Spectroscopy in the ionisation continuum. Vibrational preionisation in H2 calculated by multichannel quantum-defect theory

Abstract

Multichannel quantum-defect theory has been used to calculate the effect of vibrational–rotational preionisation on the total and partial oscillator-strength distributions and photoelectron angular distribution in H₂ for excitation between 800 and 750 Å. The total oscillator-strength profiles obtained agree well with the high-resolution photoionisation data of Dehmer and Chupka. The results of these authors concerning the final vibrational state distributions obtained by exciting preionisation resonances are also in excellent agreement with the present calculations. Particular attention is given to Rydberg levels which preionise viaΔv < –1 processes. Preionisation is found to affect all partial vibrational cross-sections σ_v⁺ near narrow Δv < –1 resonances, while near broad Δv=–1 peaks only σ_v⁺=v– 1 is perturbed (v⁺ and v are the vibrational quantum numbers of the final state and of the preionised Rydberg level, respectively). Further, it is found that peaks which preionise viaΔv < –1 and fall among the higher members of a series with less vibrational energy tend to appear as “complex” resonances, consisting of a sharp central peak surrounded by an extensive structure of broader satellites.