Spectral moment rotovibrational bands

Spectral moments. Spectral moments are defined according to Eqs. 3.5 and 3.6. For the rotovibrational bands of diatom-X pairs, like H2-Ar or H2-H2, the expressions... 

We note that an elaborate set of three-body spectral moments of various rotovibrational bands was compiled elsewhere [342], the values of which are generally much greater than the data listed in Table 3.7. We point out that these more recent data were obtained at low gas densities, typically from 10 to 50 amagats. They are, therefore, barely affected by three-body interactions pressure variation of the spectral moments Mn/g2 amounted typically to just one or two percent. While measurements at small densities are desirable to minimize interference by four-body effects, reliable data of this kind must be based on measurable effects that exceed significantly the experimental uncertainties (typically ten percent). On that score, the more recent data are deficient, in our judgement. 

The binary moment relations quoted above have been successfully used for the rototranslational bands and, to some extent, also for the rotovibrational bands [342], However, it has been noted [151] that for the rotovibrational bands, all but the zeroth spectral moment are affected by the variation of the interaction potential with the vibrational excitation, an effect not accounted for in Eqs. 6.13 through 6.18, 6.21. 

H2 He He rotovibrational band. The density dependence of the H2-He enhancement spectrum in the fundamental band of hydrogen has been measured previously, using a trace of hydrogen in helium of thousands of amagats [121, 175, 142] ternary moments were measured at room temperature. The measurements suggest again greater values of the spectral moments, Table 6.7. 

We have not attempted to exhibit in great detail the effects of the rotational excitations on the induced dipole components B and those of vibrational excitation on the interaction potential because this was done elsewhere for similar systems [151, 63,295,294], The significance of the j,f corrections is readily seen in the Tables and need not be displayed beyond that. The vibrational influence is displayed in Fig. 6.20 first and second spectral moments are strongly affected, especially at high temperatures, similar to that which was seen earlier for H2-He [294], Fig. 6.23. The close agreement of the measurements of the rotovibrational collision-induced absorption bands of hydrogen with the fundamental theory shown above certainly depends on proper accounting for the rotational dependences of the induced dipole moment, and of the vibrational dependences of the final translational states of the molecular pair. 

M. Moraldi, J. Borysow, and L. Frommhold. Spectral moments for the collision induced rotovibrational absorption bands of nonpolar gases and mixtures. Phys. Rev., A 36 4700, 1987. 

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