Induced spectra and allowed transitions

Spectral moments of allowed molecular absorption bands vary in general nearly linearly with the gas density, y /q constant. At sufficiently high pressures, a small, linear increase with density of the ratio y /g is, however, discernible, e.g., [8]. This quadratic absorption component is largely due to apparent induced absorption, resulting from the long range interaction of dipoles induced by the incident radiation field [400], Moreover, a true induced absorption component is believed to exist which arises from collision-induced dipole components (Chapter 4) [146, 210]. It was argued, however, that in most measurements true induced absorption was too weak to be identified positively in this way. Recent experimental and 

Interference effects between allowed and induced phenomena have received attention [246], Most of the work reported to date is for liquids and thus outside the scope of this monograph. We mention, though, that the Fano profiles observed in the HD-X spectra, Fig. 3.36, are interesting examples of such interference. 

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