M. Constant

Dynamics of molecular reorientational motion and vibrational relaxation for symmetric top molecules in the liquid phase

Detailed studies of the molecular relaxation processes in pure liquid t‐butyl chloride (TBC) and t‐butyl bromide (TBB) and in solutions in carbon tetrachloride and in n‐hexane have been made using several experimental techniques. Single particle reorientational motions perpendicular to the symmetry axis were studied using Raman scattering and a comparison of these results with those obtained from depolarized Rayleigh scattering allowed an evaluation of the importance of correlations in orientation between pairs of molecules in the two systems, also, collision induced scattering was not found to be significant. Additional cross comparisons of related data obtained using infrared absorption and dipolar absorption were made. In all cases, the original experimental data were subjected to Fourier transform analyses so that the time evolution of the resulting correlation functions could be studied. In general, at short times the reorientational motions are inertial, but at long times exponential decay of the co...

Raman scattering: Investigation of nematic and smectic ordering

Quantitative measurements of vibrational Raman depolarization ratios have been used to study molecular orientational order in nematic and for the first time, in smectic liquid crystals. Temperature dependence of the depolarization ratios of the terminal C≡N and the central biphenyl link vibrational bands of two alkylcyanobiphenyl compounds (CBn) and a cyclohexane derivative (PCH7) has been investigated extensively. Two experimental technics have been used: a conventional triple monochromator and the Raman microprobe to overcome the spurious depolarization due to the long wave director fluctuations. The orientational order parameters <P2≳ and <P4≳ were then calculated. The results have been discussed assuming repulsive forces are dominant for the CBn compounds and using the Maier–Saupe theory in the case of PCH7.

III–V photoconductive detectors : Gain and noise studies

Abstract The steady-state gains, the dynamical gains and the noise properties of N-type GaAs planar photoconductive detectors have been extensively investigated and it has been found that, in the gigahertz range, the noise due to the illumination of the photodetectors is lower than that of an avalanche photodiode. Planar photoconductive detectors using GaInAs materials, GaAlAs-GaAs and InP-GaInAs heterojunction layers have also been studied. A monolithic integrated photoreceiver in GaAs, constituted of an intedigitated photoconductive detector and a FET is presented.

Computational and experimental study of vibrational relaxation in liquids: Symmetric top molecules

Detailed studies of vibrational relaxation in pure liquid CH3I, t‐butyl chloride (CH3)3CCl, t‐butyl bromide (CH3)3CBr and in solutions in carbon tetrachloride and in n‐hexane, over the −90 to 50°C temperature range, have been made using Raman scattering. Assuming that only phase relaxation contributes to the isotropic band shapes, the mechanism of vibrational relaxation in these liquids was first studied by investigating the characteristics of the fluctuations of the vibrational frequencies, which are caused by the intermolecular interactions. A first study allowed us to determine the correlation time of these fluctuations. The obtained values are between 0.1 and 0.5 ps and are consistent with a binary collision model. This model has been used to develop an efficient computational model to describe the experimental data relative to vibrational relaxation and orientational motions as well.

Computer Simulation of Molecular Dynamics of Anisotropic Fluids

Abstract Molecular dynamics calculations have been performed for fluid systems containing ellipsoidal particles which interact pair-wise with a modified Lennard-Jones potential. Assuming molecular parameters close to those of the CB7 compound, we have calculated both static and dynamic properties such as the internal energy, the entropy and specific heat changes at T NI, the compressibility factor, the temperature dependence of the orientational order parameters, the reorientational angular momentum and velocity time-dependent self-correlation functions, and the diffusion coefficients in both the nematic and isotropic phases. The thermodynamical data show a transition between isotropic and nematic phases and the results obtained are in qualitative agreement with the experimental data. The influence of a decenterd dipole along the molecular axis has been studied and partially bilayered smectic phases have been obtained.

Investigation of orientational behaviour of nematic and smectic a compounds using the Raman microprobe

Abstract The Raman microprobe technique, has been used to probe orientational order in oriented samples of nematic CB 5 and nematic and smectic CB 8 versus spatial localization of the analysed microvolume. In the nematic state, the influence of long-wavelength director fluctuations is pointed out and an elastic constant derived.

Electrical performance and Raman scattering characterization of GaAs planar photoconductors

Raman scattering has been used to characterize the effects of processing techniques on the photosensitive area of GaAs planar photoconductors. The longitudinal optic (LO) phonon observed in the first-order Raman spectra was studied to probe the consequences of both chemical and plasma etching on the crystalline quality of an n-doped GaAs photosentitive area. This method has also been used to examine the stress at the interface of a Si3N4 film and an n-doped GaAs surface. For this purpose, three specially designed GaAs planar photoconductors (SN 353, AN 344, SN 344) have been fabricated. Electrical measurements on the photoconductors support the results of Raman study. First, as expected, increasing the number of processes leads to slightly poorer electrical performance. Secondly, a significant strain-induced shift in the GaAs LO phonon frequency has been observed under the Si3N4 film for the AN 344 photoconductor. However, the electrical performances of this device clearly indicate no dramatic change in the value of the n-doped surface potential.