Table of contents

The energy dependence of the differential cross section for H⁺-He scattering at fixed scattering angle has been measured in a high-resolution crossed-beam experiment. The energy range investigated was from 100 meV to a few eV. In the vicinity of E_crit=0.5 eV and below, the authors observe structures in the cross sections which are attributed to the influence of orbiting scattering in this range of collision energies. These results represent the first observation of orbiting structures in an ion scattering experiment. The experimental data are compared with theoretical calculations.

Electron-photon angular correlations have been measured for electron impact excitation of the first excited ¹P and ³P states of krypton and of the first excited ³P state of xenon. In the case of these heavy atoms two independent measurements were necessary to determine the excitation parameters lambda and chi for these states.

A single-centre formalism is used to calculate the integrated elastic cross section in the energy range 0<or=E<or=10 eV using the fixed nuclei approximation. The interaction between the e⁺ and N₂ is taken to be a sum of static plus phenomenologically cut-off polarisation potentials. The cut-off parameter is adjusted so that the calculated cross sections are in agreement with recent measured values below the positronium formation threshold. The resulting wavefunctions are used to calculate the effective number of annihilation electrons.

A precise evaluation of the fine structure of non-penetrating Rydberg states requires a relativistic wavefunction for the relative motion of the Rydberg electron and the core. The authors show that Dirac wavefunctions can only be used in the case of spinless cores. For all other cases, the authors present new wavefunctions and apply them to helium Rydberg states. For comparison, lithium states are also discussed.

A J dependence is revealed in the isotope shifts of the 5d²D_{3/2, 5/2} to 6_p²P_3/2 transitions in Ba ions, using collinear fast-beam laser spectroscopy. The experimental ratio of the field shift for the 5d²D_3/2 to 6p²P_3/2 transition to that for the 5d²D_5/2 to 6p²P_3/2 transition is 1.0465+or-0.0010. This result is compared with relativistic Hartree-Fock calculations.

For pt.II see ibid., vol.15, no.2, p.219-31 (1982). The satellite structure in the argon L X-ray emission spectrum has been calculated using correlated initial- and final-state wavefunctions based upon a frozen-core model for each state developed previously. Strong correlation effects in the final state give rise to intense satellite structure similar to the photoelectron satellites seen in valence-shell ionisation. The intensities differ from the photoelectron intensities as a result of relaxation in the transition process. Final-state correlation satellites investigated in this work contribute a total intensity equal to 84% of the diagram line intensity to the spectrum. Multiply-excited correlation satellites also exist and contribute a total intensity estimated to be 6.0% of the diagram line intensity for the initial excited-state populations chosen in this study. A few initial-state correlation satellites are also predicted with this distribution contributing about 0.3% intensity to the spectrum. The theoretical predictions are compared with current experimental work. Evidence is presented that the incident energy dependence of main-line to satellite-line intensities recently reported by Bonnet et al. (1980) is not due to variations in final-state correlation satellite intensities as originally proposed by these workers. The present paper highlights the important role of electron correlation effects in determining X-ray satellite structure and provides an insight into the complexity of the calculations required.

A recently proposed relaxation theory of Stark broadening by ion perturbers is shown to be valid for the case of charged radiators. In addition, the straight-line-path assumption for the radiator and for the ion perturbers is examined and shown to be a good approximation for the first few members of the hydrogen isoelectronic sequence. Results of sample calculations of the Stark profiles of the Lyman- alpha and Balmer- alpha lines of He II are presented which show significant dynamic effects at electron densities as high as 10¹⁸ cm^-3. Extrapolation of the results to the Paschen- alpha (4686 AA) line suggests that the discrepancy between experiment and the unified-theory calculations is primarily due to the static ion-radiator approximation of the earlier theory.

The photoabsorption cross sections of BF₃, BCl₃, and BBr₃ were obtained in the region between 190 and 280 eV using synchrotron radiation. Above the B K ionisation threshold, a broad band due to shape resonance was observed for BF₃, while no counterpart was observed for BCl₃ and BBr₃. Instead, a new strong band was found below the B K ionisation threshold in the spectra of BCl₃ and BBr₃. The origin of the shape resonances is discussed by comparing the spectra of these three molecules. Assignments of the B K spectra of BF₃, BCl₃ and BBr₃ and of the Cl L spectrum of BCl₃ are given with the aid of ab initio calculations.

Ab initio CI calculations are presented for the 1¹ Sigma , 2¹ Sigma , 1³ Sigma , 2³ Sigma , 1¹ Pi and 1³ Pi states of MgHe for energies up to 10 eV. All the states are purely repulsive. The lowest ³ Sigma and ¹ Pi states cross at around 3 au and remain nearly degenerate down to 2 au. In the region from 3.2 to 2 au the 1³ Sigma state is being progressively Rydbergised to the 4p_z level and this change is reflected in the dipole moment function which changes sign. The matrix element of the spin-orbit operator is calculated in the region of the crossing and is reduced from the decoupled Mg atomic value due to the partial Rydbergisation of the 3p electron.

The cross sections for the processes p+H(1s) to p⁺H(nlm) and alpha +H(1s) to alpha +H(nlm) have been calculated in the Vainshtein et al. (1964) approximation for incident energies in the range 50 keV to 1 MeV taking the full interacting potential. It is shown that by knowing only the asymptotic (n>>1) cross sections one easily obtains the discrete excitation results for moderate and high n values.

The cross sections for one-electron capture by the ground-state Ar²⁺(³P) ions, sigma ₂₁, and by the metastable Ar²⁺(¹D) ions, sigma ₂₁*, have been measured independently by an optical attenuation method (OAM). Both the cross-sections are of the order of 10^-14 cm² and sigma ₂₁* is about 1.3 times as large as sigma ₂₁. The metastable fraction in the primary-ion beam has been estimated at 0.29+or-0.16 from the OAM combined with the beam attenuation method. The OAM provides a useful means to determine one-electron capture cross sections for the ground and metastable ions, in particular when both the cross sections are nearly the same. The validity of the present method is discussed in detail.

An isotope selective technique has been used to determine line polarisations and excitation cross sections in the range 7.5 to 14.0 eV for the electron impact excited 185 nm transition of atomic mercury. A stepwise excitation technique was used involving electron excitation followed by single-mode laser excitation. The role of nuclear spin in the collision process was investigated by making polarisation measurements on the I=0 and I=¹/₂ isotopes.

Total cross sections for electron impact excitation of the lowest autoionising levels in Li, Na and K are calculated in the Glauber approximation. Similar calculations are also performed in the first Born approximation as well as using the method of Crothers and McCarroll (1965). The results obtained from these various approximations are compared with earlier theoretical work.

Theoretical calculations of the K-shell ionisation of atoms by relativistic electrons are presented in this work by folding the virtual-photon spectrum evaluated in the plane-wave Born approximation with the photoionisation cross section of the K shell of the atom. The authors find a fairly good agreement with the available experimental data for ₂₈Ni, ₄₇Ag and ₅₀Sn for an electron impact energy near threshold to energies much higher than the ionisation energy. The authors also compared their calculation for ₁H, ₂₈Ni, ₄₇Ag, ₅₀Sn and ₇₉Au with the existing theories and the available experimental data.

The double-escape probability at a near-zero energy E has been analysed within the classical Wannier theory. The method due to Vinkalns and Gailitis (1967) has been generalised for the case of outgoing particles of arbitrary masses and charges. Wannier's threshold law for electron-atom ionisation and Klar's (1981) recent WKB result for a positron-atom collision are obtained as special cases. The threshold behaviour of the distribution of mutual angles for the case of oppositely charged particles reads: ( theta ₁₂)_max approximately E^1/4 for all Z>0, where Z is the charge of the remaining ion. The validity of the classical model as well as the approximations made within the classical description of the near-threshold behaviour are discussed. Further possible applications of the method are also quoted.

An electron impact spectrometer incorporating a novel position-sensitive multidetector has been used to study the excited electronic states of carbon monoxide. This paper presents results for the energy region from 6 to 10.9 eV. Many new lines are observed, some of which correspond to high vibrational levels of known electronic states and some of which have been provisionally assigned to new valence states of CO. The resolution with which the measurements are made is sufficient for the effects of rotational excitation to be observed.

The results of calculations of the steady-state ionisation balance in a silicon plasma are presented. The effect of collisional de-excitation on the dielectronic satellite ratios is shown to be important for electron densities in the range 10¹⁹ to 10²⁴ cm^-3 and temperatures greater than 100 eV. This is the regime of interest for laser-produced and laser-compressed plasmas and for which the dielectronic satellite ratios have been proposed as a useful density diagnostic. The sensitivity of the He-like resonance to intercombination line ratio alpha =N(2¹P₁)A(2¹P₁;1¹S₀)/N(2³P₁)A(2³P₁;1¹S₀) to the model used for the collision rates between singly bound He-like silicon levels has been investigated. Large differences in the calculated value of this ratio result from the use of different collisional models and so it may be possible to compare the results of different theoretical predictions with experiment.

Calculations are carried out on the low ambient gas density limit to the rate coefficient alpha for termolecular ionic recombination X⁺+Y^-+Z with the ion-neutral interaction taken to be of the polarisation (r^-4) type. A table is given which covers all systems of physical interest. If M₁, M₂ and M₃ be the respective masses of the three species, alpha ₁ be the contribution to alpha from X⁺-Z collisions and alpha _T1 be the corresponding Thomson partial recombination coefficient, (M₁+M₂) alpha ₁/M₂ alpha _T1 is a function of the mass ratio parameter M₂M₃/M₁(M₁+M₂+M₃). Using the results of calculations by Flannery (1980) it is found that the functional form for the polarisation interaction is around the same as that for a hard-sphere interaction and is closely similar to that for the symmetric resonance charge transfer case.