Absolute optical oscillator strengths for electronic excitations of noblae gas atoms and diatomic molecules

• Main Author: Chan, Wing Fat
• Language: English
• Published: 2008
• Online Access: http://hdl.handle.net/2429/2886

A new high resolution dipole (e,e) method is described for the measurement of absolute optical oscillator strengths (cross sections) for electronic excitation of free atoms and molecules throughout the discrete region of the valence shell spectrum. The technique, utilizing the virtual photon field of a fast electron inelastically scattered at negligible momentum transfer, avoids many of the difficulties and errors associated with the various direct optical techniques which have traditionally been used for absolute optical oscillator strength measurements. In particular, the method is free of the bandwidth (line saturation) effects which can seriously limit the accuracy of photoabsorption cross section measurements for discrete transitions of narrow linewidth obtained using the Beer-Lambert law (formula removed)). Since these perturbing “line saturation” effects are not widely appreciated and are only usually considered in the context of peak heights a detailed new analysis of this problem is presented considering the integrated cross section (oscillator strength) over the profile of each discrete peak. Using a low resolution dipole (e,e) spectrometer (~1 eV FWHM), absolute optical oscillator strengths for the photoabsorption of the five noble gases He, Ne, Ar, Kr and Xe have been measured up to 180, 250, 500, 380 and 398 eV respectively. The absolute scales for the measurements of helium and neon were obtained by TRK sum rule normalization and it was not necessary to make the difficult determinations of photon flux or target density required in conventional absolute cross section determinations. Single point continuum normalization to absolute optical data was employed for the measurements of argon, krypton and xenon due to the closely space in the subshells of these targets which cause problems in the extrapolation procedures required for TRK sum—rule normalization. The newly developed high resolution dipole (e,e) method (0.048 eV FWHM) has then been used to obtain the absolute optical oscillator strengths for the valence discrete excitations of the above five noble gases with the absolute scale normalized to the low resolution dipole (e,e) measurements in the smooth ionization continuum region. The measured dipole oscillator strengths for helium excitation (1 ¹S—n ¹P, n=2-7) are in excellent quantitative agreement with the calculations reported by Schiff and Pekeris (Phys. Rev. 134, A368 (1964)) and by Fernley et al. (J. Phys. B 20, 6457 (1987)). High resolution absolute optical oscillator strengths are also reported for the autoionizing resonances, corresponding to the double excitation of two valence electrons and/or single excitation of a inner valence electron, of the above five noble gases. High resolution absolute optical oscillator strengths (0.048 eV FWHM) for discrete and continuum transitions for the photoabsorption of five diatomic gases (H₂, N₂, 0₂, CO and NO) throughout the va1ence shell region are reported. The absolute scales were obtained by normalization in the smooth continuum to TRK sum rule normalized data determined using the low resolution dipole (e,e) spectrometer. (For full abstract, view file)