The dependence of the cross section of the rotationa,I relaxation of ~~lecul~s on t~e energy discrepancy (AE) in collision is clearly observed in vanous ty~es of.colhslo~al mteracttons, such as those of dipole-dipole, quadrupole-dipole, and London dlsperslo~. It IS ~ound t~at the ~ dependence is quite different according to the order of collisional mteractlOn. ThIS result IS explained by the conception of the resonance function in the Anderson-Tsao-Curnutte an~ t~e quantum Fourier transform theories. A new physical interpretation of the resonance functIOn IS also given on the basis of both theories.

The cross sections of the transverse relaxation of NH 3 colliding with various spherical perturbers are measured and examined as a function of molecular parameters, such as ionization energy, polarizability, and mass. The observed cross sections show good correlation with these molecular parameters. An empirical formula in the unit of Å 2 , ϭ25.5 ͑␣ 2 M ͒ 1/6 , is obtained, where is the cross section, ␣ is the polarizability of the perturber in the unit of Å 3 , and M is the reduced mass of the colliding molecules in the unit of g/mol. This expression can be used to evaluate the cross sections of collisions between NH 3 and other spherical perturbers.