Nature and Energy Redistribution of Highly Vibrationally Excited Polyatomic Molecules

This thesis is a primarily experimental study of the spectroscopy and dynamics of highly vibrationally excited molecules in the ground and excited electronic states. Linear spectroscopic studies have been performed on CH-stretching overtones of medium and large sized molecules in the gas and solid phases. Also new time-resolved methods based on nonlinear uultiphoton ionization spectroscopy have been applied on the picosecond time-scale to the study of ultrafast intramolecular vibrational-energy redistribution (IVR) in isolated molecules. The first observation of high-energy CH stretching overtones of molecules in low-temperature solids is reported. Spectra of the νCH = 5 transitions of many aromatic molecules at ~2K reveal homogeneously broadened bands which are assigned to inequivalent local modes by studies of same isotopically substituted molecules. Typical linewidths give population lifetimes of ~50 fs for aromatic compared to 100-200 fs for methyl CHs in these large molecules. The polarization ratios of durene CH overtones are in good agreement with those predicted with the localized bond-mode transition moments. Overtone spectra of some gas-phase deuterated methanes and tetramethyl compounds have been studied. Spectra of CHD3 have shown severe vibrational mixing at νCH = 6 and 7 with an apparent tuning of the Fermi-resonance between |νCH> and discrete stretch-bend combination states, e.g., |(ν-1)CH, 2bend>. Nonetheless, the lifetime of the CHD3 overtones must be long (≥5 ps) as evidenced by the narrow resolution limited linewidths. The splitting of the |6,0>± states of CH2D2 is inferred to be <10 cm-1 and shows that the coupling of the CH oscillators is relatively weak in agreement with theoretical prediction. In general, the results on the CH overtones indicate a significant role of CH stretch-normal mode coupling in the spectra and dynamics. Picosecond pump and probe multiphoton ionization is developed for the study of IVR in isolated molecules. Experimental evidence for IVR is presented. Nonexponential decay of vibronic states of S1 trans-stilbene is analyzed in terms of a population rate equation and time-dependent Franck-Condon factor description of IVR. The thermally-averaged IVR time constant is about 2 ps. Also, the excess vibrational energy dependence of the S1 trans-stilbene isomerization rate is determined for total vibrational energy of 2000 to 4500 cm-1 and good agreement is found with calculated thermally averaged rates based on independent results on jet-cooled molecules.