Lin-Fan Zhu

Inelastic x-ray scattering study on the single excitations of helium

The inelastic x-ray scattering technique has been used to study the excitation mechanism of atomic helium in this work. The squares of the form factor for the single excitations of 1s2 ? 1snl (n ? 6) of helium have been determined by the high resolution (70 meV) inelastic x-ray scattering. The present squares of the form factor measured by inelastic x-ray scattering provide an independent cross-check to the ones measured by high-energy electron energy loss spectroscopy. It is found that the squares of the form factor for the single excitations of 1s2 ? 1snl (n ? 6) of helium measured by inelastic x-ray scattering are in excellent agreement with the ones measured by high-energy electron energy loss spectroscopy, which confirms the quality of these two experimental methods. The excellent agreement of the present measurements with the theoretical calculations also gives a rigorous test of the theoretical method. This work demonstrates that inelastic x-ray scattering is a powerful tool to study the excitation mechanism in atomic or molecular systems at the third-generation synchrotron radiation light source.

High-resolution inelastic X-ray scattering studies of the state-resolved differential cross-section of Compton excitations in helium atoms

The state-resolved differential cross sections for both the 1s{sup 2} {sup 1}S{sub 0}{yields}1s2s {sup 1}S{sub 0} monopolar transition and the 1s{sup 2} {sup 1}S{sub 0}{yields}1s2p {sup 1}P{sub 1} dipolar transition of atomic helium had been measured over a large momentum transfer region by high-resolution inelastic x-ray scattering (IXS). The almost-perfect match of the present measurement with the theoretical calculations gives a stringent test of the theoretical method and the calculated wave functions. Our results demonstrate that high-resolution IXS is a powerful tool for studying the excitations in atoms and molecules.

Application of a multichannel detection system to the high-resolution fast electron energy loss spectrometer

A microchannel-plate based resistive-anode position sensitive detection system is applied to a high-resolution fast electron energy loss spectrometer. The biasing voltage divider network for the detector is investigated. The measurement efficiency is increased by about 20 times, and to some extent, the energy resolution is also improved. The static mode and the scanning mode of the spectrometer are discussed in detail. In the scanning mode, a spectrum with higher energy resolution can be obtained through a coarse scanning. As a testing experiment, the optical oscillator strength spectrum for the Rydberg series of helium is measured and the results are compared.

Which is the most stable one in WSin (n = 1-4)? A density functional investigation with pseudo-potential model

Abstract Geometries and stabilities of the clusters WSin (n=1–4) were investigated by the density functional method. Geometry optimizations and energy calculations were carried out at the B3LYP level with the LanL2DZ basis set for the tungsten atom and LanL2DZ, LanL2DZ+STO-3G and LanL2DZ+6-31G basis sets sequentially for the silicon atoms. Higher basis sets are used for the silicon atom, slightly lower total energies are obtained while modest changes of their geometries. From the calculations with different basis sets, WSi3 is expected as the most stable one according to the relatively higher dissociation energy among these clusters. Compared with the experimental result [Phys. Rev. Lett. 86 (2001) 1733], WSin (n=1–4) have the same stability order as that of WSin+ (n=1–4). In addition, charge transfer from tungsten 6s orbital and silicon 3s orbital to tungsten 5d orbital was found. Good agreements are achieved between the calculated ionization potentials and experimental results.

High resolution electron energy loss spectroscopy with two-dimensional energy and momentum mapping

High resolution electron energy loss spectroscopy (HREELS) is a powerful technique to probe vibrational and electronic excitations at surfaces. The dispersion relation of surface excitations, i.e., energy as a function of momentum, has in the past, been obtained by measuring the energy loss at a fixed angle (momentum) and then rotating sample, monochromator, or analyzer. Here, we introduce a new strategy for HREELS, utilizing a specially designed lens system with a double-cylindrical Ibach-type monochromator combined with a commercial VG Scienta hemispherical electron energy analyzer, which can simultaneously measure the energy and momentum of the scattered electrons. The new system possesses high angular resolution (<0.1°), detecting efficiency and sampling density. The capabilities of this system are demonstrated using Bi2Sr2CaCu2O(8+δ). The time required to obtain a complete dispersion spectrum is at least one order of magnitude shorter than conventional spectrometers, with improved momentum resolution and no loss in energy resolution.

Cross sections for the valence shell excitations of nitrous oxide studied by fast electron impact

The generalized oscillator strengths, differential cross sections, as well as the integral cross sections for electron impact excitation of the B (1)Delta, C (1)Pi, and D (1)Sigma(+) states of N(2)O have been determined at an incident electron energy of 2500 eV. A generalized oscillator strength analysis reveals that the B (1)Delta<--X (1)Sigma(+) transition is dominated by the quadrupolar component. From the comparison to the previous experimental results, it is found that the first Born approximation is not satisfied for the C (1)Pi excitation while it is valid for the D (1)Sigma(+) excitation at an incident electron energy of 200 eV. The BE-scaled integral cross section for the B (1)Delta excitation from its threshold to 5000 eV was calculated based on its generalized oscillator strength, and the present integral cross sections for the excitation of the C (1)Pi and D (1)Sigma(+) states are in good agreement with the calculations using the BEf-scaling approach.

Electron-impact study for the and ( n = 3 - 6) excitations in helium

Differential cross sections and generalized oscillator strengths for the and = 3 - 6) excitations overlapping neighbour non-dipole excitations in helium have been measured by an angle-resolved electron energy loss spectrometer at an impact energy of 1500 eV in a scattering angular range of . These data are first reported at such a high impact energy. Integral cross sections for the above (n = 3 - 6) excitations were also determined. Meanwhile, the absolute optical oscillator strength density spectrum in the valence region of helium was established at the same impact energy in a mean scattering angle of , and the absolute optical oscillator strengths for these discrete transitions corresponding to the valence region of helium are reported. All these experimental results are compared with previous appropriate published data.

Study of dielectronic recombination at the CSRm using lithium-like Ar15+ ions

The main cooler storage ring (CSRm) of the HIRFL facility in Lanzhou, China is equipped with an electron-cooler and denotes an ideal platform for dielectronic recombination (DR) experiments. In order to fully understand our DR experimental setup and especially the electron energy detuning system, we have performed a DR calibration experiment using the Li-like argon ions at the CSRm because Ar15+ has a simple electronic structure and the DR spectrum can be calculated with an ultra-high precision and be compared with the existing experimental data. The experiment was carried out over the center-of-mass energy range 0–32 eV that includes all DR resonance associated with 2S1/2 → 2p1/2 and most of the 2S1/2 → 2p3/2 excitations. We present the details of the experimental technique and the DR experimental resonance spectrum of the Ar15+.

Optical oscillator strengths for valence-shell and Br-3d inner-shell excitations of HCl and HBr

Absolute optical oscillator strength density spectra for valence-shell excitations of HCl and HBr, as well as for Br-3d inner-shell excitations of HBr, have been determined by high-resolution electron-energy-loss-spectroscopy method in the dipole limit. Absolute optical oscillator strengths for the discrete transitions of HCl and HBr are reported and compared with the previous results determined by the photoabsorption method.

OSCILLATOR STRENGTHS OF VIBRIONIC EXCITATIONS OF NITROGEN DETERMINED BY THE DIPOLE (γ,γ) METHOD

The oscillator strengths of the valence-shell excitations of molecular nitrogen have significant applicational values in studies of the Earths atmosphere and interstellar gases. In this work, the absolute oscillator strengths of the valence-shell excitations of molecular nitrogen in 12.3?13.4 eV were measured by the novel dipole (?, ?) method, in which the high-resolution inelastic X-ray scattering is operated at a negligibly small momentum transfer and can simulate the photoabsorption process. Because the experimental technique used in the present work is distinctly different from those used previously, the present experimental results give an independent cross-check to previous experimental and theoretical data. The excellent coincidence of the present results with the dipole (e, e) and those that were extrapolated indicates that the present oscillator strengths can serve as benchmark data.