The absolute values of the cross sections for the emission of the dispersed fluorescences from neutral fragments, i.e. excited hydrogen atoms and radicals, produced in the photoexcitation of CH₄[1], NH₃[2] and H₂O[3] have been measured as a function of incident photon energy in the range 12-41 eV. We have aimed at investigating the dynamics and spectroscopy of the doubly excited states of these isoelectronic molecules.
The measurements were carried out at the BL-20A, the Photon Factory, IMSS, KEK equipped with 3 m normal incidence monochromator. The fluorescence due to the photoexcitation of the molecules emitted along the electric vector of the incident synchrotron radiation was dispersed by a uv and visible monochromator. The dispersed fluorescence was detected by a liquid-nitrogen-cooled charge-coupled device, which covers the range of 280 nm width. The procedure to obtain the absolute values of the cross sections for the fluorescences from the measured fluorescence spectra was described in detail in [1].
Let us focus on the cross sections for the Balmer fluorescences, H(n → n’ = 2). Some peaks are shown in the cross section curves for each Balmer fluorescence, all of which are attributed to the superexcited states of CH₄, NH₃ and H₂O. The oscillator strengths for the Balmer fluorescences originating from the precursor superexcited states were obtained through integrating the fluorescence cross section curves since the fluorescence cross sections give the oscillator strength distributions for the fluorescence. It has been found that
(1) the singly excited states, (2a₁)^-1(mo), and doubly excited states labeled D1 and D2 contribute to the formation of H(n >= 3) in the inner valence range, where ‘mo’ refers to a molecular orbital,
(2) f_Bal(D2), the oscillator strength for the Balmer fluorescences originating from the doubly excited D2 states, is twice f_Bal((2a₁)^-1(mo)) for CH₄, and f_Bal(D2) is comparable to f_Bal((2a₁)^-1(mo)) for NH₃ and H₂O, although the excitation of two electrons by single-photon absorption should be much weaker than the excitation of a single electron within the independent electron model,
(3) f_Bal falls off as n increases following f_Bal = an^-b, where n is the principal quantum number of the upper level of the hydrogen atom,
(4) the energies of the doubly excited D1 and D2 states against n follow the Rydberg-like formula proposed by our group.

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