Table of contents

Far-configuration-mixing effects on hyperfine-structure (HFS) off-diagonal elements are computed in Sc I and Ti I using the multiconfigurational Hartree-Fock method. They are about two orders of magnitude larger than the Hartree-Fock values of the pure-configuration off-diagonal elements. Taking them into consideration improves the HFS interpretation in the 3d4s² configuration of scandium I.

Previous study of the eigenvalue spectrum of time-dependent Hartree-Fock theory has involved the examination of the poles of linear response functions obtained by solving coupled perturbation equations. It is shown how the standard method for finding eigenvalues of a single differential equation may be applied in generalized form to the coupled eigenvalue equations to yield the eigenfunctions themselves rather than the matrix elements. A calculation is given for He(1s2p)¹P.

Highly excited Rydberg and autoionized levels were investigated by using selective absorption spectroscopy and the space charge amplification method for detecting ions. The one-photon absorption spectrum from the ground state 6s²¹S₀ has been recorded by scanning the emitted continuum from a mercury lamp with a 2 m Czerny-Turner monochromator. With the allowed resolution of 0.1 AA, 6snp ¹P₁⁰ Rydberg states were observed up to n=45. Furthermore, two-photon absorption spectroscopy was performed using a 0.150 cm^-1 tunable dye laser radiation over the range 4200-5600 AA. Even-parity states J=0 and J=2 belonging to the series 6sns and 6snd up to n=46 and n=77, respectively, and new autoionized levels belonging to 5dns and 5dnd configurations were observed.

Measurements have been made of time-resolved fluorescence from n²S_1/2 and n²D_3/2 states of ¹³³Cs (n=8 to 14) over a range of vapour densities covering the onset of collisional depopulation. Lifetimes for the free atom and depopulation cross sections are deduced. Lifetimes are proportional to (n*)^x with x=2.52+or-0.06 (S states) and 2.62+or-0.06 (D_3/2 states). Cross sections are proportional to (n*)⁴. Hyperfine-structure interaction constants for the ²D_3/2 states determined by the method of quantum beats agree with the recent results of Belin et al. (1976).

Feneuille and Schweighofer (1975) have used a three-level model to discuss the observation of the Autler-Townes effect in a two-step resonance experiment. A numerical investigation is presented in which the effect of the neighbouring non-resonant hyperfine levels is taken into account. Firstly all the levels are treated as non-degenerate, and then degeneracy is allowed for. It is found that both the inclusion of the neighbouring states and allowance for their degeneracy cause a large change in the expected upper-state population, especially at high light intensities. It is concluded that the resonant three-level model is a poor model in this situation.

The zero-energy Born exchange amplitude for electron-helium scattering in the forward direction has been calculated using a multi-parameter Hylleraas-type wavefunction. Up to 125 terms are used and the result is extrapolated to be 3.9430+or-0.0016. The present calculation substantiates the conclusion reached in several recent papers that the zero-energy form of the Gerjuoy-Krall dispersion relation (1960, 1962) does not hold for e^--He scattering.

An attempt is made to reproduce theoretically the results of recent measurements of low-energy positron-helium total cross sections by Kauppila et al. (1976) which disagree with the earlier experimental results of Canter et al. (1973) for positron energies below 6 eV. Positron-helium s-wave phaseshifts are calculated for a 14-term helium wavefunction. The results are only very slightly different from those obtained by Humberston (1978) for a less accurate helium wavefunction, and give cross sections in good agreement with the values of Canter et al. A significant discrepancy therefore remains between the theoretical cross sections presented here and the experimental results of Kauppila et al. which, it is claimed, cannot be resolved by further improvements in the theoretical results.

He⁺-He collisions are studied in the range of low energy and small scattering angle. The essential features observed experimentally by Brenot et al. (1975) and by Lorents et al. (1966), such as perturbations in the elastic and resonance charge-exchange differential cross sections and the phase, relative amplitude and threshold behaviour of the differential cross sections for excitation, are reproduced. A quantum-mechanical molecular treatment of the collision, including the lowest diabatic ² Sigma _g and ² Sigma _u states and the first excited ² Sigma _g state which dissociates into He⁺(1²S)+He(2³S), is considered.

Results of a first application of the variable-screening model in scattering calculations for slow ion-atom collisions are presented. Adiabatic molecular orbitals generated from the model's effective one-electron Hamiltonian are employed in a coupled-state impact-parameter calculation of cross sections for K-shell excitation in Ne⁺+O collisions. The reliability of these orbitals is assessed by comparing these results to similar results based on Hartree-Fock orbitals and orbitals corresponding to the Coulomb field of two effective nuclear charges.

Close-coupling quantal calculations of the fine-structure excitation of K 4p_1/2 due to collisions with helium are performed with a Baylis potential (1969) and a Pascale and Vandeplanque potential (1974). The discrepancy with the most recent experimental results is not removed, but measurements at higher temperatures could be a test of the interatomic potentials.

The authors report observations of asymmetric non-Lorentzian wings of impact-broadened lines of ionized emitters in a high-density plasma. The only explanation appears to be a quantum-mechanical interference effect. The effect may be of consequence in opacity calculations in dense plasmas.

Considers the transfer of optical coherence from one level pair to another. The effect is isolated from the general collision expression derived by Berman (1976) and it is argued that it is best observed in transient measurements. The free decay of a four-level system is solved analytically and two regimes are found: one with oscillations displaying a beat and another with two decay rates. At the border the exponential dependence is modified. The implications for free induction decay and echo experiments are presented. Both phenomena can be used to detect the collisional transfer of coherence, and some experimental aspects of their observation are discussed.

Calculations are presented which include exactly the effects of fringing fields on the electric-field-induced quantum beats of atomic hydrogen. The results explain some recently observed phaseshifts of the quantum beats relative to the predictions of the 'sudden approximation'. The general features produced by finite rise-time and overshoot are discussed.

The Z/(Z+1) approximation for the calculation of Auger electron energies is examined, and, in its present form, it is shown to be inappropriate for calculations of free atoms. A modified formula, using removal energies of free ions of charge Z+1, is as shown to be generally superior to the traditional formula, and is usually comparable in accuracy with Hartree-Fock calculations for Auger transitions of the order of 1 keV or less. For cases involving two final-state holes in different shells transition energies of low-energy Auger transitions can be estimated to about 1 eV.

Observations of the 3d photoabsorption spectra of isoelectronic Eu²⁺ and Gd³⁺ are reported. Their theoretical positions and relative intensities based on the 3d¹⁰4f⁷ to 3d⁹4f⁸ transition array are given and found to be in good agreement with the experimentally observed principal features. The 3d photoabsorption of the trivalent Eu is discussed and compared with that of trivalent Yb.

The Hanle effect of the resonance line 326.1 nm (5¹S₀-5³P₁) of Cd has been studied under the influence of a magnetic RF field rotating with frequency omega ₀ in a plane perpendicular to the static magnetic field H. This non-resonant RF field causes a shift of the Hanle signal that is large compared with the natural linewidth for strong RF fields. The measured line shifts are in good agreement with the semiclassical theory of Dodd and Series (1961). As predicted by the theory a coherence resonance could be observed near H approximately=-H₀=- omega ₀/ gamma .

The hyperfine-structure interaction constant of 6s²6d²D_3/2 state of thallium I has been established as having a negative sign. Its magnitude has been measured to be 41.8+or-1.1 MHz. The mean lifetime of this state has also been measured by the Hanle effect. The result 6.8+or-0.4 ns is in excellent agreement with other values quoted in the literature, derived by other methods.

By means of photoelectron spectroscopy and a synchrotron radiation source, the branching ratios for the production of the ²D_3/2 and ²D_5/2 states of the Xe⁺ ion have been determined in the energy range 70-130 eV. When combined with earlier measurements on the absolute partial cross section for the 4d photoionization, absolute cross sections have been obtained for the separate spin-orbit components. The ²D_5/2:²D_3/2 branching ratio exhibits a variation with energy that deviates from nonstatistical values that are predicted by recent theoretical calculations.

The time evolution of the ionization probability of a simple one-dimensional model atom under the influence of a harmonic electric field is studied by the numerical solution of the time-dependent Schrodinger equation. The conditions are clarified under which tunnelling or multiphoton absorption may be regarded as appropriate mechanisms. The standard tunnelling formulae are found to give poor quantitative estimates of ionization probability. The duration of the radiation pulse is found to be of critical importance, as multiphoton absorption is seen to require a certain minimum time to develop.

Differential elastic cross sections for e-H(1s) collisions at impact energies between 30 eV and 200 eV have been calculated by semiclassical and quantal methods. Both methods are 1s-2s-2p close-coupled with exchange neglected, and so differ only in the collision treatment. The discrepancy between the semiclassical and quantal results increases as scattering angle theta increases and decreases as impact energy increases. The semiclassical results are inferior to the first Born approximation when the momentum transfer exceeds about 0.5 au.

The spin polarization of electrons elastically scattered by a xenon atomic beam has been measured between scattering angles of 20 degrees and 155 degrees for electron energies of 150, 200, 300, 400, 550, 700, 900 and 1200 eV. The pressure dependence of the depolarization due to multiple scattering was investigated, and the influence of the angular resolution on the measurements is discussed. Comparison is made at all energies with Walker's calculations (1971) based on the solution of the relativistic Dirac equation with and without electron exchange. The agreement between theory and experiment is improved when exchange is fully included, but the importance of exchange decreases towards higher energies, although it makes a significant contribution at 700 eV.

A method is presented for the calculation of the radial coupling matrix elements which occur in the molecular approach to atomic collisions. The formalism is based on the earlier work of Bratos (1958) and provides a local method of evaluating analytically radial couplings between approximate wavefunctions when a Gaussian basis is used. The cases of CI and SCF molecular calculations are treated explicitly. Some examples of strong and weak radial couplings are chosen to show the efficiency of the procedure and an approximate formula is tested for the case of coupling between SCF orbitals.

The choice of free electronic wavefunctions in the description of K-shell ionization by protons is discussed. The previously known discrepancies between PWBA and SCA results are shown to be entirely due to two different choices of electronic wavefunctions. Calculations in the SCA framework with Hartree-Fock-Slater wavefunctions are reported. Some general features of the SCA calculations are discussed.

Cross sections are computed for the low-energy fine-structure excitation of singly ionized carbon (²P electronic ground state) by atomic hydrogen (²S electronic ground state) in a close-coupling formulation; recent accurate potential energy curves are employed. The effects of rotational coupling and of the fine-structure energy defect are investigated. The excitation cross section is found to be larger than previously computed. The cooling function is calculated for temperatures of astrophysical interest.

The radiative electron capture (REC) for ³²S ions in the energy range from 5 to 115 MeV is investigated using several targets. Angular distributions of REC X-rays are studied at bombarding energies of 20, 30 and 115 MeV with beryllium, carbon and nickel as targets. With the latter target an excitation function is also measured in the energy range from 5 to 115 MeV, as well as the dependence of the position and the width of the REC peak on the incident energy. The experimental results agree with those expected from a dipole REC process.

Differential elastic scattering cross sections for a beam of metastable neon (Ne*) incident on ground-state Ne atoms have been measured in the centre-of-mass energy range 7-53 eV. Two peaks identified as rainbow maxima were observed at tau =E theta approximately=45 and 67 eV deg. Oscillatory structure identified as coherent scattering on a gerade-ungerade (g-u) pair of potential curves was also observed. Use of the calculated potentials of Cohen and Schneider (1974) and a semiclassical theory for g-u oscillations allows observed features in the cross section to be attributed to particular molecular states, but difficulties due to complicated interference between scattering on the relevant potentials preclude any unique determination of improved potentials without supplementary information.

The effects of nuclear spin on the observed relaxation constants of ²P_1/2,3/2 alkali-metal resonance levels are calculated. While the primary interest is in obtaining correction factors for the Hanle effect measurements, the nuclear spin effects on experimental measurements by other techniques are also discussed and it is shown that some apparent discrepancies in the literature may be resolved. The experimental data for sodium and rubidium perturbed by the rare gases are discussed critically as examples of small and large fine structure respectively.

The A² Pi -X² Pi system of gaseous AgO has been photographed in emission from a hollow-cathode discharge. Following the rotational analysis of the ² Pi _3/2-² Pi _3/2 component of the 1-0 band and the assignment of new bands at the long-wavelength end of the system to nu '=2, it is found that state A has only a shallow potential minimum. Although in A² Pi , B₀(² Pi _3/2)<B₀(² Pi _1/2), as expected for an inverted state, the anomalous order in the ground state B₀(² Pi _3/2)>B₀(² Pi _1/2) is confirmed. Nevertheless it is concluded that most probably the ground state is an inverted ² Pi state.

A theory of motional narrowing is presented which is the basis for further study of the narrowing problem by computer simulations. For a general inhomogeneous z component of magnetic field h(r) with Delta ² identical to ( gamma ²h²), predictions are made for the half-width Gamma approximately= Delta ² tau _c and the frequency shift delta approximately -( gamma ³h³) tau _c² where tau _c is the correlation time. The lineshape is traced from the region of good averaging, Delta tau _c<<1, through the region of poor averaging, Delta tau _c>1.

For pt.I see ibid., vol.10, no.5, p.931 (1977). A simple model applicable to a class of high-resolution Zeeman spectroscopy experiments is simulated using Monte Carlo techniques. For individual Legendre polynomial field gradients, h approximately rⁿP_n with n=1-4, the authors find the autocorrelation function (h( tau )h(t+ tau )), correlation time, relaxation function and lineshape for a range of the narrowing parameter 0.1<< Delta tau _c<<10.0. The bounds placed on the correlation time, _min tau _n<or= tau _c<or= tau ₁, prove to be useful in estimating the width and frequency shift in arbitrary field gradients.

For pt.II see ibid., vol.10, no.5, p.941 (1977). A method using the field distribution function f(h) in the absence of motion is presented which allows an estimate of the frequency shift accompanying motional averaging for practical experimental situations. Analytic calculations of f(h) are made for the Legendre gradients rⁿP_n(cos theta ), n=1, 2 and 3. Monte Carlo plots of f(h) are presented for a field characterized by a sum of both n=1 and 2 Legendre gradients. Relaxation functions obtained by Fourier transformation of the analytic f(h) expressions show good agreement with simulation results.

A simple model, combining the Born approximation and classical perturbation theory, is derived for small- and moderate-angle scattering ( theta <or=60 degrees ) into all states. Agreement within 33% is obtained with close-coupling and Glauber calculations. Comparable agreement is obtained with the recoil measurements on CsF, CsCl and KI but poor agreement is found with the recent relative measurements on KI by Rudge, Trajmar and Williams (1976). Using a hard-sphere model for wide-angle scattering, simple formulae for the total and momentum-transfer cross sections into all states are derived.

The mobilities of atomic neon ions in the ²P_1/2 and ²P_3/2 states have been measured in neon at 77, 78 and 294K and compared to the theoretical predictions of Cohen and Schneider (1975). The absolute values of the mobilities have been found to disagree with those calculated by as much as 10% which is well outside the experimental uncertainty. Theory and experiment agree that (i) at low average centre-of-mass energies the mobility of Ne⁺(²P_1/2) in Ne is smaller than that of Ne⁺(²P_3/2) in Ne, (ii) the difference in the mobilities decreases as the ion energy increases and (iii) both ion species show a negative temperature dependence of their mobility.