Abstract
It is well known that in principle energy-resolved and time-resolved experiments provide complementary information. A few examples are reviewed where the time evolution of rapidly relaxing molecular systems is or could be better studied by energy-resolved experiments: (1) the evolution of a pair of resonances coupled with each other through a set of continua, (2) an irreversible relaxation of the intermediate state in the two-photon excitation of a polyatomic molecule, and (3) the derivation of a “dissociation time” from the angular distribution of fragments from photodissociation by linearly polarized light. In the first case it is shown how all parameters necessary for a time-dependent description may be obtained from the measurement, as functions of the excitation wavelength, of both the total absorption cross section across the resonances and the partial cross section for a single exit channel. In the last two cases the relevance of a description in terms of an “eigentime” of the molecule is demonstrated.