L. Sarkadi

Coupled-states model calculations of L-subshell ionisation probabilities for Ne on Yb, Pt collisions

The deviation from first-order perturbation theory in the description of the impact parameter dependence of L-subshell ionisation probabilities has been estimated by model calculations of 0.9 MeV amu-1 Ne on Yb and Pt collisions. The applied model takes into account the strong distortion of the L shell by solving a truncated set of coupled equations which describe the time evolution of the L-substate amplitudes. According to the model the subshell couplings lead to a large reduction of the L1-subshell ionisation probabilities, but they alter the L3 probabilities to a lesser extent. As a result of crossing of the probability curves predicted by the relativistic first-order theory at small impact parameters disappears, in accordance with the experimental observation of Dexheimer et al (1986). The possibilities of further improvements in the theoretical description are discussed. The evaluation procedure applied by Dexheimer et al to extract the ionisation probabilities from the measured X-ray yields is also analysed as a source of the remaining discrepancies between experiment and theory.

Measurements of L X-ray production and subshell ionisation cross sections of gold by light- and heavy-ion bombardment in the energy range 0.4-3.4 MeV

The L X-ray production and subshell ionisation cross sections of gold have been measured by proton, alpha particle, carbon, nitrogen and oxygen bombardment in the energy range 0.4-3.4 MeV. These data are compared with the predictions of the plane-wave Born approximation including the binding energy, Coulomb deflection and relativistic corrections. Satisfactory agreement has been obtained between theory and experiment except for the cross sections of the L2-subshell ionisation induced by heavy ions (12C, 14N, 16O). The theory underestimates these cross sections by almost one order of magnitude.

Systematic study of helium-induced L shell ionization cross sections

Abstract An experimental data set consisting of about 500 published cross section values has been applied to test the performance of various theoretical approximations for the description of the helium-induced L-subshell ionization of heavy elements (Z2 = 66−92). The large discrepancy observed previously between the L2-subshell data and the predictions of the ECPSSR model has been greatly reduced including the effect of the collision-induced intrashell transitions. Further improvements in the description of the data, particularly for the L3 Subshell, have been achieved modifying the binding energy correction of the ECPSSR theory according to the recent suggestion by Vigilante et al. The use of more realistic electronic wave functions than those applied in ECPSSR is discussed as a possible way to remove the remaining discrepancies.

Higher order processes in L-shell ionization

Abstract The time-dependent perturbation theory has been applied to the description of the L-shell ionization of atoms by heavy charged particles in the independent-particle model approximation. A second order correction factor to cross sections calculated in a first order theory (e.g. PWBA) has been derived considering transitions at an average impact parameter and with minimum energy transfer as dominant ionization processes in low-velocity collisions. Numerical calculations have been performed for light and heavy (Z1 ⩽ 8) ion impact ionization of gold in the energy range 0.15—2.0 MeV u . The results are in satisfactory agreement with the experimental data for the L3-to-L2 and L1-to-L2 subshell ionization cross section ratios. The model seems to account also for the anomalous behaviour of the L3-subshell alignment observed recently at heavy-ion impact.

Study of the L-shell ionisation of gold by 3.0-18.2 MeV nitrogen-ion bombardment

The angular distributions of the L X-ray line intensities of gold have been measured by nitrogen-ion bombardment in the 3.0-18.2 MeV energy region. From the measured data absolute subshell ionisation cross sections and the anisotropy parameter of the Ll X-ray line have been deduced. Comparing the experimental data with the first-order perturbation theories significant deviations have been found, especially for the L2-subshell ionisation cross section and for the anisotropy parameter of the Ll X-ray line.

L-shell ionization by low-energy protons and alpha particles

Comparison between theoretical and experimental values for L-subshell ionization cross sections by low-energy protons and α particles has been made graphically. The theoretical values are calculated by the relativistic plane-wave Born approximation, including the corrections for the binding-energy and Coulomb-deflection effects. The experimental data are expressed as a ratio to the calculated values and plotted against a reduced velocity parameter. It is found that agreement between theory and experiment is not so good as in the case of the K-shell ionization cross sections. ForL1 and L3 shell, most experimental data are in agreement with the calculated values within errors of 60%. However, the measured values for L2 shell are systematically larger than the calculated ones. Possible reasons for the discrepancy are discussed.

Interference effects in electron emission from H2 by particle impact

A simple method for the description of the coherent electron emission from two H atoms of the H2 molecule induced by particle impact is suggested. The method is based on a formalism that separates the cross section for the electron emission into an atomic part describing the independent emission from the two H atoms, and a factor giving an account of the interference caused by the coherent emission from the two centres. This separability allows the use of a classical ionization theory to determine the atomic part of the cross section. Calculations applying the classical trajectory Monte Carlo (CTMC) theory have been carried out for 68 MeV amu−1 Kr33+ on H2 collisions. A reasonable agreement has been found between the CTMC results and the recent experimental data obtained by Stolterfoht et al.

K-shell ionization by protons and alpha particles

Abstract Comparison between theoretical and experimental values for proton- and α-induced K-shell ionization cross sections on various targets have been made graphically. The theoretical values are calculated by the relativistic plane-wave Born approximation, including the effects of increased binding energy and Coulomb deflection. The experimental data are expressed as the ratio to the calculated values. It is found that the theoretical predictions are in satisfactory agreement with the experimental values with some exceptions and except for the data for low-energy projectiles on high- Z 2 targets. For the latter case, the theory overpredicts the ionization cross sections, and possible reasons for the discrepancy are discussed.

L3-subshell alignment calculations in the second-order Born approximation for light- and heavy-ion impact on Au

The L3-subshell alignment parameter for 1H, 2H, 4He, and 14N impact ionisation of gold has been calculated in the energy range of 0.15-2 MeV amu-1 using an improved version of the recent second-order L-shell ionisation model of Sarkadi and Mukoyama (1980). The calculations show that the second-order contribution is significant even for light ions, and that its inclusion brings the theory into better agreement with experiment. For nitrogen-ion impact, in accordance with the observations of the heavy-ion experiments, the present model predicts sign reversal and large positive values for the alignment parameter at low collision velocities. To get further improvements in the description of the collision-induced L3-subshell alignment the need for coupled-channel calculations is emphasised.

L-shell Coulomb ionization

Abstract A brief review of the most important features of the L-shell ionization of atoms by charged particle impact is given. It is shown that the direct Coulomb theories successfully describe the ionization processes as long as the perturbing effect of the projectile is small. For stronger perturbations induced by low velocity heavy ions, however, serious disagreement has been observed between the experimental cross section data and the theoretical predictions which indicates the failre of the hypothesis of the independent ionization of the individual atomic L sublevels.