Measurements of the fluorescence cross sections in the photoexcitation of CH4, NH3 and H2O in the vacuum ultraviolet range

Abstract

The cross sections for the emission of dispersed fluorescences in the ultraviolet and visible ranges by excited hydrogen atoms and radicals in the photoexcitation of CH4, NH3 and H2O were measured recently as a function of incident photon energy in the vacuum ultraviolet range, i.e. approximately 15–40 eV, and most of them were placed on an absolute scale. The fluorescence cross sections were discussed in terms of the fluorescence oscillator strengths. It is remarkable that the oscillator strengths for the Balmer fluorescences originating from the doubly excited states amount to the magnitude comparable to those from the nearby singly excited states in the inner valence range. In CH4, the former amount to twice the latter. The independent electron model, in which the double photoexcitation should be much weaker than the single photoexcitation, seems to be violated in the superexcited states of CH4, NH3 and H2O in the inner valence range, and the degree of the breakdown is largest in CH4.