SPECTRAL-LINE SHAPE ARISING FROM COLLISIONAL INTERFERENCE BETWEEN ELECTRIC-DIPOLE-ALLOWED AND COLLISION-INDUCED TRANSITIONS

A theory is developed to describe the spectral line shape due to interference between electric-dipole-allowed and collision-induced transitions in pure rotational molecular spectra. Motivation was provided by experimental data available for HD-inert gas systems. This theory is based on a master-equation approach to induced spectra employed by Alber and Cooper [Phys. Rev. A 33, 3084 (1986)]. The active molecule is considered to be immersed in a bath of perturbers. An expression for the absorption coefficient is obtained within the binary collision approximation that contains terms due to allowed, induced, and interference contributions. Effects due to m mixing, J mixing, and successive collisions are included. Low-order approximations of the theory eventually reduce to results of earlier efforts, namely, the pioneering description of collisional interference by Herman, Tipping, and Poll [Phys. Rev A 20, 2006 (1979)] and refinements to it through consideration of rotational level mixing. The principal attribute of this approach is the treatment of allowed and collision-induced transitions in a consistent manner.