David F. Underwood

Excited State Hydrogen Bond Dynamics: Coumarin 102 in Acetonitrile−Water Binary Mixtures

The time dependent change in the intermolecular response of solvent molecules following photoexcitation of Coumarin 102 (C102) has been measured in acetonitrile-water binary mixtures. Experiments were performed on mixtures of composition x(CH3CN) = 0.25, 0.50, 0.75, and 1.00. At low water concentrations (x(H2O) < or = 0.25) the solvent response is consistent with previous measurements probing dipolar solvation. With increasing water concentration (x(H2O) > or = 0.50) an additional response is found subsequent to dipolar solvation, exhibited as a rapid gain in the solvents polarizability on a approximately 250 fs time scale. Monte Carlo simulations of the C102:binary mixture system were performed to quantify the number of hydrogen-bonding interactions between C102 and water. These simulations indicate that the probability of the C102 solute being hydrogen bound with two water molecules, both as donors at the carbonyl site, increases in a correlated fashion with the amplitude of the additional response in the measurements. We conclude that excitation of C102 simultaneously weakens and strengthens hydrogen bonding in complexes with two inequivalently bound waters.

Three-photon near-threshold photoionization dynamics of isooctane

The electron survival probability following three-photon (9.3 eV total) near-threshold photoionization of neat isooctane is measured with sub-50 fs time resolution. The measured dynamics are nonexponential in time and are well described by a diffusion-controlled electron-cation recombination model. Excitation-power-dependent studies indicate that the unperturbed three-photon threshold ionization is only observed for pump irradiance below 0.5 TW cm2. At excitation fields above this level, the signal is no longer cubic in the excitation irradiance, and the observed electron survival probability dramatically changes, decaying as a single exponential in time.

Polarization-dependent detection of impulsive stimulated Raman scattering in α-quartz

The stimulation and detection of both the A- and E-symmetry Raman lines in α-quartz using ultrafast impulsive stimulated Raman scattering is presented. Using a flexible experimental set-up, each nuclear oscillatory mode is selected and analysed in the time domain separately. The mode selection is made possible with optical summing of the fields involved to reduce noise and assure accurate synchronization. The experimental procedure is compared to optical heterodyne detection Raman-induced Kerr effect spectroscopy (OHD-RIKES) and the data is compared with standard frequency-domain measurements.

The role of stress in the time-dependent optical response of silicon photonic band gap crystals

Amorphous silicon inverse opals with a complete photonic band gap have been studied with optical pump-probe spectroscopy. The pump-generated free carriers cause the reflection near the band gap to be lowered for the first few picoseconds due to induced absorption. After ∼5 ps, this effect disappears and an unexpected blue spectral shift is seen in the photonic band gap. The shift appears consistent with photoinduced stress caused by the thermal expansion mismatch between the silicon and its native oxide. However, simple mechanical models fail to quantify this behavior.

Time Resolved Direct Probing of the Change in the Local Solvent Response Following Excitation of a Solute

The change in the low frequency (0.1 − 500 cm−1) non-resonant Raman response following excitation of a dye molecule in solution has been measured as a function of time after resonant excitation of a solute using a two-dimensional mixed resonant, non-resonant time domain spectroscopy. This method provides a direct measurement of the change in the spectrum of the local environment as a dynamic event proceeds in a Condensed phase.

Polarization-dependent detection of Impulsive Stimulated Raman Scattering in alpha-quartz

We have, for the first time, stimulated and detected both the A- and E-symmetry Raman lines in alpha-quartz using ultrafast Impulsive Stimulated Raman Scattering. Using a heterodyne detection scheme along with a Transient Grating set-up, we match the prediction of stimulated Raman theory. Experimentally-determined lineshapes will be presented and discussed.