Translational spectrum, interaction-induced molecules

Collision-induced absorption from free pairs of molecules appear as broad lines or bands located at the wavenumbers of the pure-rotation or vibration-rotation transitions in the participating individual molecules. Figure 3.3.6 shows the spectrum for H2-H2 collisions (Bachet et al., 1983) [see also Courtin (1988)]. In the far infrared (below 200 cm ) a weak translational band is also present. In H2 the prominent features in planetary atmospheres occur at the pure-rotation / = 0 -> 2 and 1 3 transitions located at 354 and 587 cm . The widths of collision-induced features are extremely large, about 100 cm or more, because the time during the collision in which the partners are interacting is very short ( 10 seconds or less). The width of a spectral line is related to the reciprocal of the collision duration. 

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