“…However, for this particular problem, symmetry properties make null several terms, and those surviving in Eq. (12) are those that correspond: (a)To an equivalent of the Golden Rule of quantum mechanics averaged over the nonequilibrium ensemble ± they are terms with coecients s À1 , M 1 and M 2 , but it must be noticed that the two with M 1 vanish because they correspond to processes that are not allowed because energy conservation in the scattering event is not possible (we recall that the frequency of the phonons of the bath is lower than that of the vibrations under consideration), and M 2 gives a negligible contribution in this case of a very narrow frequency-dispersion relation x q ; (b) To terms, those with coecient K, that are the fourth-order contribution in Born's perturbation series averaged over the nonequilibrium ensemble.…”