Richard E. Wilde

A memory–functions approach to vibrational relaxation in dense media

We propose a new method to study some effects concerned with vibrational relaxation in dense molecular media. Our approach makes use of the memory‐functions technique to result in a clear physical description of the behavior in the system. The crucial point in this study is the introduction of a noninteracting reference system with regard to which the perturbation parameters in the real system are derived. These parameters consist of a time‐modulation parameter τ and an interaction parameter labeled κ2 which accounts for the real‐system spectral fourth moment. We show how the last parameter may be obtained from spectral measurements on the system. The time parameter which describes (by means of a Poisson distribution) a stochastic process for the random force acting on the molecules remains as an adjustable parameter. We compare our results with some existing simplified calculations and note the possibility of estimating the range of τ from such calculations. We apply our method to some new experimental d...

Vibrational correlation‐function modeling in the context of the Zwanzig–Mori formalism. Application to liquid ethane

The relationship between the memory‐function approach to correlation‐function modeling and the Mori continued‐fraction scheme suggests that unsuspected dynamical couplings may require higher‐order procedures to adequately model some correlation functions. The application of first‐, second‐, and third‐order memory‐function procedures and their significance is discussed. Some guidelines for the experimentalist are given, and the problem of the truncation of isotropic Raman bands is discussed. The spontaneous Raman‐scattering spectrum of the ν3 band of C2H6 has been reexamined for sample temperatures in the range 93–168 K. Both the isotropic and anisotropic spectra have been obtained, and the vibrational‐correlation function has been modeled within experimental error. The need to go to third order in the modeling procedure and the values of the second moments of the IVH spectra suggest a vibrational–rotational coupling. The depolarization ratio for the ν3 band was observed to be 0.21.

Correlation function modeling via the third‐order memory function: Application to ethane

In order to understand the vibrational correlation function of the ν3 band of liquid C2H6, it has been necessary to include third‐order memory functions in the modeling procedure. The analysis indicates that the vibrational dephasing process may be a complex one involving both the spinning motion around the threefold symmetry axis and the tumbling motion of the axis itself. Information about the molecular dynamics in the temperature range 93–168 K has been obtained out to 7 ps.

Microwave rotational spectrum of methyl azidoformate

The R branch, a dipole microwave rotational spectra of four isotopic species of methyl azidoformate have been assigned over the range 18–40 GHz. The assignment includes vibrational satellite series as well as the vibrational ground state transitions. The methyl barrier to internal rotation as determined from the E–A doubling data for the first excited state of the methyl torsion is V3=400±10 cm−1. The methyl barrier for the first excited state of another vibrational mode, methyl ground state, is determined to be V3=370±10 cm−1. Only one conformation, that with the azido group and the methyl group cis to the C=O group, was found to be measurably populated at room temperature. A partial vibration–rotation anaylsis has been performed with extremely limited success.

Vibrational dephasing of heavy phosphine in the liquid and plastic crystalline states

Vibrational correlation functions obtained from the spontaneous Raman scattering of the P–D stretching vibration of liquid and solid PD3 have been modeled using a memory‐functions approach. It has been shown that the modulation of the frequency is fast and most probably arises from the rotational motions of the molecules. Information about the molecular dynamics has been obtained out to 7 ps.

Detection of phase transitions in heavy silane by i.r. spectroscopy

Abstract An i.r. spectroscopic study of SiHD 3 isolated in SiD 4 reveals an apparent second-order phase transition at 38 K. In the lowest-temperature phase the SiD 4 molecules probably occupy two sites of C 1 symmetry. Between 38 K and 67 K the SiD 4 molecules probably occupy one site of C s symmetry.

A statistical analysis of the ν2 and ν3 modes of methyl iodide

The ν2 and ν3 Raman-band profiles of methyl iodide have been carefully measured as a function of temperature. Both I iso(ω) and I VH(ω) profiles have been extracted and Fourier transformed to provide the corresponding autocorrelation functions. The second moments of the bands have been determined to ±15 per cent. The vibrational-correlation and memory functions have been analysed by a memory-function modelling procedure and by an autoregressive time-series analysis. In order to separate the effects of resonant vibrational energy transfer and dipolar interactions from pure dephasing processes, CH3I has been studied in the solvents CD3I, CH3F, n-pentane, and CS2. Evidence is presented for intermolecular vibration-rotation interaction in the ν2 and ν3 modes. A large contribution to the dephasing by resonant vibrational energy transfer has been found for both modes, with dipole-dipole interaction contributing a lesser amount. Our analysis suggests that static inhomogeneity contributes a constant 8–10 cm-2 to ...

MATRIX-ISOLATED SILANE. INFRARED SPECTRA.

The infrared spectra of silane and its deuterated species isolated in solid neon, argon, krypton, xenon, nitrogen, carbon monoxide, and methane have been obtained. In addition, argon—krypton mixed‐matrix studies and oxygen‐doping experiments have been done. It is shown that silane occupies at least two types of defect sites. There is no evidence for free or nearly free rotation of silane in any of the matrices.

Rayleigh scattering in Sr0.61Ba0.39Nb2O6

Rayleigh scattering (λ=488 nm) is observed in single‐crystal Sr0.61Ba0.39Nb2O6 (SBN:61). In particular, the scattered intensity is measured as a function of temperature across the ferroelectric–paraelectric phase transition. The results show that the scattered intensity can vary by as much as two orders of magnitude in passing through the transition. The particular form of the intensity‐vs‐temperature curve is found to depend on the poling history of the crystal. The unpoled crystal exhibits a rapid change from strong scattering below the transition (ferroelectric state) to weak scattering above the transition (paraelectric state). The measured inflection point is near 63 °C upon heating and near 58 °C upon cooling. In this case the scattering is predominantly attributed to the presence of high‐density ferroelectric domains, which vanish above the transition. If the crystal is thermally poled (by cooling through the transition with an electric field applied), it shows no measurable domain scattering but does exhibit strong central‐peak behavior at 65 °C upon heating and 59 °C upon cooling. Despite repeated cycling through the transition, the previously thermally poled crystal consistently displays central‐peak behavior. The results suggest that thermal poling in SBN followed by thermal depoling produces a significant increase in the average ferroelectric domain size when compared to the unpoled crystal.

Vibrational dephasing in liquid SiH4

The Raman spectrum of the v 1 band of SiH4 in the liquid phase has been obtained as a function of temperature. The band profile has been Fourier transformed to provide vibrational-autocorrelation and memory functions. These functions have been analysed by a memory-function modelling procedure and by an autoregressive analysis. The lorentzian and gaussian bandwidths obtained from a Voigt-profile analysis are compared with the bandwidths calculated using a hard-sphere, binary-collision theory.