Knowledge Bank

The relationship between the direct (i.e. beam) measurements of inelastic excitation rates for rotational energy transfer and the bulk gas phase measurements, such as pressure broadening, NMR spin-lattice relaxation etc. is investigated within the framework of the infinite order sudden approximation (IOS). It is shown that the IOS approximation combined with the usual correlation function approach gives rather simple formulas for a variety of relaxation cross sections in terms of the excitation rates $k(0\to j)$. The possibility of using these formulas to deduce excitation rates from spectroscopic measurements is discussed. Some applications of the theory to the calculations of relaxation processes in atom-molecule and molecule-molecule gas mixtures are presented.