S. R. Greenfield

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 120u2009°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...

Nanosecond time scale dynamics of pseudo-nematic domains in the isotropic phase of liquid crystals

Abstract The slow reorientational dynamics of isotropic methoxybenzylidene-butylaniline (MBBA) were measured, using a transient grating optical Kerr effect experiment, over a wide temperature range (49.4°–119.7°C). The range of validity of the Landau—de Gennes theory for hydrodynamic relaxation in ordered fluids is determined. The results are compared to previous experiments on pentylcyanobiphenyl (5CB). Both liquid crystals conform to the Landau—de Gennes theory until the pseudo-nematic domain correlation length falls below three molecular lengths.

Optical dephasing of a near infrared dye in PMMA: photon echoes using the superconducting accelerator pumped free electron laser

Abstract A superconducting accelerator pumped free electron laser is used to perform picosecond photon echo experiments on the near infrared dye 1,1′,3,3,3′,3′-hexamethylindotricarbocyanine iodide (HITCI) in poly(methyl methacrylate) (PMMA). The temperature dependence of the optical dephasing is measured from 1.5 to 10 K. At the lowest temperatures a temperature dependence, T −1.4 , characteristic of two-level system glass dynamics is observed. Above 5 K, the optical dephasing is exponentially activated with an activation energy of 15 cm −1 . This is the same activation energy reported for rhodamine B in PMMA, demonstrating that 15 cm −1 corresponds to an intrinsic glass mode. These are the first photon echo experiments, and to our knowledge, the first nonlinear optical coherence experiments, performed using a FEL as a source.

Superconducting, transition, and normal state photoresponse in YBCO observed at different temperatures

The photoresponse of YBCO films was measured with a novel technique between 7 K and 200 K. YBCO was exposed to variable-fluence (<or=3 mu J) 300-fs 665-nm 2-kHz laser pulses. Photoinduced impedance changes in the samples biased with a DC current exhibited a transient voltage signal. Typically the voltage signal had a sharp (<50-ps) rise followed by a more gradual decay. Below T/sub c/, the response did not exhibit a simple exponential relaxation time. Relaxation occurred by a serial combination of fast ( approximately 200-700 ps) and slow ( approximately 1-ns) decay time constants. At T/sub c/, the photoresponse was the highest, and the relaxation time constant was typically greater than 1 ns. Above T/sub c/, the relaxation voltage signal was smaller in amplitude and followed a simple exponential dependence. The relaxation time constant for YBCO films on MgO was greater than 2 ns and was about 1 ns for YBCO films on LaAlO/sub 3/. The results yield a signal well below T/sub c/ (a quantum response); in addition, the negative photoresponse signal corresponds to recombination in the superconducting state. With the Corbino sample geometry, the potential of positive response due to flux flow has been minimized. A response linear with current was observed. The response below 0.9 T/sub c/ is temperature independent. At and above T/sub c/, a thermal response due to absorption of the laser energy was observed.

Local structure dynamics in liquids: non-hydrodynamic orientational relaxation of 2-ethyl naphthalene

Abstract The rotational dynamics of 2-ethyl naphthalene have been studied from 2°C to 100°C using subpicosecond transient grating optical Kerr effect techniques. After ultrafast librational dynamics have decayed, there remains (⪢3–4 ps) a triple-exponential decay of the orientational anisotropy. The slower two decays exhibit hydrodynamic temperature dependences, corresponding to rotational diffusion. The fastest decay is not hydrodynamic, being temperature independent from 2°C to 40°C. This decay is attributed to the relaxation dynamics of local structures in the liquid.

Anomalous photoresponse of metal films

Current biased samples are illuminated by short pulses of laser light and voltages are measured for a range of temperature 7 K<T<350 K. Classically, no response is expected for times greater than 10 ps after the pulse arrival. However, transient signals with temperature‐independent decay times are observed in Ni and Nb samples, and a similar signal with diminished amplitude is observed in a Au sample.

Picosecond infrared vibrational photon echoes and lifetime measurements using the Stanford superconducting-accelerator-pumped free-electron laser

The first infrared vibrational photon echo experiments conducted in a liquid and a glass are reported. The experiments were performed on the CO stretching mode of tungsten hexacarbonyl at 5.1 micrometers (1960 cm-1) in 2-methyltetrahydrofuran over the temperature range 300 K to 16 K using picosecond pulses from the free electron laser at Stanford University. In addition, the first vibrational population relaxation measurements spanning a temperature range that takes a system from a liquid to a supercooled liquid to a glass are reported.

Ultrafast dynamics of nematic liquid crystals in the isotropic phase

Measurements of orientational relaxation over 6 decades in time have been made on the liquid crystal Methoxy Benzylidene butyl aniline (MBBA) using a Transient Grating Optical Kerr effect experiment (TG-OKE). The Slower dynamics have been shown to fit to Landau-de Gennes modified Debye Stoke Einstein Hydrodynamic equation. The faster dynamics show a power law behavior that is temperature independent for 43 degree(s) above the nematic-isotropic phase transition. The slower dynamics deviate from Landau-de Gennes behavior at the same temperature that the faster dynamics become temperature dependent. This is attributed to the domain size, the factor controlling the slow decay, becoming small enough that the local structure is disturbed. Two possible sets of processes are proposed for the power law dependence of the faster dynamics. A parallel process, the Forster direct transfer model, where there is a distribution of potential surfaces for the system to propagate along and the serial process (or Hierarchically constrained dynamics model) where some degrees of freedom are suppressed unless other degrees of freedom are in particular states. These results are compared to earlier work on pentylcyanobiphenyl(5CB). The same behavior is seen in both 5CB and MBBA.