Intracollisional interference

The induced dipole moment of the HD-X systems, with X = He, Ar, H2, HD, is well known from the fundamental theory, for the purely rotational bands and also for most fundamental bands [59]. To the induced dipole, the permanent dipole moment of HD has to be added vectorially, accounting for the linear variation with density which differs from the density variation of the induced dipole components [391]. According to the theory of intracollisional interference (as the process was called, to be distinguised from the intercollisional interference considered elsewhere in this monograph), interference occurs for those induced components that are of the same symmetry as the allowed dipole, namely A1A2AL = 0110 and 1010 [178, 179, 321, 389]. These induced components are always parallel or antiparallel to the allowed dipole, causing constructive or destructive interference. 

The observed cross sections for the 18s (0,0) collisional resonance with v E and v 1 E are shown in Fig. 14.12. The approximately Lorentzian shape for v 1 E and the double peaked shape for v E are quite evident. Given the existence of two experimental effects, field inhomogeneties and collision velocities not parallel to the field, both of which obscure the predicted zero in the v E cross section, the observation of a clear dip in the center of the observed v E cross section supports the theoretical description of intracollisional interference given earlier. It is also interesting to note that the observed v E cross section of Fig. 14.12(a) is clearly asymmetric, in agreement with the transition probability calculated with the permanent electric dipole moments taken into account, as shown by Fig. 14.6. 

Intercollisional interference is a many-body process. Poll (1980) has pointed out that, no matter how low the gas densities actually are, this many-body effect will always have to be reckoned with, for principal reasons. In more practical terms, at low densities intercollisional dips are generally reasonably well separable from the intracollisional profiles, because intercollisional profiles are relatively sharp while intracollisional ones are rather diffuse. In other words, a reasonably clear distinction between binary and many-body profiles is straightforward in low-density recordings. For this reason, separate theoretical discussions of the intra-and intercollisional processes are convenient and quite natural. 

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