We review several methods for the calculation of vibrational spectra from ab initio molecular dynamics (AIMD) simulations and we present a new implementation in the trajectory analyzer TRAVIS. In particular, we show mass-weighted power spectra, infrared spectra, and Raman spectra with corresponding depolarization ratios, which are based on time-correlation functions of velocities, dipole moments, and polarizabilities, respectively. Using the four organic molecules methanol, acetone, nitromethane, and pinacol as test systems, we compare the spectra from AIMD simulations of the isolated molecules in gas phase to static calculations relying on the harmonic approximation and to experimental spectra recorded in a nonpolar solvent. The AIMD approach turns out to give superior results when anharmonicity effects are of particular importance. Using the example of methanol, we demonstrate the application to bulk phase systems, which are not directly accessible by static calculations, but for which the AIMD spectra also provide a very good approximation to experimental data. Finally, we investigate the influence of simulation time and temperature in the AIMD on the resulting spectra.

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The real and imaginary parts of individual tensor elements of the nonresonant third-order nonlinear optical susceptibility of simple liquids, carbon tetrachloride and benzene, have been characterized for the first time by transient grating optical heterodyne detected impulsive stimulated Raman scattering (TG-OHD-ISRS). Optical heterodyning is achieved through additional scattering induced by a thermal grating. Vibrational modes to 1000 cm-I are impulsively prepared and probed using 15 fs laser pulses. This technique allows for a precise determination of frequencies and dephasing times of intramolecular Raman-active vibrations. It is shown that the symmetry of these modes defines the measured phase of the oscillatory modulations of the various linearly detected tensor components of ~(~' ( t ) .Advantages to conventional techniques, such as optical heterodyne detected Raman induced Ken effect spectroscopy or homodyne detected impulsive stimulated Raman scattering, are demonstrated. Further detection of the real or imaginary part of the susceptibilities using phase locked femtosecond optical pulses is discussed.

A spectroscopic investigation of the vibrational dynamics of water in a geometrically confined environment is presented. Reverse micelles of the ternary microemulsion H2O/AOT/n-octane (AOT = bis-2-ethylhexyl sulfosuccinate or aerosol-OT) with diameters ranging from 1 to 10 nm are used as a model system for nanoscopic water droplets surrounded by a soft-matter boundary. Femtosecond nonlinear infrared spectroscopy in the OH-stretching region of H2O fully confirms the core/shell model, in which the entrapped water molecules partition onto two molecular subensembles:  a bulk-like water core and a hydration layer near the ionic surfactant headgroups. These two distinct water species display different relaxation kinetics, as they do not exchange vibrational energy. The observed spectrotemporal ultrafast response exhibits a local character, indicating that the spatial confinement influences approximately one molecular layer located near the water−amphiphile boundary. The core of the encapsulated water droplet is similar in its spectroscopic properties to the bulk phase of liquid water, i.e., it does not display any true confinement effects such as droplet-size-dependent vibrational lifetimes or rotational correlation times. Unlike in bulk water, no intermolecular transfer of OH-stretching quanta occurs among the interfacial water molecules or from the hydration shell to the bulk-like core, indicating that the hydrogen bond network near the H2O/AOT interface is strongly disrupted.

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