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

Abstract

Differential cross sections and generalized oscillator strengths for the 3 1 S and n 1 P (n = 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 1.0-6.0 • . These data are first reported at such a high impact energy. Integral cross sections for the above n 1 P (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 0 • , 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.

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

“…The calculations of Cann and Thakkar [8], which are in excellent agreement with the R-matrix calculations [7], are not shown for clarity. It is clear from figures 3-8 that the present IXS results for these excitations are in excellent agreement with the theoretical calculations [7] and the high-energy EELS ones measured at the impact energies of 1500 and 2500 eV [10,23,24] considering the mutual experimental errors. The agreement can be easily understood because of the intrinsic connection between IXS and high-energy EELS as discussed above.…”

“…Therefore, the reliable DCSs and GOSs of helium are often used as the standard data to normalize the DCSs and GOSs for other atoms and molecules [11,12]. The previous experimental and theoretical investigations about the dynamic parameters of helium were concentrated on the photoabsorption cross sections [13], the lifetimes [14,15], the DCSs [9,[16][17][18][19][20][21][22] and the GOSs [7,8,10,[23][24][25]. The above-mentioned DCSs and GOSs were normalized to the elastic DCSs [16,18,19,22] or the DCSs of the 2 1 P state [10,17,20,21,24,25].…”

“…The previous experimental and theoretical investigations about the dynamic parameters of helium were concentrated on the photoabsorption cross sections [13], the lifetimes [14,15], the DCSs [9,[16][17][18][19][20][21][22] and the GOSs [7,8,10,[23][24][25]. The above-mentioned DCSs and GOSs were normalized to the elastic DCSs [16,18,19,22] or the DCSs of the 2 1 P state [10,17,20,21,24,25]. Since photoabsorption cross section is measured at near zero momentum transfer, only fragmental information of the initialand final-state wavefunctions of a dipole-allowed transition can be obtained.…”

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

“…Within the errors, all of the present results measured by the high-resolution dipole ( γ, γ ) method are in excellent agreement with those measured by Chan et al 14, Feng et al 13 and Zhong et al 12 with the dipole (e, e) method and with the results obtained by Skerbele and Lassettre56. Generally speaking, the results of the dipole (e, e) method are slightly larger than the above mentioned calculated ones, especially for the values of 6 1 P and 7 1 P transitions, whereas our dipole ( γ, γ ) method results are in better agreement with these calculations.…”

“…Different types of experimental techniques, including the photoabsorption method1234, electron energy loss spectroscopy (EELS)5611, dipole (e, e) method78912131415, lifetime method1617, and selfabsorption181920, have been used to measure the absolute OOSs of atoms and molecules. Among them, the photoabsorption method and the dipole (e, e) method are most commonly used.…”