Ximeng Chen

Nonadiabatic dynamics in energetic negative fluorine ions scattering from a Si(100) surface.

The dependence of the negative-ion fractions on incident energy and angle is reported for 8.5-22.5 keV F(-) ions scattered from a Si(100) surface at a fixed scattering angle of 38°. The negative-ion fraction increases monotonically with incident velocity for specular scattering. In particular, the variation of the fraction with incident angle is bell shaped for a given incident energy. We interpret this variation using the incident-velocity effect at short distances where the yield of negative ions depends on the number of initial neutrals. It strongly indicates that at short distances, a dynamical equilibrium population is never achieved. This nonadiabatic feature is supported by simple calculations using modified rate equations.

X-ray emission of Xe 30+ ion beam impacting on Au target

X-ray emission from Xe 30+ ions at 350–600 keV impacting on an Au target was investigated at the Heavy Ion Research Facility at Lanzhou. Characteristic X-rays of Xe ions at energy of about 1.65 keV were observed. This X-ray emission is induced by the decay of very high Rydberg states of Xe ions. It was also found that the yield of such characteristic X-rays is decreasing with increasing the projectile kinetic energy. Simultaneously, the yield of the characteristic Au X-rays from the M shell increases also. These phenomena are qualitatively analyzed with the classical Coulomb over the Barrier Mode (COBM) for highly charged ions interacting with solid state surfaces.

Charge exchange of keV O? ions transmitted through Al2O3 nano-capillaries

The transmission of 10–18 keV O− ions through Al2O3 nano-capillaries has been investigated experimentally. From the angular distribution of the transmitted particles, O0 and O− and O+ ions can be distinguished in the mixed scattered beams. The fractions of the scattered ions increase with both the tilt angle and the incident energy, which indicates strongly that there is dynamical charge exchange in the scattering of negative ions on the insulator surface. Moreover, it is found that the scattered neutrals (O0) are only more or less guided along the capillary axis, which is greatly different from the case of the guiding effect of highly charged ions.

M -subshell ionization of Th and U by alpha particle bombardment

The M x-rays of Th and U have been produced by alpha particle bombardment in the energy range of 2.5-4.5 MeV. N6,7 → M5(Mα), N6 → M4(Mβ), N5 → M3(Mγ), N4 → M2 and N2 → M1 lines were selected to deduce M-subshell x-ray production cross sections. The radiative transition rates and the Coster-Kronig yields and M-subshell fluorescence yields were used to convert the x-ray production cross sections into M-subshell ionization cross sections. The deduced M-subshell ionization cross sections were compared with the ECPSSR predictions. The discrepancies of our experimental results from the theoretical calculations are discussed.

L-subshell ionisation of 71Lu, 73Ta and 79Au by 1.5–4.5 MeV alpha particles

Abstract The L X-rays of 71 Lu, 73 Ta and 79 Au were produced by 1.5–4.5 MeV alpha particle bombardment. The emitted L α , L γ1 and L γ2,3 lines were selected to convert X-ray production cross sections into ionisation cross sections. The ionisation cross sections obtained were compared with the predictions of the ECPSSR theory. A reasonable agreement of L 3 -subshell ionisation cross sections between the present experiment and the ECPSSR theory is found. The departures of the measured L 2 -subshell ionisation cross sections from the ECPSSR theory are discussed.

L-shell x-ray yields and production cross-sections of molybdenum induced by low-energy highly charged argon ions*

L-shell x-ray yields of molybdenum bombarded by highly charged Arq+ ions (q = 11-16) are measured. The x-ray production cross-sections are extracted from the yields data. The energy of the incident Ar ions ranges from 200 to 350 keV. After the binding energy correction, experimental data are explained in the framework of binary-encounter-approximation (BEA). The direct ionization is treated in the united atom (UA) limit (Lapicki and Lichten 1985 Phys. Rev. A 31 1354), not in the separate atom (SA) limit. The calculation results of BEA (Gacia and Fortner 1973 Rev. Mod. Phys. 45 111) are much lower than the experimental results, while the results of binding energy modified BEA are basically in agreement with the experimental results.

A coincidence experimental setup for investigating multiple electron processes

The coincidence experimental setup for studying multiple electron processes at low energies established at the Institute of Modern Physics was presented. The setup includes time-of-flight spectrometer, position sensitive detector, and multi-parameter data acquisition system. Presented were a TOF spectrum for Ar12+ on Ar collisions, a position spectrum obtained in O2+ + He collisions, and a two-dimension spectrum for Ar8+ + Ar collisions at 104 keV. The experimental results were analyzed in detail and 12 subprocesses were identified from the two-dimension spectrum.

ONE AND TWO ELECTRON TRANSITIONS IN ENERGETIC ION AND HELIUM COLLISIONS

Abstract The single ionization (SI) and double ionization (DI), single and double ionization accompanying one electron loss (SS) of the projectile and the single and double electron capture of the projectile from target atom are investigated for 2.0–8.0 MeV Na q + –He and C1 q + –He collisions. The cross-section ratio of the double ionization (DI) to that of the SI, the ratio of SS+DI to SS+SI, and ratio of double capture (DC) cross-section to that of single capture (SC), are measured by means of time-of-flight technique. The velocity and charge state dependences of the measured cross-section ratios are studied and discussed. The obtained capture cross-section ratios are compared with the theoretical calculations using the classical over-barrier model.

The ionization of helium by 1.5–7.5 MeV partially stripped Cq+ ions

Abstract The double to single ionization cross section ratios of helium by partially stripped carbon ions have been measured for projectile charge states ranging from +1 to +4 and impact energies from 1.5 to 7.5 MeV. The effective charge effect in partially stripped ion-helium collision is presented. It is found that the effective charge qeff increases as the impinging energy increases and qeff shows a modest dependence upon the projectile charge state in the present energy range. The projectile charge state, projectile and target electronic state dependences of the effective charge effect is discussed using orbital interpenetrating. The nCTMC calculations were carried out to investigate the energy dependence of the effective charge. By introducing effective charge the cross section ratio of helium obtained in the present work is in good agreement with the ( V q eff ) −2 law.

Production and decay of K -shell hollow krypton in collisions with 52–197-MeV/u bare xenon ions

X-ray spectra of K-shell hollow krypton atoms produced in single collisions with 52 - 197 MeV/u Xe54+ ions are measured in a heavy-ion storage ring equipped with an internal gas-jet target. Energy shifts of the K{\alpha}_1,2^s, K{\alpha}_1,2^(h,s), and K\b{eta}_1,3^s transitions are obtained. Thus, the average number of the spectator L-vacancies presented during the x-ray emission is deduced. From the relative intensities of the K{\alpha}_1,2^s and K{\alpha}_1,2^(h,s) transitions, the ratio of K-shell hollow krypton to singly K-shell ionized atoms is determined to be 14 - 24%. In the considered collisions, the K-vacancies are mainly created by the direct ionization which cannot be calculated within the perturbation descriptions. The experimental results are compared with a relativistic coupled channel calculation performed within the independent particle approximation.