State preparation overtone excitation

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) ... 

In recent experiments lasers have been used to excite unimolecular reactants. In overtone excitation, states in which an MH or MD bond contains n quanta of vibrational energy are prepared by direct single-photon absorption. Here, M is a massive atom in contrast to H or D. For states with large n, the energy in the bond may exceed the molecule s unimolecular threshold. Overtone excitation of CH and OH bonds has been used to decompose molecules (Crim, 1984, 1990). One limitation of this technique is the very weak oscillator strengths of overtone absorptions. 

Figure 10 The left-hand panel shows the excitation scheme used in a typical doubleresonance overtone photofragment spectroscopy experiment. The particular scheme shown here is used to prepare HOCl molecules with six quanta of OH stretching and many quanta of rotation, just above the dissociation threshold. The right-hand panel depicts sample double-resonance spectra of the HOCl(6ui <— 2vi) band. Each spectrum in the plot originates from a selected (J, K ) rotational state. Courtesy of T. R. Rizzo.

The concept of intramolecular vibrational energy redistribution (IVR) can be formulated from both time-dependent and time-independent viewpoints (Li et al., 1992 Sibert et al., 1984a). IVR is often viewed as an explicitly time-dependent phenomenon, in which a nonstationary superposition state, as described above, is initially prepared and evolves in time. Energy flows out of the initially excited zero-order mode, which may be localized in one part of the molecule, to other zero-order modes and, consequently, other parts of the molecule. However, delocalized zero-order modes are also possible. The nonstationary state initially prepared is often referred to as the bright state, as it carries oscillator strength for the spectroscopic transition of interest, and IVR results in the flow of amplitude into the manifold of so-called dark states that are not excited directly. It is of interest to understand what physical interactions couple different zero-order modes, allowing energy to flow between them. A particular type of superposition state that has received considerable study are A/-H local modes (overtones), where M is a heavy atom (Child and Halonen, 1984 Hayward and Henry, 1975 Watson et al., 1981). 

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