The realization of the dipole (γ, γ) method and its application to determine the absolute optical oscillator strengths of helium.

Long-Quan Xu,Ke Yang,Lin-Fan Zhu,Nozomu Hiraoka,Ku-Ding Tsuei,Ya-Wei Liu,Dong-Dong Ni,Xu Kang

doi:10.1038/srep18350

Abstract

The dipole (γ, γ) method, which is the inelastic x-ray scattering operated at a negligibly small momentum transfer, is proposed and realized to determine the absolute optical oscillator strengths of the vanlence-shell excitations of atoms and molecules. Compared with the conventionally used photoabsorption method, this new method is free from the line saturation effect, which can seriously limit the accuracies of the measured photoabsorption cross sections for discrete transitions with narrow natural linewidths. Furthermore, the Bethe-Born conversion factor of the dipole (γ, γ) method varies much more slowly with the excitation energy than does that of the dipole (e, e) method. Absolute optical oscillator strengths for the excitations of 1s2 → 1 snp(n = 3 − 7) of atomic helium have been determined using the high-resolution dipole (γ, γ) method, and the excellent agreement of the present measurements with both those measured by the dipole (e, e) method and the previous theoretical calculations indicates that the dipole (γ, γ) method is a powerful tool to measure the absolute optical oscillator strengths of the valence-shell excitations of atoms and molecules.

Highlights

oscillator strength (OOS) for a definite excitation measured by the photoabsorption method is always smaller than the real value, and the line saturation is even more serious for the stronger and narrower transition
The electron inelastic mean free path (IMFP)[34,35], which is a parameter of fundamental importance to a range of fields including electron microscopy, Auger electron spectroscopy, x-ray photoelelctron spectroscopy and photoelectron diffraction, can be obtained from the optical energy-loss function (OLF) determined at q2 ≈ 0 by the inelastic x-ray scattering (IXS)[36]
We propose and realize the high resolution dipole (γ, γ) method to measure the absolute OOSs of valence-shell discrete excitations for gaseous atoms and molecules by utilizing the inelastic x-ray scattering technique at q2 ≈ 0

Summary

Introduction

OOS for a definite excitation measured by the photoabsorption method is always smaller than the real value, and the line saturation is even more serious for the stronger and narrower transition. The dipole (e, e) method has the advantage of being free from the line-saturation effect in determining the optical oscillator strength because of the nonresonant nature of the electron-impact excitation process. In addition to the above-mentioned photoabsorption and dipole (e, e) methods operated at an optical limit, the OOS can be determined by extrapolating the measured generalized oscillator strength (GOS) in q-space to q2 ≈ 05,6,11. We propose and realize the high resolution dipole (γ, γ) method to measure the absolute OOSs of valence-shell discrete excitations for gaseous atoms and molecules by utilizing the inelastic x-ray scattering technique at q2 ≈ 0. The diferential cross section (DCS) of the IXS for gaseous atoms and molecules can be written as[28,38,39,40] (the atomic unit is used throughout this paper): dσ dΩ γ

Methods

Results

Conclusion