Fig. 1-1 Calculated cumulative reaction probability for the F + HD reaction Cumulative reaction probability is the integrated probabilities for reaction over all existing vibrational and rotational states of the initial and final molecules undergoing reaction. Two sharp resonances are found at 0.2433 eV (peak M) and 0.2658 eV (peak N). These can be assigned as F...HD Van der Waals complexes occurring in the tunneling region and dissociating into the product with an extremely high reaction probability.

Fig. 1-2 Laser-induced decomposition of Van der Waals complex leading to chemical reaction Quasibound states of Van der Waals complexes (AB...C) and (A...BC) including the respective resonance states can be found at the entrance and exit of the reaction path potential barrier. Some resonance states can transfer by an interference effect, including the tunneling mechanism, into the product region with an extremely high reaction probability (cross-section).

Atoms and molecules condense at low temperatures through various kinds of attraction forces operative between all atoms and molecules when they are close together. Among them, the Van der Waals force, which is quite weak, occurs as the result of correlated motions of electrons in the adjacent atoms and molecules. We investigated theoretically the role of the Van der Waals force in chemical reactions and predicted an occurrence of a new type of laser induced chemical reaction. For the F + H2 HF + H reaction, which is important in understanding reaction dynamics, many researchers have calculated probabilities for the reaction at various energies of the reacting system using the potential energy surface obtained by ab initio M.O. methods. It is noted that the reacting system, having energy less than the potential barrier, can react through the quantum mechanical H-atom tunneling phenomenon. However we are the first to consider the weak Van der Waals interaction in this calculation and to study in detail quasibound states localized in the reactant and product Van der Waals regions, i.e. (F...H2) and (HF...H) formed at the entrance and exit of the potential energy surface, respectively. Interestingly, we found that, when the energy levels of the reacting system (F + H2, D2 and HD) coincide with levels of the quasibound states, quantum interference effects play a decisive role in determining reaction probabilities (Fig. 1-1). The quasibound states include many internal states originating from rotational, vibrational and electronic states of each molecule, and if each state occurs in the tunneling region, having a dissociation channel into the stable product, it easily undergoes line-broadening, and consequently significant interference. This phenomenon can be applied to control chemical reaction (Fig. 1-2). Van der Waals complexes of the type AB(ground state)...C may be prepared for example, in a molecular beam, and excited by an appropriate laser to yield AB(excited)...C. This may lead to dissociation into A + BC with an extremely high probability due to the interference effect with tunneling. Thus, DF(deuterium fluoride) may be selectively separated from a mixture of H2, HD, and D2.

Reference T. Takayanagi et al., Van der Waals Resonances in Cumulative Reaction Probabilities for the F + H2, D2, and HD Reactions, J. Chem. Phys., 109(20), 8929 (1998).

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