Vibrational excitation, effect reactions

As a general rule, the rates of those reactions that are endoergic for ground-state reactions are dramatically enhanced when excited-state reactants are employed. Electronic excitation and vibrational excitation may or may not have similar effects. In the case of vibrational excitation, the reactions may occur on a single electronic potential surface. In the case of electronic excitation, a single-surface path between electronically excited reactants and ground-state products may not exist because of the prevailing symmetries. 

Tunable, narrow bandwidth lasers provide a very convenient method for obtaining selective vibrational excitation. A number of reactions of infrared laser-excited species have been studied to date, and are Usted in Table I. Also included is the frequency of the pump laser, the ground-state heat of reaction, and the effect of vibrational excitation on reaction rate. Reactions in Table I are listed according to increasing molecular complexity. 

The effect of reactant vibrational excitation on reaction oxtss-sectitMis and rate constants has also been studied for a numbo of exothermic, ion- nolecule reactions. The reaction ... 

G. C. Schatz, A quasiclassical trajectory study of reagent vibrational excitation effects in the OH 4- H2 H2O 4- H reaction,... 

An extensive review of the literature reveals that the only studies of vibrational effects in insertion chemistry have focused on reactions of 0(1D)175-177 and C(1D)177,178 with H2. Since there is no potential energy barrier to insertion in these systems, reaction proceeds readily even for unexcited reactants.179 Since the efficiency of vibrational excitation was 20% in both studies, due to the large cross-sections for ground state reactions, only small changes were observed in the experimental signal. From an analysis of the product distributions, it was concluded that while H2(v = 0) primarily reacted via an insertion mechanism, direct abstraction seemed to become important for = 1). For 0(1D), this is similar to behavior at elevated collision energies.180... 

The general requirement for charge transfer is that the transfer should occur from a molecule of higher ionization potential to a molecule of lower ionization potential that is, that the reaction be exothermic. In general, it is also accompanied by some vibrational excitation.43 In the condensed phase the ionization potential may be modified somewhat by collective effects but the gas phase value can be used to a first approximation to determine the reaction energetics. 

Third, ultrashort laser pulses can provide for selective electronic or vibrational excitation of molecules, which will probably allow one to effect a high selective excitation of certain vibrational degrees of freedom, that is, to effect mode-selective photochemical reactions. 

Direct ionization produces a staircaselike structure in the plot of ion current as a function of photon energy, where the height of each step is proportional to the probability of production of a certain vibronic state of the ion. Such favorable cases of staircaselike structure have been observed for ammonia87 and acetylene.88 The structure in ammonia is attributable to excitation of successive vibrational levels of the out-of-plane bending mode of the ion and in acetylene, to excitation of the C-C stretching mode. As a result, these molecules are favorable candidates for studying the effects of vibrational excitation on the cross sections for ion-molecule reactions. 

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