Bimolecular reactions, vibrational enhancement

Similarly, in the case of bimolecular reactions, Zare s group [466] confirmed theoretical predictions and demonstrated experimentally [467-469] that by exciting either the OH or the OD bond in HOD one can selectively enhance product forma- tion in a subsequent H + HOD reaction. Specifically, when the OH bond is excited , the reaction yields H2 + OD, whereas when the OD bond is excited, H reacts with HOD to form the HD + OH product. In these experiments, the OH was prepared either by overtone excitation [57, 58] to the fourth vibrational level v = 4 or by excitation to the u = 1 state by Raman pumping [102]. As yet to be verified experimentally is the computational prediction of Manz et al. [124, 125] that strong optimized pulses can also achieve selective excitation of higher lying vibrational) ... 

Triggered by the discovery of substantial vibrational enhancement of reactivity considerable experimental and theoretical effort was devoted to the exploration of the role of vibrational energy in bimolecular reactions. On the other side the role of rotational energy and the vector properties of j were only marginally investigated. One obvious reason for this is that only minute effects due to the usually much smaller energy of rotation were expected and first experimental results supported this... 

For the first two time regions mode-locked or pulsed lasers must be used. In most experiments performed so far, pulsed CO2 lasers or chemical lasers were utilized for vibrational excitation of reactant molecules. As a specific example of a laser-enhanced bimolecular reaction we consider the reaction... 

For reasons given in Section 1.2.2, there have been few direct experiments on bimolecular reactions involving molecules with more than one quantum of vibrational excitation. However, the energies associated with single quanta are comparable with the activation energies of many elementary atom-transfer reactions, so the resultant rate enhancement can be considerable and revealing. In this section, data on the reactions (and parallel relaxations) of diatomic hydrides such as Hg, HX (X = F, Cl, Br, I), and OH, are reviewed first, and then some examples are provided of measurements on reagents excited by CO2 laser photons. 

In summary, vibrational nonequilibrium effects in bimolecular reactions are important only if a substantial fraction of the total reactive flux occurs via the excited state and if vibrational relaxation is slow. The effects are enhanced during the induction period preceding attainment of the steady-state condition. Hence, it may be surmised that the ignition delay of shock-... 

---