Vibrational dynamics in complex systems investigated with non-linear ultrafast infrared techniques

Ultrafast non-linear infrared techniques were used to study the vibrational dynamics in both moderately sized molecules in the collisionless gas phase and in hydrogen-bonded liquids. The experiments on hydrogen-bonded liquids were carried out using multi-dimensional infrared techniques related to COSY in NMR spectroscopy.; For the gas phase experiments, infrared ps pump-probe experiments are presented for the P, Q, and R rotational branches of the asymmetric CO stretching mode of tungsten hexacarbonyl (1997 cm−1) in the collisionless gas phase. The pump-probe decays are tri-exponentials (140 ps, 1.3 ns, and >100 ns) in contrast to single exponential decays observed in supercritical fluids and liquid solvents. The 1.3 ns decay component is the vibrational energy relaxation (VER) time. The long-lived signals result from an increase in the occupation numbers of low frequency modes (internal heating) that causes a shift of the vibrational spectrum. The fastest decay is produced by spectral diffusion. Spectral diffusion is caused by the time evolution of the complex low frequency thermal population distribution, which differs from molecule to molecule.; Experimental results and calculations are presented to explain hydrogen bonding in both methanol-OD and water samples. The experiments are carried out using ultrafast (<50 fs) infrared multidimensional stimulated vibrational echo correlation spectroscopy with full phase information and spectrally resolved pump-probe spectroscopy.; In the methanol experiments, hydrogen bond breaking allows data to be collected for times much longer than the vibrational lifetime. Both the pump-probe data and the correlation spectra indicate that the hydrogen bonds that remain after vibrational relaxation are weaker than exist in the sample at thermal equilibrium. The correlation spectra indicate that the vibrational relaxation selectively breaks the strongest of the hydrogen bonds.; The water experiments are carried out on HOD in H₂ O. The experimental data are compared to MD simulations using the TIP-4P and SPC/E models of water through calculations based on response function theory. The TIP-4P model is shown to produce dynamics that are significantly too fast. The SPC/E model includes a slowest component of 980 fs, while a triexponential fit to the experimental data indicates a slowest component of 1.2 ps.