A semiclassical model of hne couphngs for IR absorption or Raman scatterrng IS proposed Thor model which Uses a simple traJectoty descrtptton leads to accurate state-to-state rotatIonal Integral cross secttons as shown by comparison m11.'1 close-cot.phng and coupled-stares results for Nz-Ar Its tracrablhty allows apphcatton to molecules hawng a large number of populated rotattonal hncs as m many practical apphcattons 1 Introduchon A wide variety of experimental methods can be apphed to the study of intermolecular forces (pressure broadening, rotational relaxation, etc.) In many cases, the connection of the intermolecular potential to experiment is through various cross sections which relate the values of appropriate tensors after a collisron to values before A review of the most important relaxation experiments according to their tensor character K has been gven by Gordon et al. [l] _ For atom&atom systems, in a semiclassical approximation where one treats the translation classically while retaining a quantum S matrix descnption of internal states, the onentation averaged cross sections for a given uutial lanetic energy Ek may be expressed as [ 1,2] : u(j,.16.jaj6 ,Ei) = J 2nb db l/2 c (-l)'a~fi'-"fl-mfl' n mff,m6 For K = 0 and QI = JS, Q' = fi', eq. (1) reduces to the cross section for transioon between tiferent j-states Then, by averaging over a Bollzmann velocity distribution functron, the same equation gives the off-diagonal elements of the matrix descibing the interference effects between the twq spectral lmes uijl + UfJi and LJ,~: + uf& i7 isotropic Raman spectra, provided the S matrix elements are assumed bldependent of vibrational states. Electric dipole absorption spectia correspond to K = 1. Here too, by averaging over the velocity titribufion, diagonal elements lead to the huewidths and off-diagonal elements to the mterference cross sections