On the possibility of prolonging the lifetime of CH overtones through double-resonance excitation

Abstract

On the possibility of prolonging the lifetime of C-H overtones through double- resonance excitation A. Lami and G. Villani Istituto di Chimica Quantistica ed Energetica Molecolare de1 CNR Via Risorgimento, 35 - 56100 Pisa (Italy) The possibility of utilizing laser sources to promote a given chemical reaction is entirely based on the hope that in this way one can prepare and mantain a sufficient population in an excited state satisfying the following two requirements: i) it must belong to the reactant valley in order to be accessible with a non negligible probability by photon absorption from the ground state (due to the Franck-Condon principle); ii) it must exhibit a propensity to evolve in to the product valley. Condition ii) is the most difficult to be met and the usual fate of an excited state in the reactive region of a polyatomic is to rapidly become a more or less ergodic misture of all the excited states having the same energy, on the reactant side. Once this has occurred, every selectivity is lost and the products yield is completely determined by statistics. Despite the hardness of the task, several theoretical schemes have been proposed l-l 11 to attain a laser control of selected reactions. Our proposal [ll] was to use a double-resonance irradiation, where one frequency is used to excite the selected state and the other one to modify the internal dynamic leading to “ergodization”. Here we reconsider this “coherent trapping” scheme and show that it can be used to make the lifetime of a given C-H overtone longer, thus enhancing the rate of reactions involving tunneling from a C-H group 19,121. The hydrogen tunneling reactions seem to us to be among the best candidates to exhibit laser induced “mode selectivity” since, by overtone excitation one may excite directly the vibration that goes into the reaction coordinate. In the next section we expose the theoretical basis of what we called “coherent trapping control”