Translational spectrum, interaction-induced

The translational motions and spin dynamics of conduction electrons in metals produce fluctuating local magnetic hyperfine fields. These couple to the nuclear magnetic moments, inducing transitions between nuclear spin levels and causing nuclear spin relaxation. The translational motions of electrons occur on a very rapid time scale in metals (<10 s), so the frequency spectrum of hyperfine field fluctuations is spread over a wide range of w-values. Only a small fraction of the spectral intensity falls at the relatively low nuclear resonance frequency (ojq 10 s ). Nevertheless, the interaction is so strong that this process is usually the dominant mode of relaxation for nuclei in metallic systems, either solid or liquid. 

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. 

---