An issue of central importance in chemical reaction dynamics is the nature of the energy transfer processes within and between reactants and products. At a fundamental level, bond rupture and formation can be understood in terms of the transfer of energy into the reaction coordinate, causing a bond to break, and then relaxation of the energy away from the newly formed bonds in the product molecules to the other degrees of freedom of the system. By their very nature, these processes are highly anharmonic, making their detailed characterization a formidable challenge. In recent years, spectroscopists have taken on the challenge of trying to characterize the quantum states of a molecule at the high vibrational energies corresponding to the chemically interesting regime. At these energies, the density of states becomes extremely high and the coupling between the states very strong, the result being that the vibrations can no longer be characterized in terms of simple isolated local or normal modes. In the extreme limit, where RRKM theory (Wardlaw and Marcus 1987) applies, there is rapid energy redistribution that, at least approximately, samples the available states statistically, allowing us to overlook many of the fine details. Although we are still far from having a complete understanding of the quantum state dynamics of systems in this regime, the recent progress that has been made in both experiment (Felker and Zewail, 1985; Go et al., 1990; Parameter 1982, 1983; Smalley 1982) and theory (Stuchebrukhov and Marcus 1993; Uzer 1991) is helping to better define the important processes. Ultimately, the detailed characterization of all the intramolecular couplings in a molecule would provide us with a basis for understanding the chemistry at a fundamental level, in both the statistical and nonstatistical regimes. After all, energy transfer from one vibrational mode of a molecule to another is determined by the intermode couplings, which, in the ground electronic states of molecules, are predominantly due to anharmonic and/or coriolis effects. Of course, the problem becomes even more challenging when one moves from the realm of isolated molecules to solvated systems.
Energy transfer within the hydrogen bonding network of water following resonant terahertz excitation
