Table of contents

The excitation threshold and oscillator strengths for the 3p⁶3d²D to 3p⁵3d²²P⁰ transition in Sc²⁺, Ti³⁺ and V⁴⁺ ions of the potassium sequence have been calculated using both Hartree-Fock (HF) and configuration interaction (CI) wavefunctions. For Ti³⁺, the HF length value of the oscillator strength (f_l) is close to the recent value of Griffin et al. (1982), although for all three ions, the HF velocity value (f_v) is only about half the length value. The CI values of f_l and f_v are in much closer agreement with each other, but differ substantially from the HF values. This demonstrates the importance of including CI for these transitions. The authors predict the positions of the multiplets 3p⁵3d²(¹D)²P⁰ and 3p⁵3d²(¹S)²P⁰ in V⁴⁺ to be 317500 cm^-1 and 376000 cm^-1, respectively, above the ground state.

From the spectroscopic data of non-penetrating Rydberg states of C I, N I, O I and Ne I the authors have extracted the polarisability and the quadrupole moments of the ionic core. The values so obtained from different Rydberg nl levels are mutually consistent to within 1%. Comparison with the values from ab initio calculation shows agreement to a few per cent. Further, some empirical relations between certain Slater integrals are shown to be the direct consequence of our treatment.

Experiments on collisionally assisted two-photon absorption (TPA) in calcium vapour buffered by argon gas have been performed using two tunable dye lasers. Both direct TPA and stepwise transitions through the collisionally populated resonance level. 4s4p ¹P₁, have been observed. Detailed study of the lineshapes of the stepwise transitions to the 4p²¹S₀, 4p²¹D₂ and 4s6s ¹S₀ levels reveals large differences in the strengths of the Ca-Ar interatomic potentials for these levels. These differences probably arise from configuration mixing.

Introduces a simple model of laser-induced negative ion photodetachment, incorporating the Wigner power law, in order to discuss non-perturbatively the effects of strong exciting laser fields near to threshold. The authors find that the threshold can be smeared out and shifted slightly, as might be expected. More interesting is the possibility of observing power law (t^-3) and other deviations from the exponential decay of bound electron probability near to threshold.

The authors have demonstrated the production of molecular Rydberg state in K₂ by means of two-step photoexcitation with help of thermionic detection in a disc-shaped heat-pipe oven. Preliminary results concern a spectroscopic investigation of the very high 'Rydberg region'. Results of the pressure shift of Rydberg lines are also presented.

Theoretical estimates of ionisation and charge exchange cross sections confirm an earlier suggestion that the production of Li²⁺ in collisions between Li⁺ ions at intermediate energies takes place chiefly via ionisation.

Radiative transitions of electrons to the K shell in the supercritical atom Z=188 are investigated. This system may be transiently produced in ₉₂U+₉₆Cm collisions with bombarding energies close to the nuclear Coulomb barrier. For the total transition time the authors obtained tau =8.8*10^-19s. The dominance of transitions with magnetic multipolarity is pointed out.

Screening by the centrifugal field condenses the effect of molecular anisotropy into a few channels of the single-centre adiabatic partial-wave expansion of the low-energy scattered electron wavefunction. Applications to e+N₂ and e+HCl are discussed.

The energies and wavefunctions of thirteen doubly-excited and nine triply-excited resonances of Be⁺ are calculated using the saddle point technique. These results are used to make identifications in the Be⁺ Auger spectra observed by Rodbro et al. (1979). For all within the experimental uncertainty quoted. For the triply-excited resonances, fourteen Auger lines are identified. These identifications are compared with those found for triply-excited lithium. The strong similarities found between these systems provides supporting evidence that both identifications are probably correct.

In l^Ns configurations the coupling of the s electron causes each parent term of the l^N core to split into two Russell-Saunders terms. Far-configuration mixing ('crossed-second-order effects') leads to different electron charge densities at the nucleus for these two terms and thus to different field isotope shifts. With the Hartree-Fock computer code of Froese Fischer (1976), the difference in the non-relativistic electron charge densities of those pairs of terms belonging to the parent with the highest multiplicity and the highest L value, was calculated for the configurations np^N(n+1)s (n=2, 3, 4, 5, 6), nd^N(n+1)s (n=3, 4, 5), and nf^N(n+2)s (n=4, 5). In the d series the crossed-second-order effect is generally an order of magnitude larger than in the p and f series with the same principal quantum number. The crossed-second-order effect increases with n and is the same for isoelectronic sequences. There is good agreement between theoretical and experimental field shift differences.

Electric-field-induced higher-order anticrossing signals S-D and S-F were studied in the fine structure system n=4 of ⁴He⁺. The following fine structure intervals were measured (theoretical values by Erickson in brackets): 4²D₃2/-4²S₁2/:20143.8+or-3.7(20143.27) MHz; 4²D₅2/-4²S₁2/:27454.7+or-1.5(27459.02) MHz; 4²F₅2/-4²S₁2/:27437.9+or-2.4(27446.06) MHz; 4²F₇2/-4²S₁2/:31100.5+or-1.7(31103.84) MHz. Combining these values further intervals are obtained: 4²D₅2/-4²D₃2/:7310.9+or-4.0(7315.75) MHz; 4²F₇2/-4²D₅2/:3662.6+or-3.0(3657.78) MHz; 4²D₅2/-4²F₅2/:15.8+or-2.8(12.96) MHz. In most cases experiment and theory differ by more than one standard deviation.

The Stark shift of electric field induced anticrossing signals 4S-4D and 4S-4F has been measured for the hydrogen-like He⁺ ion. An electric field range of approximately 50-200 V cm^-1 was covered, where the Stark shift is small compared with the fine structure, and therefore, a predominantly quadratic function of the electric field. Within the experimental accuracy of up to +or-2% the measured Stark constants are in agreement with the theoretical values obtained from diagonalisation of the energy matrix of the n=4 fine structure system in combined electric and magnetic fields.

Absorption line profiles of neutral argon spectral lines have been investigated using a GaAlAs diode laser as a light source. The broadening and the shift coefficients for the 811.5 nm line, which terminates on the lowest metastable state 1s₅, have the values (1.42+or-0.14)*10^-20 and (4.4+or-0.4)*10^-21 cm^-1 cm³, respectively. From the broadening coefficients for the 810.4 and 840.8 nm lines, which terminate on the resonance states 1s₄ and 1s₂, the oscillator strengths for the resonance transitions at 106.7 and 104.8 nm have been determined to be (49+or-5)*10^-3 and (207+or-20)*10^-3, respectively. The shift coefficient for the 810.4 nm line was about a half of the value for the 811.5 nm line and no shift was observed for the 840.8 nm line in accordance with the theory of Lewis (1967).

He atoms excited to the 1s3d ¹D and 1s4d ¹D levels were produced by collisions of 10-40 keV He⁺ ions with He atoms and the splitting of the magnetic depolarisation signals of the impact radiation in electric fields was investigated. The authors measured amplitude ratios of the observed level-crossing signals, which are equivalent to the ratio sigma ₀⁽⁴⁾/ sigma ₀⁽²⁾ of the multipole components of the excitation matrix. Using excitation cross sections and polarisation fractions measured by de Heer et al. (1965), the excitation cross sections of the Zeeman sublevels of the 4¹D levels are evaluated. By introducing a parametrical plots of the excitation matrix the energy dependence of the excitation cross sections sigma _{mod m mod} of the Zeeman sublevels as well as of the multipole components sigma ₀^(k) is displayed.

Photoionisation by a strong laser tuned to the neighbourhood of an autoionising level is discussed. An explicit model for the continuum dipole strength is used which reproduces the important features of a real atom. Within the context of this model the problem is solved exactly. The exact solution is related to the usual pole approximation via a Taylor expansion which also yields an improved pole approximation. Population trapping is observed to occur and the general condition for it is derived. An interpretation of population trapping as the strong field relic of the Fano minimum is given. Some remarks are made on the consequences of population trapping for electron-ion scattering in the presence of a laser.

The authors study doubly resonant multiphoton ionisation induced by two laser fields. The first field produces a near-resonant multiphoton transition between the ground state g and a discrete state 0. The second field is nearly resonant with a one-photon transition between 0 and an autoionising state 1. The spontaneous emission of both excited states is taken into account. The authors discuss and compare the spectral profiles expected for ionisation probability and fluorescence from state 1 when the first step laser is scanned.

The authors examine the problem of two-photon excitation of a three-level detector atom by antibunched radiation from a strongly driven two-level source atom. They show that two-photon transitions are possible and that a commonly used effective two-photon coupling model has to be used with caution in quantum optical problems.

The absorption spectrum of a weak probe radiation by a two-level atom strongly driven by an intense radiation field is examined. Using a Green's function method developed for calculations of general multiphoton processes, the author obtains some earlier results in which the spectrum has regions (in the probe frequency domain) of amplification and regions of attenuation. Such a spectrum is shown explicitly to be due to a coherent interference effect arising from the interaction of the atom with the vacuum field. Furthermore, an equation that relates the area under the amplification part of the spectrum to that under the attenuation part is obtained. This indicates that amplification cannot occur if a very-wide-band probe beam is used instead of a narrow-band one. At high driving intensities the absorption spectrum is shown explicitly to be inversely proportional to the Rabi frequency and has a dispersion-like behaviour near the Rabi sidebands at driving resonance.

The vibrational and rotational structure in the D-X system of the HgCl molecule has been studied by photographing it in the third order of a 10.6 m concave grating spectrograph. Rotational constants in the (0, 0) and (0, 1) bands have been determined from the partially resolved rotational structure. Variation of intensity in the bands of different sequences have been explained on the basis of Franck-Condon factors estimated from Morse curves for the two combining states. Vibrational constants for the isotopic molecule HgCl³⁷ have also been determined.

Various comments by Matsuzawa (1981) on validity criteria for application of the impulse approximation to A-B(n, l) collisions are examined and are shown to be without basis. The weak-binding and quasi-classical conditions are discussed and a connection between classical momentum and angular momentum changes is developed.

Coupled-channel calculations have been carried out for p+Li⁺ collisions in the 70 to 400 keV amu^-1 energy range and for p+Li²⁺ for energies between 50 and 200 keV amu^-1, in the authors' 'perturbative one-and-a-half-centre' (POHCE) formulation. This work includes the first test of the POHCE method for a system with more than one electron. Various effective one-electron target potentials for Li⁺, and their effect on the scattering cross sections, are discussed. The authors' p+Li⁺ cross sections for capture and ionisation, as well as their sum (total electron loss), are compared with the experiments of Gilbody's group and with other theoretical calculations. Their ionisation result is in good agreement with experiment; it would have been up to 50% too high without the POHCE modification of the single-centre method. Their capture result is lower than the measured cross section, as it was for p+He⁺. By time reversal their calculations also give capture cross sections for Li²⁺+H and Li³⁺+H, for capture into each state in the lithium-centred part of the basis.

The authors have studied electron capture processes into excited states for ⁴He²⁺ ions (15-150 keV) colliding with Li atoms, by means of optical methods. From the measured emission cross sections they derive state-selective capture cross sections into various nl states of the He⁺ ion formed during the collision. At low energies (E_CM<or=20 keV) the n=3 states are very selectively populated. The distribution over 3s, 3p and 3d states is statistical (within the experimental errors). A detailed comparison is made with the theoretical values of Bransden and Ermolaev (1981). The overall agreement between theory and experiment is very good.

Emission cross sections have been measured for the 2p to 2s resonance line and the 3d to 2p line of lithium produced by He²⁺ ion impact in the energy region of 15-600 keV. For the 2p level the corresponding excitation cross sections are evaluated. The cross sections obtained are compared with experimental and theoretical data for electron impact at the same velocity. In general the cross sections for electron impact are relatively the largest.

Differential, integral and momentum transfer cross sections have been determined for the elastic scattering of electrons by CH₄. With the help of a crossed electron beam, molecular beam apparatus using the relative flow technique, the ratios of the elastic differential cross sections (DCS) of CH₄ to those of He were measured at electron impact energies of 3, 5, 6, 7.5, 9, 10, 15, and 20 eV for scattering angles from 30 to 140 degrees . From the ratios, the absolute DCS were determined by utilising the known DCS of He. Because pure rotational excitation could not be resolved, the elastic DCS are the sum of elastic and pure rotational excitations at room temperature. Integral and momentum transfer cross sections were calculated from the DCS. The CH₄ DCS are similar in size and in angular behaviour to those of Ar at impact energies below 7.5 eV, and also agree approximately with a close-coupling calculation for electron scattering from polyatomic molecules. A broad maximum in the integral and momentum transfer cross sections is located at 7.5 eV. The angular distributions at energies up to about 7.5 eV are characteristic of a d-wave type of resonance.

The five-fold differential cross section for the (e, 2e) reaction is derived using a relativistic plane-wave Born approximation. It is shown that (e, 2e) reactions with electron impact energies of a few MeV should provide a sensitive test of the wavefunctions of the inner shells of heavy elements. Calculations are presented for the K shells of gold and thorium using relativistic and non-relativistic Hartree-Fock theories. These show that relativistic effects are significant in the bound-state wavefunctions of heavy elements, and that they can be measured in a measurement of the differential cross section.

The direct and indirect (i.e. autoionising) contributions to the collisional ionisation cross section of N²⁺ have been calculated in the Coulomb-Born and distorted-wave approximations for incident electrons with up to twice the outer-shell ionisation energy. The results of these two methods differ by less than 10%. Autoionisation is found to exceed the direct contribution by about a factor of two in agreement with the approach suggested by Burgess et al. (1977). The present results are compared with those of other theoretical calculations and with experimental data.

The authors have measured the spin-dependent ionisation asymmetry in electron impact ionisation of sodium over the energy range 5.6 to 37 eV in a crossed beam experiment with polarised electrons and polarised sodium atoms. The observed ionisation asymmetry A=( sigma ^{up arrow down arrow} - sigma ^{up arrow up arrow} )/( sigma ^{up arrow down arrow} + sigma ^{up arrow up arrow} ) reaches a peak value of approximately 0.30 and shows relatively little variation in energy up to three times the ionisation energy. Then the ionisation asymmetry starts to decrease with increasing electron energy. From the ionisation asymmetry the authors obtain information about the ratio of the interference to the total ionisation cross section and also about the ratio of the triplet to the singlet ionisation cross section. A comparison of the authors' measurements is made with theoretical results of a Born approximation and predictions following from threshold laws.

A new procedure is proposed to derive an analytical expression of the high-energy limit of the differential cross section for small-angle electron-molecule scattering. An integral transform of the spherical Bessel functions, which appear in the multipole expansion of the Born series, allows one to evaluate the second Born integrals analytically without explicit use of wavefunctions.