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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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The time and frequency resolved optical response of wild-type green fluorescent protein (wt-GFP) has been measured at room temperature following 30 fs, 400 nm photo-excitation. In the wavelength range covering the stationary fluorescence spectrum of the protein, the stimulated emission rises on a time scale of roughly 20 ps due to excited-state proton-transfer (ESPT). The rise can be described phenomenologically by a sum of two exponentials. A long-time isosbestic behavior on the blue edge of the stationary emission implies a barrier for ESPT which is significantly larger than thermal excitations. In addition, an instantaneous component to the stimulated emission appears within the time resolution of our experiment. This observation is indicative of nonvertical cross-well transitions that prepare the proton-transferred configuration of the excited state directly from the equilibrium geometry of the ground-state neutral species during photo-excitation. Finally, transient absorptions around 500 nm and 650 nm can be observed, which are attributed to transitions from different protonated forms of the excited-state of GFP to higher lying electronic configurations, S n . The entire optical response of GFP is quantitatively simulated using a dynamic model that includes: (i) an energy-dependent rate coefficient for ESPT, (ii) intra-and intermolecular transfer of excess vibrational energy (IVR and VET), and (iii) an additional non-radiative decay pathway for the initially prepared Franck-Condon state leading to internal conversion via motion along a torsional coordinate. In particular, the nonexponential nature of the ESPT originates from overlapping time scales of reactive and non-reactive elementary processes following optical excitation.

A high-quality depolarized Raman-spectrum is obtained in the frequency range 0 p o p 600 cm À1 by Fouriertransformation of time-resolved dual-color heterodyne-detected optical Kerr-effect data of liquid water at 0 C. The time-resolution was sufficient to fully capture the restricted translational and part of the hindered rotational region of the Raman spectrum. This low-temperature spectrum is used to test the applicability of stochastic line broadening theories. A conventional Kubo line shape analysis indicates that restricted translational modes involving hydrogen-bond bending and stretching motions are predominantly in the slow modulation limit at temperatures close to the melting point. However, a pronounced residual fine structure exists which cannot be fully accounted for by the theory in its standard form. Instead, we propose to apply a modified Kubo model based on truncating its continued-fraction representation at a finite order N including a convolution with a quasi-static structural inhomogeneity in the liquid. In particular, a quantitative agreement of our experimental data with such an inhomogeneous N-state random-jump model is interpreted with a discrete size distribution of aggregates which can interconvert on a time scale of about 500 fs by breaking and making of hydrogen bonds.

The ultrafast relaxation dynamics of the well-known solvated electron in liquid ammonia solutions are investigated with femtosecond near-infrared pump-probe absorption spectroscopy. Immediately after photoexcitation, the dynamic absorption spectrum of the electron is substantially red-shifted with respect to its stationary spectrum. A subsequent dynamic blue shift of the pump-probe spectrum occurs on a timescale of 150 fs. The data are understood in terms of ground-state "cooling" and can be quantitatively simulated by an intuitive temperature-jump model employing a dynamically evolving Kubo line shape for the electronic resonance. A simple estimate implies that, on average, the electron in the liquid is coordinated to six nearest-neighbor ammonia molecules. An equivalent analysis of the data based on a bubble-formation/cavity-contraction mechanism is briefly outlined.

The population relaxation of the OH-stretching vibration of HOD diluted in D2O is studied by time-resolved infrared (IR) pump-probe spectroscopy for temperatures of up to 700 K in the density range 12 1 OH stretching transition with a 200 fs laser pulse centered at approximately 3500 cm(-1). Above 400 K these spectra show no indication of spectral diffusion after pump-probe delays of 0.3 ps. Over nearly the entire density range and for sufficiently high temperatures (T > 360 K), the vibrational relaxation rate constant, kr, is strictly proportional to the dielectric constant, epsilon, of water. Together with existing molecular dynamics simulations, this result suggests a simple linear dependence of kr on the number of hydrogen-bonded D2O molecules. It is shown that, for a given temperature, an isolated binary collision model is able to adequately describe the density dependence of vibrational energy relaxation even in hydrogen-bonded fluids. However, dynamic hydrogen bond breakage and formation is a source of spectral diffusion and affects the nature of the measured kr. For sufficiently high temperatures when spectral diffusion is much faster than energy transfer, the experimentally observed decays correspond to ensemble averaged population relaxation rates. In contrast, when spectral diffusion and vibrational relaxation occur on similar time scales, as is the case for ambient conditions, deviations from the linear kr(epsilon) relation occur because the long time decay of the v = 1 population is biased to slower relaxing HOD molecules that are only weakly connected to the hydrogen bond network.