The factor C 4( D x ) describing the intensity distribution at large detunings D x from a vibrational
band centre is calculated for classical linear rotators in a bu€ er monoatomic gas. A
formalism developed uses the theory of moments and is appropriate for a spectroscopy of any
rank. The invariant form of the interaction potential and the irreducible tensor approach are
exploited to derive the third- and fourth-order Poisson brackets of spectroscopic quantities.
The anisotropy of the pair distribution function is analytically incorporated in the formulas
for zeroth and second moments of the torque spectra appearing in the wing distributions. By a
similar technique, the relaxation rates of rotational energy and momentum are studied. Useful
analytical relations are found in the fast-collision limit. Calculations performed for CO+ He
and CO2+ He systems permit one to check the accuracy of the current models based on the
in® nite order sudden approximation and are in reasonable agreement with available experimental
IR and NMR data.