which was predicted by Farago (see abstr. A9432 of 1974), should be measurable for caesium at incident electron energies below about 15 eV.

It is pointed out that the so-called 'displaced threshold effect', recently reported by Hicks et al. (see abstr. A71444 of 1973) may lead to indirect excitation of atomic states under electron impact resulting in the occurrence of structures in the excitation curves. Experimental results on the excitation of helium by electron impact have been re-analysed and evidence is found that some structures indeed may be due to the proposed indirect excitation mechanism.

It is shown that the post-collision interaction model, previously proposed to explain the energy shifts of electrons ejected from autoionizing states of He excited by low-energy charged particles, can be extended to include cases where very low-energy scattered electrons are recaptured to form singly excited bound states. Electron excitation functions of three band states of He are presented over the range 57 to 60 eV. The photon yield from He over the incident energy range 56 to 61 eV is interpreted in the light of the above findings.

Collision strengths have been calculated for electron impact excitation of forbidden lines of Ne²⁺ by solving systems of coupled integro-differential equations. Configuration interaction type wavefunctions are used for the target states in the wavefunction expansion. Transitions between the ground configuration terms ³P, ¹D and ¹S are considered. The terms for which the dominant configurations are 2s²2p⁴ and 2s2p⁵ have been included. The technique is also used to make bound state calculations for Ne⁺. The effective quantum numbers obtained are in better agreement with experiment than in earlier work.

Crossed beam experiments have been performed to study elastic scattering and vibrational excitation in e-H₂O collisions for the energy range from 1 to 10 eV. The rise of the cross sections at low energies (below 2 eV) is interpreted in terms of a direct scattering mechanism, whereas the broad enhancement of the cross sections around 6 to 9 eV is attributed to H₂O^- states intersecting the Franck-Condon region in this energy range. Theoretical estimates for the energies of H₂O^- states are not in agreement with these findings.

Asymptotic screening parameters obtained by fitting the exact first order results for (4₁ⁿ+r₂ⁿ) with n=-1, 1, and 2 are compared with those obtained by energy optimization, for configurations 1s1s' and 1s2p. The Hylleraas principle for the second order energy shift is presented in a form which involves the operator commutator technique previously used in applications of the Dalgarno interchange theorem.

Energy values for and dipole transition probabilities among 23 low lying 3p⁵ ns, np, nd excited states (J=1) of Ar I have been determined. The calculation was ab initio and used non-relativistic, frozen core Hartree-Fock wavefunctions in configuration interaction with the inclusion of both Coulomb and spin-orbit interactions. The energy values lie within a few per cent of the experimental values. Of 72 transition probabilities computed, comparison data, either experimental or theoretical, exists for 54, and a broad pattern of agreement is obtained.

The evolution of a (n=2) hydrogen atom passing through a linearly increasing or decreasing electric field is calculated in a three-state approximation. The observable fraction of 2p_1/2, 2s_1/2 and 2p_3/2 quantum state in the resulting coherent mixture is calculated. The amplitude of the oscillations, which appear when the field parameters are varied, is shown to depend only on the rate of change of the field, as seen by the atom. Maps of oscillation amplitudes and average values are given for the metastable atom. Comparison is made with the sudden and adiabatic approximations.

The transition arrays 1s²2s-1s2s2p and 1s²2p-1s2p² in Li-like ions are calculated by means of relativistic Z-expansion combined with second order calculations of the electron electrostatic interaction. The results of the ab initio calculations for 6<or=Z<or=29 agree very well with experiment and with the calculations of Gabriel (see abstr. A3332 of 1973).

A formalism is proposed for extending the random phase approximation to open shell systems. This method is based upon use of a central field model to obtain zeroth order energies and wavefunctions and utilizes the equations-of-motion approach of Rowe (1972). The basic equations of the usual random phase approximation are preserved, so that many of the characteristics of the solutions of the extended problem can be obtained using traditional arguments. Characteristics and limitations peculiar to the open-shell problem are discussed.

Experimentally determined mass attenuation coefficients for C₂H₄, CO₂, N₂, O₂, CF₄, Ne, H₂S, HCl, Ar, air, Mg, Al, SiO₂, (C₂H₅)₃PO₄ at a series of energies from 4.508 to 25.192 keV are presented in tabular form, together with estimates of total random and systematic uncertainty. Full details of the experimental method are given. This data set enables deduction of a coherent data set for the elements from Z=6 to Z=18 with better accuracy, internal consistency and reliability than any previously published data in this energy and atomic number range.

By curve fitting routines, values of total attenuation cross section and photoelectric cross section for the elements C, N, O, F, Ne, Mg, Al, S, Cl, Ar in the energy range from 4.508 to 25.192 keV, for Si from 8.041 to 25.192 keV and for P from 11.210 to 25.192 keV have been derived, based on experimental measurements of high accuracy. These smoothed values have estimated 99% confidence limits of +or-1%. This constitutes a substantial improvement on existing experimental data. The impressive agreement of these results with the theoretical data of Storm and Israel (1970) lends weight to the newer theoretical techniques.

New measurements of Cs resonance lines show certain asymmetries in the line wings. The suggestion for the explanation of this behaviour is given by means of quasistatic theory.

Exact equations determining the positions of the longitudinal and transverse magnetic resonances of a doublet are derived. These equations are simply related. Analytical expressions for the Bloch-Siegert shift and the first three transverse resonances are given which apply over the whole range of RF field strengths with 1% accuracy. The expressions for the three quantum resonances are correct to 3%.

In an optically pumped ⁸⁷Rb vapour, multiple quantum transitions in the ground state Zeeman sublevels are examined as a function of the radiofrequency field intensity. The experimental results are compared with existing theoretical approaches. Good agreement is found, for the resonance shifts, with the theoretical analysis based on the Hamiltonian solution for the atom plus RF field system. A more refined theory must be developed for interpreting line broadening results.

Accurate measurements of the threshold behaviour for excitation of the states K(4²P₁2/) and K(4²P₃2/) have been performed for K+Ar, Kr, and Xe. Emission due to the decay of excited rare gas states has been studied for the same systems between 50 and 250 eV. The results show that the simultaneous excitation of both potassium and rare gas atom is not important in populating K(4²P). Excitation mechanisms are discussed on the basis of the molecular orbital model.

The total elastic scattering cross section for electrons on neutral atoms is evaluated in the first Born approximation via tabulated values of the form factors. The theoretical results are compared with experiment for helium, neon and lithium. For lithium, comparisons are also made with Glauber and polarized-Glauber models. A simple model is presented for low-energy total cross sections in lithium, and its predictions compared with experiment. Elastic differential cross sections for helium and neon are briefly discussed.

It is shown that the first-order results of the recent many-body theory of inelastic scattering (see abstr. A25430 of 1971) can be derived by a direct application of the distorted-wave and random phase approximations to the usual expression for the inelastic scattering amplitude. The result is derived both in the second quantized formalism and by the standard application of the distorted-wave approximation coupled with the random phase approximation (RPA). The RPA (or time-dependent Hartree-Fock theory) provides the transition density between the initial and inelastically excited states. Possible generalizations of the procedures are discussed.

Cross sections and collision strengths for electron impact excitation of the resonance lines of Mg⁺, Ca⁺, Sr⁺ and Ba⁺, calculated in the Coulomb distorted wave, Coulomb Born and Coulomb Bethe approximations, are presented. A technique for calculating contributions to the total collision strength from large values of incident electron angular momenta is discussed. A comparison of the results with three-state close-coupling calculations indicates that the Coulomb Born and Coulomb distorted wave approximations agree well for energies greater than about 8 times threshold, while comparison with recently measured absolute cross sections shows that the Coulomb Born and Coulomb distorted wave results agree well with experiment only for energies above about 80 times threshold.

Inner-shell ionization cross sections of silver and gold have been calculated from a result which uses the relativistic Sommerfeld-Maue wavefunction for the ejected electron. The results have been compared with the results of Das (see ibid., vol.7, 923 (1974)), which use the non-relativistic Coulomb wavefunction for the ejected electron. The comparison shows that there is only a few percent lowering in total cross section under these improved calculations and the overall trend of the curves for total cross section and the conclusion of Das are maintained.

The authors study the scattering of Lyman alpha by hydrogen, nitrogen, argon, krypton and xenon. By using a calculated value for the Rayleigh scattering cross section for hydrogen, values for other gases are deduced. In the case of xenon it is found that the scattering is dominated by the formation of xenon molecules and a value of 17.1 Mbarn for this species is found.

A partial analysis reveals two violet-degraded sub-bands having P, Q and R branches with P heads at 18576 and 18646 cm^-1. The upper states are badly perturbed. Effective values of B" are 4.956 and 5.22 cm^-1 and of B' are about 5.15 and 5.4 cm^-1 respectively. The type of transition is discussed and it is concluded that it probably involves quartet states (but possibly sextet) not of type Sigma and that Delta Lambda =+or-1.

Expressions for the distribution of ion energy among thermal, drift and random field components are given in terms of the ion drift velocity and the field derivative of the ion mobility. Comparison with available Monte Carlo calculations indicates that the expressions are accurate to high field strengths, without restrictions on the ion-neutron mass ratio or force law.