Excited-state preparation and relaxation in the vibrational quasicontinuum.

The preparation of highly vibrationally excited states in polyatomic molecules is analyzed by quantum mechanically modeling the vibrational quasicontinuum. Our model includes 10 000 coupled states in an energy range of 200 cm-1, grouped into ten ‘‘tiers’’ according to their first-order couplings to other states. Our results reveal that state preparation in the quasicontinuum is dominated by the presence of ‘‘intramolecular vibrational relaxation (IVR) resonance’’ states embedded in the quasicontinuum. This hypothesis is confirmed by calculations using the method of optical potentials of the resonance positions and widths. These IVR resonance states correspond to high-energy vibrational motions which are only partially localized and which relax slowly into the dense state background, and which are possibly analogous to unstable classical periodic orbits. [ABSTRACT FROM AUTHOR]