Time- and frequency-resolved spontaneous emission: Theory and application to the NO2 (X)over-tilde (2)A `/(A)over-tilde (2)A ` conical intersection

We present a theoretical scheme for the calculation of time- and frequency-resolved spontaneous emission spectra of nonstationary states prepared by a laser pulse, considering explicitly the effect of the frequency filter and the time gate of the measurement instrument. Our scheme treats in a perturbative manner the matter-radiation interaction taking into account the states radiative lifetimes, and utilize the eigenstates of the molecular Hamiltonian up to the maximum excitation energy. We study the fluorescence of a nonstationary state of NO2 created by a Gaussian pulse mainly on the (A) over tilde (2)A' excited adiabatic potential, following an absorption from the ground adiabatic electronic state (X) over tilde (2)A'. We analyze the (X) over tilde (2)A'/(A) over tilde (2)A' conical intersection effects on the spectra and dynamics in a (2)A(1)(ground)/B-2(2) (excited) diabatic electronic representation. We have pointed out that the wave packet emits more strongly at times corresponding to partial recurrences, i.e., when it returns to the region of space where it was initially, and that the whole spectrum is red-shifted. The nonadiabatic interactions between the electronic states bring the wave packet from the bright B-2(2) state to the quasi-dark (2)A(1) one, and thus they quench the oscillations of the total emitted energy. Moreover, they cause the broadening of the part of the wave packet that remains on the upper diabatic surface, and this results in a further quenching of the emission. On the contrary, the nonadiabatic interactions have a negligible effects on the times at which the emission peaks occur. The striking effect of the duration of the interval in which the time-gate is opened on the time- and frequency-resolved emission is investigated and discussed. (C) 2000 American Institute of Physics. [S0021-9606(00)01034-5].