Table of contents

Perturbation theory is used to calculate the dipole polarizability of the 1s²2s ²S states of the lithium isoelectronic sequence. Good agreement is obtained with the recent bruekner-Goldstone many-body calculation of Chang, Pu and Das (1968) on lithium.

Classical calculations suggest that there are small irregularities in the electron or proton ionization cross sections of complex atoms. These irregularities are due to the shape of the electron velocity distribution of the atomic electron.

An electron spin resonance spectrum corresponding to S=¹/₂, I=³/₂, and showing orthorhombic symmetry has been detected in single crystals of gamma -irradiated zinc bromate. First-order analysis of the spin Hamiltonian produced the parameters (i) g_xx=2.004, g_yy=2.054, g_zz=2.067, mod A_xx mod =447 G, mod A_yy mod =163 G, mod A_zz mod =120 G; (ii) g_xx=2.005, g_yy=2.056, g_zz=2.068, mod A_xx mod =483 G, mod A_yy mod =172 G, mod A_zz mod =121 G. The paramagnetic centres characterized by these parameters are tentatively identified as the ⁷⁹BrO₂ and ⁸¹BrO₂ radicals.

It is shown that the Kohn variational principle for scattering amplitudes need not be explicitly modified when a trial function with a discontinuous derivative is used.

In a previous paper in this series (abstract A22979 of 1969) it is shown that complex quantum defects mu may be used to describe properties of resonances in electron-ion collisions; they are such that the usual Rydberg formula, E=E₂-z²/(n- mu )² gives the complex energies at which the scattering matrix has poles. In this formula, E₂ is the energy of an excited state of the ion. Calculations are made for the e-Be⁺ system, including the 2s and 2p states of Be⁺. Complex quantum defects are calculated below the 2p threshold and above that threshold, and are found to vary smoothly as functions of the energy.

The need to include both spin eigenfunctions in the wave function in order to consider the hyperfine structure of lithium is demonstrated. An approximation to the orbital functions is first determined using only the two-determinant spin eigenfunction. The energy functional obtained from a linear combination of the two spin eigenfunctions is then minimized only with respect to their relative weighting. This process simultaneously improves the energy and the hyperfine structure over their Hartree-Fock (HF) values. Similar calculations are performed for members of the isoelectronic sequence, up to Z=36. For large Z, the relative amount of the less significant spin eigenfunction appears to behave as Z^-12/.

Recently, according to the Q-space definition of Feshbach-type autoionization states, a multiconfiguration energy-bound theorem (minimum principle) has been given by Perkins for such states of two-electron atoms. The multiconfiguration stabilization method of Holoien is consistent with the energy-bound theorem requirements and is thus an upper-bound procedure. To test the upper-bound method, and its convergence in the limit from above to the Feshbach projection method, further extensive variational calculations have been carried through. Results obtained for many of the lowest-lying states of the specific symmetry ¹S^e of helium below the n=2 threshold are given.

A discrepancy between the form of the observed angular distribution functions for multiple scattering of medium-energy electrons in argon and that expected from the form of the single collision function is investigated. The discrepancy is formulated in terms of a sequence bias which is shown also to account for the differences between the observed and expected behaviour of two other aspects of the shapes of the electron tracks. These aspects are the distribution of the sign of successive observed collisions in the projected track and the distribution of the area enclosed between the projected track and its chord. A sequence bias is then shown to be operating in the simulated tracks of electrons in a square lattice by an investigation of the same three aspects of the shapes of the tracks. It is therefore suggested that the discrepancies are due to a residual statistical geometrical order in the gas.

The recent thermal conductivity data of several homonuclear diatomic molecules and a number of binary gas mixtures have been compared with the recent theories of thermal conductivity which take into consideration inelastic effects. For the pure gases the results show the uncertainty in the prediction of the temperature dependence of rotational relaxation times from thermal conductivity data. For binary mixtures the experimental data show the inability of the theory of Monchick, Pereira and Mason (1965) to represent the cross-relaxation effect on thermal conductivity.

The method of Harris and Michels is used to evaluate the single-centre wave functions of a diatomic molecule using a two-centred LCAO-MO-SCF function. The static potential between the molecule and an incident electron is then calculated. The results for N₂ are given.

For pt. I see ibid. vol. 3, no. 5, 636 (1970). The theory of electron scattering by closed-shell diatomic molecules, assuming that the molecule is constrained to be in its ground electronic state, is developed. To simplify the analysis it is assumed that the molecule does not vibrate or rotate during the collision and it is then shown how rotational excitation cross sections may be obtained from the asymptotic form of the scattered electron wave function. A general computer code based on this formalism and using single-centre wave functions has been written and it used to study e^--N₂ collisions at low energies. A low energy ² Pi _g resonance is found in agreement with recent experiments.

An investigation of one- and two-electron capture and one-electron loss processes was carried out for Mg⁺, Fe⁺ and Cu⁺ on gases Ne, Kr and N₂ in the energy range of 5-30 keV. The measurements were made under single-collision conditions using the beam technique. The characteristic behaviour of sigma ₁₂ cross section, for the investigated metal ions, in the velocity region v<10⁸ cm s^-1 is discussed. The theoretically calculated cross sections for the one-electron capture process are compared with those obtained experimentally. The two-electron capture process, leading to the formation of negative ions, was investigated for the case of Fe⁺ on Kr. The analysis of the experimental results revealed the important role of two-step processes on the formation of Fe^- in the gas target thickness range of 10^-3-10^-2 torr cm. The experimentally obtained sigma ₁₁ cross-section values for that case are compared with the theoretically calculated values.

The ionization produced by 5-45 keV helium atoms during passage through thin targets of H₂, N₂, O₂, CO, CO₂ and CH₄ has been investigated by the mass-spectrometric analysis of the secondary ionic products of single collisions. The method ensured that all secondary product ions were collected and recorded with a high and uniform efficiency. Absolute cross sections for the production of particular fragment ions, derived from measured relative abundances, are found to be considerably higher than some previously reported values. It is shown that the possible influence of fast metastable atoms on the measured cross sections is very small. In spite of the large energy defects associated with possible formation processes for different fragment ions, at the lowest impact energies dissociative ionization is shown to be the dominant ionizing mechanism for all the gases investigated.

Electron collisional excitation cross sections of all allowed resonance transitions in Si II have been calculated using semi-classical perturbation theory. Allowance has been made for violation of the unitary condition on the S matrix due to coupling between all resonance states and the ground state. These cross sections were used to calculate the relative intensities of allowed and forbidden ultra-violet resonance lines of Si II emitted both by the sun and ZETA. The allowed line intensities are very sensitive functions of electron temperature while the forbidden lines are sensitive monitors of the electron density. Comparison with observation suggests the (rate averaged) cross sections are accurate to within a factor of two.

Vibration-rotation spectra and dielectric dispersion are combined to reveal the microdynamics of polar molecules in dense phases. A new general relation between the dielectric dispersion and the microdynamics is introduced with the aid of microscopic angular autocorrelation and cross-correlation functions. Experimental autocorrelation functions of CHCl₃ and HCl and DCl in tetrachlorides are presented for illustration and simple models are given.

The collision-induced absorption spectrum of gaseous CO₂ has been measured in the far infrared at temperatures of 200 K, 293 K, 323 K and 373 K. The peak of the induced absorption band is observed to shift to lower frequency and to increase in magnitude as the temperature is reduced. Values of the absorption coefficient measured at frequencies across the band, and the integrated band absorption coefficient are presented as functions of temperature. The data have been used in two ways to obtain a value of the quadrupole moment of CO₂, yielding the mean values of (6.4+or-1.0)*10^-26 esu cm² and (7.6+or-1.0)*10^-26 esu cm². A discussion of possible mechanisms of absorption in addition to that due to the quadrupole- induced dipole moment is given.

The band spectrum of TiBr has been excited in emission in a radio- frequency discharge and photographed under high dispersion. Two band systems of TiBr have been analysed in the regions lambda lambda 4265-4185 AA and lambda lambda 3850-3790 AA, and attributed to A ⁴ Pi -x⁴ Sigma and C⁴ Pi -x⁴ Sigma transitions respectively.

The emission spectrum of the FeBr molecule is excited in a high-frequency discharge tube through a mixture of heated iron metal and bromine vapour. The spectrum which has been photographed under high dispersion (1.25 AA mm^-1) reveals a new system of violet-degraded bands in the region lambda lambda 3650-3760 AA. The system is interpreted as arising from a ⁴ Pi -⁴ Sigma transition analogous to the system II of FeCl in the region lambda lambda 3500-3650 AA, analysed by previous workers. The vibrational assignments of the band heads are confirmed from a study of the observed bromine isotope effect.

Using a low-pressure arc source the band spectrum of La¹⁸O has been obtained. The heads between 5200-6300 AA were analysed and the vibrational constants are: sigma _e=17884.89 cm^-1, omega _e'=696.00 cm^-1, omega _e' chi _e'=1.83 cm^-1, omega _e"=775.53 cm^-1, and omega _e" chi _e"=2.06 cm^-1. With these figures the recalculated positions of the bands are in excellent agreement with the experimental values.