Coherent pulse sequence induced control of selectivity of reactions

We present a novel approach, suggested by Tannor and Rice, to the control of selectivity of photochemical reaction products. The central idea is that in a two-photo or multiphoton process that is resonant with an excited electronic state, the resonant excited state potential energy surface can be used to assist chemistry on the ground state potential energy surface. By controlling the delay between a pair of ultrashort (femtosecond) laser pulses, it is possible to control the propagation time on the excited state potential energy surface. Different propagation times, in turn, can be used to generate different photochemical products. There are many cases for which selectivity of product formation should be possible using this scheme. Our examples show a variety of behaviors ranging from virtually 100% selectivity to poor selectivity, depending on the nature of the excited state potential energy surface. Branching ratios obtained using a swarm of classical trajectories are in good qualitative agreement with full quantum mechanical calculations.