Autoionization linewidth

For NO, the separation between the first two excited states of NO+ is considerably larger than for Nj, giving nm-m > 2. A linewidth of 500 cm-1 has been observed in the ultraviolet spectrum of NO (Takezawa, 1977), for a 3p7r Rydberg state converging to the NO+ b3II excited state. If that width is caused exclusively by electronic autoionization and not by predissociation, it corresponds to a value of about 0.05 for the I parameter. 

In the preceding sections, we have assumed that an absorption line has a Lorentzian shape. If this is not true, then the linewidth cannot be defined as the full width at half maximum intensity. Transitions from the ground state of a neutral molecule to an ionization continuum often have appreciable oscillator strength, in marked contrast to the situation for ground state to dissociative continuum transitions. The absorption cross-section near the peak of an auto-ionized line can be significantly affected by interference between two processes (1) direct ionization or dissociation, and (2) indirect ionization (autoionization) or indirect dissociation (predissociation). The line profile must be described by the Beutler-Fano formula (Fano, 1961) ... 

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