Direct observation of a mode-selective (non-RRKM) van der Waals reaction by picosecond photofragment spectroscopy

Abstract

The idea of directing reactions by selectively exciting particular vibrational modes with lasers (laser-selective chemistry) has stimulated much experimental and theoretical research aimed at exploring its feasibility and the deviation from statistical RRKM behavior in large molecules. In the absence of collisions, the yield and rate of a reaction will be determined by the specific rate of intramolecular vibrational-energy redistribution (IVR) from the initial mode excited to the other modes in the molecule and by the rate of bond breaking (vibrational predissociation). If k_(lvR) is larger than k_(VP), then the chemistry is from a statistically formed distribution of vibrational states and a statistical behavior (RRKM) may prevail, even if a laser is used to selectively excite the initial state of the reagent. It is this interplay between IVR and VP that determines the mode selectivity of the reaction. Excitation of the molecule with short enough pulses, short compared to k_(lVR)^(-l) and k_(vP)^(-l), should allow us to view these processes in real time and hopefully establish conditions for nonstatistical behavior.

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