Christopher J. Fecko

Local hydrogen bonding dynamics and collective reorganization in water: Ultrafast infrared spectroscopy of HOD/D2O

We present an investigation into hydrogen bonding dynamics and kinetics in water using femtosecond infrared spectroscopy of the OH stretching vibration of HOD in D(2)O. Infrared vibrational echo peak shift and polarization-selective pump-probe experiments were performed with mid-IR pulses short enough to capture all relevant dynamical processes. The experiments are self-consistently analyzed with a nonlinear response function expressed in terms of three dynamical parameters for the OH stretching vibration: the frequency correlation function, the lifetime, and the second Legendre polynomial dipole reorientation correlation function. It also accounts for vibrational-relaxation-induced excitation of intermolecular motion that appears as heating. The long time, picosecond behavior is consistent with previous work, but new dynamics are revealed on the sub-200 fs time scale. The frequency correlation function is characterized by a 50 fs decay and 180 fs beat associated with underdamped intermolecular vibrations of hydrogen bonding partners prior to 1.4 ps exponential relaxation. The reorientational correlation function observes a 50 fs librational decay prior to 3 ps diffusive reorientation. Both of these correlation functions compare favorably with the predictions from classical molecular dynamics simulations. The time-dependent behavior can be separated into short and long time scales by the 340 fs correlation time for OH frequency shifts. The fast time scales arise from dynamics that are mainly local: fluctuations in hydrogen bond distances and angles within relatively fixed intermolecular configurations. On time scales longer than the correlation time, dephasing and reorientations reflect collective reorganization of the liquid structure. Since the OH transition frequency and dipole are only weakly sensitive to these collective coordinates, this is a kinetic regime which gives an effective rate for exchange of intermolecular structures.

Isotropic and anisotropic Raman scattering from molecular liquids measured by spatially masked optical Kerr effect spectroscopy

Spatially masked optical Kerr effect (SM-OKE) spectroscopy is a nonresonant femtosecond pump–probe technique capable of measuring isotropic contributions to the transient birefringence of molecular liquids. In conjunction with traditional optical-heterodyne-detected optical Kerr effect spectroscopy, polarization-selective SM-OKE measurements are used to experimentally measure the anisotropic and isotropic third-order nonlinear response of CS2, acetonitrile, methanol, and water. These two responses, which allow the intermolecular dynamics to be separated by symmetry, form a complete and independent basis for describing the polarization dependence of nonresonant third-order experiments. The Fourier transform spectral densities of these responses are presented for each liquid and are interpreted in terms of the molecular and interaction-induced contributions to the many-body polarizability. The molecular contributions are suppressed in the isotropic response for all liquids, while the line shape in the inter...

Polarization-selective femtosecond Raman spectroscopy of low-frequency motions in hydrated protein films

Abstract We investigated the vibrational dynamics of proteins in amorphous hydrated films of lysozyme and myoglobin using polarization-selective time-domain Raman spectroscopy. The anisotropic spectra for these proteins all have a broad peak due to librational motion of side chains at 90 cm −1 and a background that may arise from bound water. The isotropic spectrum of lysozyme is similar to that of myoglobin, and has peaks at 240 and 500 cm −1 that are likely due to secondary structure fluctuations. These results suggest that low-frequency deformations of the protein molecule may contribute to the solvation dynamics of proteins in aqueous solution.

Dynamics of hydrogen bonds in water: Vibrational echoes and two-dimensional infrared spectroscopy

Water hydrogen bond dynamics are studied by interpreting the OH frequency correlation function of HOD in D2O obtained from vibrational echoes with molecular dynamics simulations. Heterogeneous hydrogen bond dynamics are investigated using 2D IR spectroscopy.

A unified analysis of ultrafast vibrational and orientational dynamics of HOD in D2O

Broadband infrared spectroscopy measures the ensemble-averaged vibrational dephasing, lifetime, and orientational dynamics of the OH stretch of HOD in D2O. Two-dimensional infrared spectroscopy probes spectral heterogeneity, revealing a slight frequency dependence to spectral diffusion.