Table of contents

The K alpha ₃/K alpha ₁ intensity ratio has been measured following a procedure that minimises instrumental errors, for platinum and thallium, by means of a detection system consisting of a Ge (LEPS) detector and various electronic devices. The spectra have been analysed with curve-fitting computer routines which include an accurate instrumental function. The final results agree, within errors, with theoretical predictions.

The author presents a quantitative estimate of photon absorption cross sections due to forced dipole excitation in noble gas atoms by intense CO₂ laser pulses. The cross sections for Xe, Kr, Ar and Ne are calculated using a dynamic model in which the electron cloud of a complex atom is treated as an inhomogeneous gas. The cross sections are used to obtain the saturation intensities in noble gas atoms at two CO₂ laser frequencies, h(cross) omega =0.118 and 0.130 eV. A comparison with observed experimental saturation intensities, for an estimated effective interaction time of about 0.2 ps, shows remarkably close agreement between the predictions of the model and the data.

Time-resolved single-photon counting has been used to study the fluorescence of crystalline sodium salicylate excited by 15-150 eV and 4-15 keV photons using the Daresbury synchrotron. A rapid decay is followed by a long tail. The pulse shape changes with photon energy. Time-dependent magnetic field effects are observed. The results are compared with data obtained using fast beta particles.

K-LL Auger electron emission from C³⁺ ions induced by electron capture collisions of C⁴⁺ and C⁵⁺ ions on Xe, H₂ and He atoms at energies of 25 and 100 keV was measured at an observation angle of 0 degrees . The peak of promptly decaying (1s2s²) ²S states directly produced in C⁵⁺-He collisions exhibited a pronounced low-energy tail, which was attributed to post-collision interactions (PCI) with He²⁺ ions recoiling at defined relative velocities. From the peak shape a lifetime of T=1.2+or-0.2*10^-14s was determined for the (1s2s²) ²S state, which is in agreement with recent calculations which give values for T of 1.337*10^-14s.

The collision system CCO²⁺+Ne has been studied by energy gain spectroscopy at a collision energy of 100 eV. Cross sections for capture to final CO⁺ states have been measured, and the authors show how the energy gain spectrum can give information on the state of the CO²⁺ projectiles. It is demonstrated that the present method is superior to other methods when it comes to placing level energies of CO²⁺ on an absolute scale.

Laser-assisted excitation of the 1 ¹S to 2 ³S transition in helium is measured in the threshold region of the cross section.

Angular dependences of the rotational excitation from j_i=1 to j_f=3 of H₂ induced by electrons with a collision energy of 0.6 or 0.2 eV are presented. Comparison with different theories shows that adiabatic-nuclei approximations are no longer valid in this energy range whereas close-coupling and off-shell calculations can qualitatively reproduce the experimental results.

Based on the concept of inner products of operators, a procedure is developed to obtain the contributions of all operators P deriving from perturbation theory as projections on a basis of orthogonal model operators. With respect to such a model operator, an operator P may possess the same spin-angular structure, a different spin-angular structure, but still a non-zero inner product and a zero inner product. Complete second-order expressions for the three-electron parameters T₁ and T₂ are obtained; calculations of these parameters in the III spectra of the iron group elements are presented. The radial integrals associated with the various excitations are evaluated using a complete set of single-particle states. A potential for generating both occupied and virtual states is defined. The Hermiticity of the potential and a physical interpretation of the off-diagonal Lagrange multipliers are discussed. The method is exemplified with a second-order calculation of the two-particle parameters in Co III (3d⁷) and Fe VI (3d³).

By evaluating the Fermi hole function Delta ^alpha (p₁₂), where p₁₂ is the momentum difference mod p₁-p₂ mod , the author investigates the consequences of introducing exchange correlation into the wavefunction in momentum space. Results are presented for the isoelectronic series of Li ²S and Be ¹S. In particular, calculations demonstrate that the probability that p₁ and p₂ will differ by either a small or a large amount is reduced. He has also found that the amplitude of the polarisation curve of the momentum-space Fermi hole is nearly independent of the value of the momentum of the reference electron, being essentially spherically symmetric for small values of p₁. As the value of p₁ increases, a displacement of the centre of the polarisation curve to greater values is observed.

The author presents a number of tensor identities which one considers should be available for future work. They are obtained by a full tensor expansion of the parity non-conserving electron-electron interaction both in terms of a contact interaction and distributed operators.

The author develops an approximate quantal treatment of the ionisation of an excited one-dimensional hydrogen atom by a strong periodic electric field. By representing the wavefunction in terms of an adiabatic basis, ionisation can be viewed as the escape of the electron over a moving barrier; the dominant ionisation mechanism is then excitation to states near the barrier minimum. For large quantum numbers and for low-frequency fields the equations of motion solved are essentially exact during most of the field period; but for a short time, whilst the moving barrier is near its minimum, the author introduces complex energies in order to represent loss to the continuum. The net result is to obtain the relative, but not the absolute, variation of the ionisation probabilities with system parameters reasonably accurately by numerically solving a small number of coupled equations.

The photoabsorption spectrum of bismuth was recorded in the 1900-400 AA region on a 3 m normal incidence spectrograph using the flash pyrolysis technique. Relative photoionisation cross sections of atomic bismuth are obtained in the 1780-430 AA region. The measurements have been used to revise, confirm and extend earlier analyses. Two new series converging on the 6p²³P₂ and four new series converging on the 6p²¹D₂ have been identified.

The evaluation of the binary-collision operator Phi ( omega ), pertaining to collisional redistribution of strong laser radiation, involves the computation of a time-ordered exponential, which only reduces to a scalar exponential in the medium-coupling limit. In this approximation one neglects the alteration of the collisional dynamics due to the presence of the laser field. A two-state atom is considered, for which the authors expand Phi ( omega ) in the ratio of the Rabi frequency Omega to the detuning from resonance. This parameter vanishes in the medium-coupling case, and is small for common gas-phase experiments. It is shown that the correction to medium coupling can again be expressed in a scalar function, which represents the temporal correlation between the interaction potential and the full time evolution during a single collision. The decay constants for collisional transitions between dressed states, which are germane to the width of the spectral lines, appear to be sensitive to the deviation from medium coupling. Quite remarkably, a non-vanishing collisional contribution survives in the limit Omega to 0 of Phi ( omega ).

The 1692 AA mercury and 2125 AA cadmium absorption bands are analysed. The temperature dependence of the profiles is studied and interpreted in the framework of the quasistatic theory. The 1692 AA mercury absorption band is attributed to transitions ending at the ¹ Sigma ⁺_u Rydberg excited-state arising from Hg(7¹S₀)-Hg(6¹S₀). Because the energy curve of the ground state is available, the excited-state potential energy curve is deduced: D_e=4800+or-50 cm^-1 at R_e=3.4+or-0.1 AA. The oscillator strength of the transition is derived to be between 3.2 and 3.5 AA. The analysis of the 2125 AA cadmium absorption band shows that this band is the superposition of two non-resolved absorption bands involving transitions ending at the ³ Sigma ⁺_u and ¹ Sigma ⁺_u Rydberg states arising from Cd(6³S₁)-Cd(5¹S₀) and Cd(6¹S₀)-Cd(5¹S₀) respectively. As the energy curve of the ground state is available, the excited-state potential energy curves are deduced: D_e(³ Sigma ⁺_u)=4910+or-50 cm^-1 at R_e=3.05+or-0.15 AA and D_e(¹ Sigma ⁺_u)=6420+or-50 cm^-1 at R_e=3.0+or-0.1 AA. The oscillator strengths of the transitions are derived to be between 2.4 and 3.5 AA and between 2.2 and 4 AA respectively.

Discrete tuning of the single longitudinal mode over the gain profile of the 457.9 nm Ar-ion laser line has been used in order to directly excite the C¹ Pi _u and 2¹ Sigma ⁺_u states. The authors have observed fluorescence spectra of the interference continuum (2¹ Sigma ⁺_u-X¹ Sigma ⁺_g) and the C-X molecular band together with collisionally induced diffuse-band emission (2³ Pi _g-1³ Sigma ⁺_u). The excitation processes leading to the observed emission are discussed.

Photoionisation spectra for krypton and argon were measured as a function of pressure (p<or=230 Torr) for photon energies below and above the first atomic ionisation limit I_g. The photocurrent spectrum below I_g evolves, with increasing pressure, from peaks at the energies of the atomic transitions, into broad overlapping bands. The upper limit for the photoresponse renders a (pressure-independent) yield function for ionisation with onsets at 12.84 and 14.49 eV for krypton and argon, respectively. The results are interpreted in terms of a simple statistical model for the decay of the Rydberg state, with energies above the onset, into a dimer-ion-electron pair.

The dissociation of the v=0 and v=1 vibrational levels of the c⁴ Sigma _u^- state of O₂⁺ has been reinvestigated using a photoion-photoelectron coincidence method. A different branching ratio for the fragmentation into O⁺(⁴S⁰)+O(³P) and O⁺(⁴S⁰)+O(¹D) has been observed for ¹⁶O⁺₂ and ¹⁸O⁺₂ in the v=0 level. This isotope effect shows that the predissociation to the O⁺(⁴S⁰)+O(¹D) adiabatic limit is mainly due to the tunnelling through the c-state potential barrier; no spin-orbit or rotational interaction seems to be involved.

The alignment of the L₃ subshell has been determined from the measured angular distribution of the L_l X-ray line of gold ionised by 0.3 MeV protons and 0.4-0.66 MeV deuterons. For deuteron impact the absolute value of the alignment parameter has been found to be larger than earlier results for proton and helium projectiles of the same velocities, indicating the presence of a projectile atomic number dependence of L₃-subshell alignment at these low projectile velocities. The ratios of L-subshell ionisation cross sections for Sm, Er and Au by 0.2 MeV u^-1 proton and deuteron impact and those for Au by 0.3 MeV u^-1 proton and deuteron impact are also reported. The Coulomb deflection factor deduced from these data has been compared with different types of Coulomb deflection factors, and it is found to be in good agreement with the Coulomb deflection factor of Anholt et al (1979) and that of Brandt and Lapicki (1979).

Projectile soft X-ray spectra (1< lambda <10 nm) emitted after a charge-exchange collision between slow (v approximately=0.4 a.u.) N⁷⁺, O⁸⁺ and Ne⁸⁺ on He and H₂ targets have been observed. These spectra allow an evaluation of the n and l distribution of projectile excited states formed by a single electron capture. The spectra in the case of O⁸⁺ and N⁷⁺ bare projectiles show evidence on double capture processes followed by autoionisation. Good agreement is obtained with theory or alternative experimental data when available.

Experiments related to the electron-potassium-atom momentum transfer cross section Q(v) are considered in which the electrical and electron thermal conductivities are measured in equilibrium and non-equilibrium pure potassium and argon-potassium plasmas. Measurements of differential cross sections are also taken into account. A convenient algorithm is used to vary Q(v) until the best fit to all data is obtained. The result represents a stable solution of the problem, so that the RMS deviation of the experimental values from the calculated ones is 20%. Values of Q(v) are recommended in the range 0.06 to 2 eV which can be used for calculations in the range of electron temperatures from 1300 to 3000 K.

Electron impact double ionisation cross sections for Ar²⁺, Ar³⁺ and Xe⁺ have been calculated in the modified binary-encounter model incorporating the effects of the Coulombic field of the target. Contributions of inner-shell ionisation and Auger emission have been included in the calculations. The calculated cross sections are in good agreement with the available experimental and theoretical results. The probable reasons behind the discrepancies observed between the experimental and present cross sections have been discussed.

The absolute triply differential cross section for K-shell ionisation by 500 keV electron impact for high atomic number (Z=73) was measured as a function of the energy sharing between outgoing electrons. By using a two-parameter set-up, electron-electron coincidences in a wide energy sharing range could be registered in the same run. Scattering angle pairs of 40 degrees /-20 degrees and 30 degrees /-30 degrees , respectively, were used in a coplanar geometry. The results show the inadequacy of the theory of Das and Konar (1974) for the present parameters.

Differential and total cross sections for excitation of the 1 ¹S-2 ¹P transition in helium under positron impact were calculated using a distorted-wave approximation. The authors have also evaluated the correlation and orientation parameters for this transition. The results are compared with previous theoretical calculations and with available experimental data. Agreement with the experimental results is poor and they question whether the correct interpretation of the experimental data has been given.

The diffusion coefficient of Li in Ar was measured at T=563 K. Free Li atoms were produced in a line source by laser photodissociation of LiI in an isothermal cell. The time change in the Li concentration was monitored by resonant laser ionisation of free Li atoms contained in a laser beam volume concentric with the initial line source. The product of the diffusion coefficient D and pressure p has the value pD=12.3+or-1.9 N s^-1 (923+or-143 Torr cm²s^-1). This is in very good agreement (5%) with a value predicted by a semi-empirical calculation.

The line profile of H_alpha is measured in plasmas produced in liner flashtubes. The electron densities are in the range 6*10¹⁷-10¹⁸ cm^-3 measured by independent methods. The full widths at half maximum of H_alpha are given. At N_e=10¹⁸ cm^-3, the experimental and the theoretical profiles are compared. The red shift is measured and varies linearly with the electron density, in agreement with some theoretical work.