Table of contents

The wavefunction of a quantum mechanical system can be evaluated in any gauge and can be expressed in terms of a complete set of eigenstates of a physical observable or of a mathematical operator. If the physical observable is expressed in the same gauge as the Hamiltonian from which the wavefunction is obtained, the expansion coefficients are gauge invariant and have definite experimental implications. This physical observable need not be the energy. If the basis set of vectors are eigenstates of a mathematical operator, the expansion coefficients are gauge dependent. This mathematical operator can be translated to a physical observable once the gauge under which the wavefunction is calculated is specified. The expansion coefficients can then be interpreted as probability amplitudes.

The alignment of the 2p state of fast one-electron oxygen after excitation by He has been calculated by the plane wave Born approximation (PWBA) and compared with recent experiments.

By the aid of translational spectroscopy the authors have studied the production of Ne⁺ ions in Ne²⁺-Xe collisions with an improved energy resolution (E/ delta E approximately 3000) obtaining a better identification of the reception channels. The interpretation of the spectrum has to be changed as compared with the earlier publication. The spectrum emphasises the importance of Ne²⁺(¹D₂) ions for both electron capture and transfer ionisation at low collision energies (<1 keV). Within the energy range studied the projectile core is not conserved during the capture reaction. However at higher collision energies the core conservation rule seems to hold better leading to a change of the final-state population of the projectile.

One consequence of the Wannier theory of correlations between electrons is that states of high orbital angular momentum can be populated efficiently by electron impact on the ground (or a low-lying excited) state of an atom, provided the incident energy is close to the ionisation threshold of the atom. This feature was first pointed out by Fano (1974), who gave a qualitative estimate that l values up to O(n¹2/) would be populated. A recent note by Drukarev (1982), based on an assumption of the nature of the wavefunction involved, provides a quantitative distribution which peaks at l_max approximately=(¹/₂n)¹2/. An alternative argument, which follows from the description of angular correlation between the electrons in O₄ group symmetry is presented and supported by numerical evidence. The peak of the distribution lies at l_max approximately=¹/₂n¹2/.

A semiclassical model of the helium atom is presented, which has as its basis the concept of classical non-linear resonances. A novel feature of the calculation is its incorporation of exchange effects, leading to singlet-triplet splittings, hitherto inaccessible by a semiclassical approach. Energies of a variety of singly and doubly excited states are given.

EUV spectra of B III are investigated by means of high-resolution fast-ion spectroscopic measurements. For the first time the decay of several even parity (1s2pnp)²P and odd parity (1s2pnd)²D⁰ core-excited levels has been identified by theoretical calculations where relativistic and mass polarisation effects are considered. The fine-structure splittings of these states are also predicted. The experimental results of this work are used to determine team energies for the B III system. Finally a level scheme for a 6.226 nm laser is proposed.

The spectrum of a silver plasma produced by a 1.5 J, 25 ns ruby laser is dominated in the 80-120 AA region by 4p-5s transitions from a number of ion stages. Many lines corresponding to the transitions 4p⁶4d-4p⁵4d5s of Ag XI, 4p⁶-4p⁵5s of Ag XII, 4p⁵-4p⁴5s of Ag XIII and 4p⁴-4p³5s of Ag XIV have been identified with the aid of Hartree-Fock calculations. Where possible, isoelectronic extrapolations were also used to assist the assignments.

The absorption spectrum of neutral antimony vapours has been photographed in the spectral range 1425-2530 AA with a 3 m normal incidence vacuum spectrograph using a flash pyrolysis technique. Over 400 spectral lines have been classified as a combination between 5 odd parity levels of the ground-state configuration and 141 even parity levels. One long and four short series of the 5s²5p²(³P_0,1,2)ns configurations are listed. Fifty-eight new levels have been added to the previous analysis.

New high-resolution measurements of the 5p photoabsorption spectrum of Ba I are reported. The spectral dispersion is fifteen times higher than in the earlier investigation of the same spectrum performed in the laboratory and enables the authors to extend and improve the classification of the complex structure into overlapping Rydberg series. The wavelength calibration was also improved with respect to earlier work by superposing the photoabsorption spectra of He and Ne, which contain many sharp and accurately known transitions, thereby achieving identical illumination of the grating for both the spectrum under investigation and the calibration lines. The continuum source was provided by the 500 MeV electron accelerator in Bonn.

Absorption of radiation by neutral atoms and molecules, and by positive ions, gives infinite series of spectral lines converging to the first ionisation threshold, and photoionisation with infinite series of resonances converging to higher thresholds. Quantum defect theory has been used in many previous studies of such processes. The authors present a systematic treatment. Results obtained include: (i) formulae which give the dependence of the dipole moment on the effective quantum numbers in closed channels; (ii) formulae, convenient for numerical work, which involve only real quantities; (iii) a parametric formula for the case of two channels, one open and one closed. The parametric formula describes a complete Rydberg series of resonances. It involves a profile factor (x+q)²/(1+x²), where x=tan( pi ( nu + alpha ))/tanh( pi beta ) and where, for the closed channel, nu is the effective quantum number and mu =( alpha +i beta ) is the complex quantum defect. For pi beta <<1 the resonances are narrow and one recovers the formula of Fano which has x=(E-E₀)/¹/₂ Gamma where E₀ and Gamma are such that the scattering matrix has a pole at an energy E=E₀-¹/₂i Gamma .

For pt.III see J. Chem. Phys., vol.78, no.78, p.2180 (1983). The fluorescence throughout the VUV and violet regions has been observed from photoexcitation of SO₂, and the absolute fluorescence cross sections of the VUV emissions have been measured at selected atomic emission lines and in the synchrotron radiation continuum from 550 to 760 AA. Emissions from the O I 1304 AA and several S I lines account for the fluorescence observed in the VUV region, while the fluorescence observed in the UV and violet regions is due to excited SO and, also, possibly to excited SO₂⁺ and SO⁺. Structure in the VUV fluorescence excitation function is attributed to predissociation of Rydberg states. The VUV fluorescence resulting from an O₂ elimination process, SO₂+hv to S*+O₂, has been observed in the present work although the threshold for this process has not been determined. The threshold corresponding to the total dissociation process, SO₂+hv to S*+2O, has been measured and found to agree reasonably with the expected value calculated from thermochemical data. The processes for the formation of S⁺ are also discussed.

Within the energy-sudden approximation the motion of the target and the projectiles can be decoupled and the transition amplitude for differential inelastic cross sections can be expressed entirely in terms of transition amplitudes involving only orbital quantum numbers. This property naturally lends itself to a purely quantal treatment of the target dynamics and a classical or semiclassical treatment of the relative motion with no further approximation: the point of this separation being that the former usually involves small and the latter large quantum numbers. For rotational excitation of the linear molecules the axial symmetry reduces the problem further to finding a set of one-dimensional transition amplitudes. The authors show how these elements can be combined to produce a simple yet effective and accurate method of obtaining inelastic differential transition amplitudes using only classical trajectories. The method has none of the problems associated with the usual purely classical descriptions; it treats interference effects and tunnelling accurately and handles target quantisation exactly.

Measurements of the electron energy distribution in krypton and xenon afterglow plasmas are used to determine the rate coefficients for superelastic collisions between excited np⁵(n+1)s Kr or Xe atoms and thermal electrons. The rate coefficient for electron impact de-excitation of the state Kr ³P₂ is beta _e2=(1.5+or-0.2)*10^-10 cm³ s^-1, for Kr ³P₁ it is beta _e1=5.2*10^-10 cm³ s^-1, for Kr ³P₀ it is beta _e0=9.8*10^-10 cm³ s^-1, for Xe ³P₂ it is beta _e2=(3.2+or-0.5)*10^-10 cm³ s^-1 and for Xe ³P₁ it is beta _e1<1*10^-9 cm³ s^-1. It is also shown that if the energy of the incident electron is in the range 0-0.2 eV, a binding of this electron with the excited atom in an autodetachment state of the negative ion, followed by its decay, is a quite possible channel of the reaction.

Cross sections for elastic scattering, rotational excitation and vibrational excitation are reported for e-H₂S scattering in the energy range 0.5-10 eV. Calculations have been carried out using the adiabatic fixed-nuclei approach. For vibration the symmetric stretch and bending modes have been considered within the nuclear impulse approximation. The fixed-nuclei scattering amplitudes have been determined for an electron-molecule interaction comprising: (i) a static potential derived from molecular ground-state near-Hartree-Fock functions; (ii) an exchange term in the free electron gas Hara-type approximation; (iii) a parameter-free polarisation potential. The convergence problems associated with scattering by a polar molecule are treated by the angular transformation approximation of Collins and Norcross (1978). The elastic and rotationally summed cross sections exhibit three distinct scattering regions: a threshold peak dominated by the A₁ scattering state; an enhancement at about 2 eV due to the B₂ scattering state-it is possible to extract the resonance parameters E_r=2.22 eV and Gamma =1.31 eV; and a broader enhancement around 6-8 eV mainly due to A₁ scattering. The B₂ resonance is also responsible for a large enhancement in the vibrational cross sections at 2-3 eV. The shape of the rotationally summed and vibrational results are in good agreement with the experimental data of Rohr (1978).

The rate coefficients, beta , for the attachment reactions of electrons with CCl₄, CCl₃F, CCl₂F₂, CHCl₃, Cl₂ and SF₆ have been measured under truly thermal conditions over the approximate temperature range 200-600K using a flowing-afterglow/Langmuir probe apparatus. The beta values obtained at 300K are 3.9*10^-7, 2.6*10^-7, 3.2*10^-9, 4.4*10^-9, 2.0*10^-9 and 3.1*10^-7 cm³ s^-1 respectively. From the variation with temperature of beta for the CCl₂F₂, CHCl₃ and Cl₂ dissociative attachment reactions, activation energies, E_a, of 0.15, 0.12 and 0.05 eV respectively were derived. The beta for CCl₄, CCl₃F and SF₆ are close to their theoretical limiting values within the temperature range investigated. While Cl^- was the only product ion observed for the reactions involving chlorine-containing molecules, both SF₆^- and SF₅^- were observed for the SF₆ reaction. The data obtained are compared with previous data and the separate influences of electron temperature and gas temperature are noted.

Explores the possibility of inducing useful gain on the vibration-rotation bands of homonuclear diatomic molecules by immersion in an electric field. General expressions are set up and two cases in particular are examined, both enhanced by motional line narrowing. A partial inversion in the gas phase favours the O branch of light molecules and the gain at breakdown scales as the square of density; for hydrogen approximately 1% cm^-1 should arise at 30 Amagat. Cryogenic liquids require a complete inversion and are predicted to have strong polarised Q-branch gain. Comparison with collision-induced data reveals that the field induced effect is much stronger.

Scattering of light by an electron embedded simultaneously in a uniform magnetic field and in a microwave field is first analysed within the framework of classical electrodynamics. It is shown that the intensities of the different components of the scattered light can be increased by the presence of the magnetic field. Then coherent quantum states of the electron in the same field configuration are constructed and are applied to the description of the scattering process using semiclassical methods. The results of the classical and semiclassical calculations are compared and discussed and various limiting cases are considered to show the connection with previously published work.