CONTROL OF THE YIELD OF PHOTOPHYSICAL PHOTOCHEMICAL PROCESSES BY EXCITATION WITH PROPERLY DELAYED ULTRASHORT PHASE-LOCKED LIGHT-PULSES - A MODEL STUDY ON THE PYRAZINE S-2-]S-1 INTERNAL-CONVERSION

We study the internal conversion S-2[B-1(2u)(pi pi*)] --> S-1[B-1(3u)(n pi*)] due to the conical intersection between potential energy surfaces in the pyrazine molecule by numerically solving (by the Lanczos method) the diabatic model proposed by Stock and Domcke. The model involves three normal modes, one promoting and two accepting. After optical excitation of the S-2 state with a single delta-pulse, the total (i.e., summed over the vibrational manifold) population of S-2 decays very rapidly but incompletely. It is shown that the fraction of the population trapped in S-2 can be significantly reduced if two phase-locked ultrashort light pulses are used, provided the time delay as well as the phase relation between them are properly chosen.