Vincent J. Newell

Anisotropic reorientational relaxation of biphenyl: Transient grating optical Kerr effect measurements

Subpicosecond transient grating optical Kerr effect measurements have been used to evaluate the reorientation of biphenyl molecules in neat biphenyl and n‐heptane solutions. Besides an ultrafast (100 fs time scale) component associated with librational damping/dephasing, two reorientational relaxation components are observed. The slow reorientation is due to rotation around the short axes of the molecule (tumbling motion), the fast reorientation is associated with internal rotation around the central C–C bond and/or rotation of the whole molecule around its long axis (spinning motion). Whereas the tumbling motion has been observed in earlier depolarized light scattering data, the time resolved Kerr data presented here are the first ones to reveal the dynamics of the fast reorientation component and the ultrafast librational dynamics. It is shown that the diffusive reorientational relaxation must be coupled to the ultrafast librational dynamics, and implications of this coupling are pointed out.

Nonhydrodynamic molecular motions in a complex liquid: Temperature dependent dynamics in pentylcyanobiphenyl

The isotropic phase of nematogenic substances above the nematic–isotropic phase transition temperature Tni is characterized by short range orientational correlation of the molecules and the existence of pseudonematic domains. This microscopic structure has a strong influence on reorientational dynamics in the liquid. We present time resolved transient grating optical Kerr effect measurements of pentylcyanobiphenyl between 35 and 120 °C, which allow us to monitor dynamics on timescales from subpicoseconds to tens of nanoseconds. The well‐known collective component characterizing the reorientational dynamics of the pseudonematic domains follows Landau–deGennes behavior up to 30–35 degrees above Tni where the theoretical correlation length ξ≊three molecular lengths. Additionally fast relaxation channels (with time constants between 1 and 300 ps) are observed which are associated with local reorientational and intermolecular dynamics within the pseudonematic domains. The local dynamics are temperature indepen...

Pseudolocal modes of guest molecules in mixed molecular crystals: Photon echo experiments and computer simulations

The temperature‐dependent optical dephasing of anthracene, 9‐methylanthracene, and 2‐methylanthracene monomers in phenanthrene host crystals has been measured using photon echo experiments. Despite large linear electron–acoustic phonon coupling, all three systems dephase because of coupling to pseudolocal modes (local motions of the guest molecule). Computer simulations of the three systems calculate the pseudolocal mode eigenvalues and eigenvectors. In contrast to previous discussions in the literature which describe pseudolocal modes as librations, the predicted eigenvalues are in reasonable agreement with the measured pseudolocal mode energies. The predicted eigenvectors are combinations of translational motion along the long molecular axis and rotational motion about the out‐of‐plane axis of the guest. Differences in site energies for various locations and orientations of the methyl group are calculated.

Polariton effects in transient grating experiments performed on anthracene single crystals

Abstract Excited state dynamics in anthracene crystals at low temperatures are studied with the transient grating technique. In contrast to work reported earlier, the excitation wavelength is in the vicinity of the polariton stop band, and there is a pronounced dependence of the time response on the wavelength. A qualitative discussion of the wavelength dependence of the grating transients is presented.

Picosecond Transient Grating Experiments on Hydrogenated Amorphous Silicon: A Model for Surface Quenching

In this article we present a room temperature experimental study of the transport and surface quenching of optically generated charge carriers in thin-film hydrogenated amorphous silicon (a-Si:H). At low temperature (4.2 K), in high quality samples, the principal mechanism for carrier quenching is radiative recombination.l Nonradiative processes such as Auger recombination and tunneling to defects play only a minor role.1 However, it has been suggested2 that surface effects play an important role at higher temperatures (77 K). Rehm et al.2a examined the effects of sample thickness and wavelength (optical penetration depth) on the decay time of the luminescence. They found dramatic effects when the sample thickness or the optical penetration depth fell below 0.3 µm.