FAST AND SLOW FLUORESCENCE DECAYS IN PYRAZINE UNDER NANOSECOND EXCITATION CONDITIONS - A RESOLUTION OF THE ENIGMA

We study the fluorescence decay behavior of the S-1(B-1(3u)) electroni c state of pyrazine following its excitation from the ground S-0((1)A( 1g)) electronic state with a few nanoseconds light pulse. Our probe of the dynamics is the time-dependent Schrodinger equation. We form supe rpositions of the eight strongest S-1 molecular eigenstates (MEs) of p yrazine with the light pulse, and then compute the total spontaneous e mission as a function of time using the known optical properties of th e MEs. Both coherent and incoherent contributions to the fluorescence decay have been observed. We find that single exponential decays exist at selected frequencies in the spectrum, corresponding to exact ME re sonances. However, most decays are biexponential owing to the off-reso nant excitation of many nearby MEs. Even resonant excitation decays be come biexponential at high power. Thus, the ''enigma'' is apparently r esolved, the fast component (and its J dependence) in the nanosecond e xcited fluorescence decay of pyrazine has its origin in the nonlinear light scattering properties of the isolated molecule. (C) 1996 America n Institute of Physics.