Table of contents

Emission bands tentatively attributed to a new ¹ Sigma ^--¹ Sigma ^- transition of neutral SiO (Bredohl et al. see abstr. A9510 of 1974) are shown to arise from the known E ¹ Sigma ⁺-x¹ Sigma ⁺ system of SiO.

New bands belonging to the c³ Pi _i-a³ Pi _r transition of SiO have been observed yielding a change of the vibrational nu ' numbering. New vibrational and rotational constants are given.

A high resolution X-ray emission spectrum of water vapour is reported and the main lines are analysed in terms of molecular orbital theory. Seven satellites are found and discussed.

It is shown that the frequency of the scattered radiation can be deduced accurately using a model in which the radiation is described as a classical electromagnetic wave. By considering the experimental facts and applying the conservation laws, the quantal nature of the interaction is exposed. The view is taken that this resides in the charge-matter system, but not in the electromagnetic radiation.

Rectangular distributions of velocities due to excitation transfers between levels of xenon and krypton are the origin of anomalous profiles observed in the 3.36 mu m line. The interpretation is based on the consideration of different mechanisms for populating low-excited levels of xenon and is supported by the recording of various profiles.

The authors analyse the convergence of the binding energy for the He-like atoms in the framework of the hyperspherical formalism. Their calculations give for the helium ground state a binding energy of -2.903 au.

In addition to previous absorption measurements of Cs resonance lines, new investigations on Na, K and Rb also showed definitely that beyond symmetrical Lorentzian central parts of the lines, the stronger components of the resonance lines have blue and the weaker components red asymmetry. This behaviour can be explained by a quasistatic treatment of the self-broadening mechanism taking interatomic interaction potentials into account. But with additional data it is now possible to give a different interpretation of the origin of observed asymmetries than suggested in the previous paper on Cs resonance lines (see abstr. A5740 of 1975). The asymmetries are shown to arise from the mutual repulsion of two potential curves of the same species.

Yttrium spectrum has been recorded on the 5 m grazing incidence spectrograph at Lund (Sweden) from about 130-440 AA. Transitions from 5d, 6d, 6s and 7s to the ground levels of YV are identified for the first time; those from 4d and 5s levels, reported earlier have been confirmed. The fifth ionization potential of yttrium, 4p⁵²P₃2/-4p⁴³P₂, is evaluated from the straight ns and nd Rydberg series as (6.047+or-0.025)*10⁵ cm^-1. The ionization potential for Sr IV is estimated to be 4.462*10⁵ cm^-1 from the isoelectronic comparison of the ionization potentials of Br I sequence. The parameters G₁ and zeta _p are calculated for ²P and ⁴P levels of 5s and 6s.

Theoretical line shapes, computed by diagonalizing numerically the semiclassical time-independent Hamiltonian in the rotating frame of reference, are compared with ODR measurements in the state 7²P_3/2 of ¹³³Cs in a strong magnetic field, which cannot be explained simply by single quantum transitions. Good agreement is obtained between experiment and theory. The computer program was applied to the calculation of multiple quantum transitions.

Experimental and theoretical results concerning the broadening of Na 3p-3s (5890-96 AA) and K 5p-4s (4044-47 AA) lines by helium are compared and discussed in the light of simplified collision models referring to the molecular Hund's coupling cases. The two lines of the sodium multiplet are almost equally broadened while those of potassium differ appreciably. This is attributed to the increased importance of the a coupling case for potassium corresponding to a reduced angular velocity of the internuclear axis during K-He collisions which is due to the much larger diameter of the potassium atom.

Angular distributions are given for the Ar 3p-4s, s' and Kr 4p-5s electron impact excitations in the energy ranges 30-120 eV and 50-60 eV respectively. A modulated crossed-beam technique was employed and an angular resolution of about 3 degrees realized in the range 15-140 degrees . Results for argon are compared with the prediction of a distorted wave calculation of Sawada et al (1971) and quite good agreement is found over the whole energy range. Agreement with the previous experimental results of Nicoll and Mohr (1933) is poor in general around the structural minima. It is not possible to compare the present krypton results with existing theories or experiments. The energy dependence of the inelastic differential cross sections at 30 degrees scattering angle have been measured in the above energy ranges together with inelastic to elastic cross section ratios.

The effective number of annihilation electrons (Z_eff), the scattering length and s-wave phase shift of the positron-Ar system have been determined empirically below the threshold of positronium formation by requiring that the calculated temperature and electric field dependence of the equilibrium annihilation rate, the lifetime spectrum and the elastic scattering cross section agree with experiment. In atomic units, the velocity dependence of Z_eff may be represented by Z_eff( nu )=15.0+45.25 exp(-30.66 nu ), and the scattering length appears to lie between -4.0 and -3.0.

The cross sections for the transfer of up to four electrons in single collisions between multiply charged argon ions (up to Ar⁷⁺) and argon atoms are measured. The cross sections ( sigma ₂₁ excepted) show no significant energy dependence in the investigated energy range from 5 to 15 keV times charge state number. Their values are between 4*10^-17 cm² for the transfer of one electron to Ar⁷⁺( sigma ₇₆).

Starting from a partial wave decomposition of the quantum-mechanical equations, a systematic development of the semiclassical theory of inelastic transitions in slow atom(ion)-atom collisions is presented. Both atomic and molecular representations are discussed. It is shown that the introduction of an average potential reduces the semiclassical equations to the well known impact parameter equations; the notions of the classical trajectory and the classical collision plane arise as a natural consequence of the semiclassical approximation. Expressions for the differential scattering cross sections for inelastic transitions are subsequently derived. The connection with the straight line eikonal approximation is discussed.

For pt. I, see ibid., vol.8, no.2, 239 (1975). As an application of the semiclassical method presented in paper I, the elastic and inelastic, direct and exchange differential cross sections for proton-hydrogen collisions are calculated using a three-term molecular basis. The results of the common trajectory method are shown to agree with experiment (in the energy range 250-1000 eV) for relatively large angles, where the straight line eikonal method begins to show significant deviations. It is emphasized that the present treatment is not much more complicated numerically than the rectilinear model. A limit of validity to the straight line eikonal method is thus obtained. The sensitivity of the results to the explicit choice of trajectory is studied. The validity of the stationary phase approximation is tested in view of its popularity in other semiclassical treatments.

S-matrix elements in the classical limit and the corresponding probabilities are calculated for the collinear charge exchange reaction H⁺+H₂( nu =5,6) to H+H₂⁺( nu =0) for centre-of-mass energies ranging from 3.5 to 4.5 eV relative to H₂⁺( nu =0). The reaction involves an electronic transition between the two lowest adiabatic singlet potential energy surfaces of the H₃⁺ system, where the ground singlet surface corresponds asymptotically to H⁺+H₂and the other surface H+H₂⁺. The two surfaces are analytically continued from their avoided intersection to an actual intersection in the complex nuclear coordinate plane. The motion of the nuclei is assumed to be classical, and an electronic transition is effected by complex valued classical trajectories switching surfaces continuously at the complex intersection. Due to this switching the classical actions, calculated along these trajectories, gain imaginary increments. The S-matrix element for a given transition between reactants and products is calculated in terms of these classical actions, where the imaginary parts of the actions contribute exponential damping to the S-matrix element.

Rate constants for charge transfer and interchange reactions of O⁺ and N⁺ ions with N₂, O₂, NO and CO have been measured in the injected ion drift tube, using helium as buffer gas. The range of mean impact energies is 0.04-2 eV. Agreement with previously reported data is, on the whole, good. Energy distributions of ions emerging from the drift tube have been measured using planar retarding grids, and the mean energies E are found to be in agreement with the expression E=1/2m₊ nu _d²+1/2m_g nu _d²+3/2kT for ion mass m₊, buffer gas mass m_g, and drift velocity nu _d.

The one-body Green's function is investigated by expanding the self-energy part up to third order. On this level some properties of the diagrams of the self-energy part are discussed. To estimate the contribution of higher orders of the self-energy part an effective time dependent interaction is introduced. It is found that the main contributions due to this interaction can be determined by considering the analytic properties of second and third order terms only. The numerical effort required can thus be strongly reduced. The theory is applied to calculate the vertical ionization potentials of B₂H₆. The calculation of the pole strengths shows that one may expect satellite lines in the photoelectron spectrum of B₂H₆ with about 10% of the intensity of the principal valence lines due to excitations accompanying ionization.

A new vibrational analysis of the TiO gamma ' band system has been made based upon 35 of Phillips' (1969) measured band-head positions for this system and a new analysis of the ground state (Phillips 1973). The band-head positions were corrected to band-origin positions which were fitted by a least-squares procedure to a second-order polynomial in the molecular constants T'_e, omega '_e, and omega _ex'_e. The resulting values were used to derive estimates of the equilibrium rotational constants, B'_e, alpha '_e, and D'_e, and improved arrays of Franck-Condon factors and r centroids.

From high-resolution spectra obtained from low-temperature discharges, the spin doubling in several vibrational levels of the x² Sigma ⁺, B² Sigma ⁺ and D² Sigma ⁺ observed states of AlO have been calculated. The results differ completely, even in sign for B² Sigma ⁺, from previous determinations. A theoretical attempt is made to calculate the gamma constant for the ground state and the result appears to confirm the authors' experimental value.

Measurement of pre-breakdown ionization currents in N₂O in uniform fields were made within the range 150*10^-17 <E/N<182*10^-17 V cm² (where E is the electric field and N the gas number density) at relatively high pressures corresponding to values of N in the range from 1.66*10¹⁸ to 19.5*10¹⁸ cm^-3. Previous published data were used to postulate a reaction scheme which was used to obtain equations in terms of which the experimental results were analysed. The values of the coefficients representing ionization and attachment obtained from this analysis are given and discussed and the information obtained about the ion-molecule reaction rates is shown to be consistent with earlier data obtained at lower values of E/N.

Close-coupling calculations have been performed for the rotational excitation of CsF by low-energy electrons. There are significant differences between these results and the values predicted by the first Born approximation. Momentum transfer cross sections are reduced by a factor of three, improving the comparison with recent experimental measurements.

The contribution of proton perturbers to the far wings of the Lyman- alpha line of hydrogen is calculated in the adiabatic one-perturber limit. The wing expression is evaluated using the electronic molecular eigenstates of H₂⁺.