James E. Griffiths

Molecular reorientations in liquid benzene: raman line shapes and 2D NMR relaxation

Abstract The interpretation of molecular reorientations in liquid benzene has been largely unsatisfactory because isotropic reorientation is usually assumed. By analyzing Raman line shapes and 2 D NMR relaxation times as a function of temperature, a picture of highly anisotropic molecular reorientation emerges. Rotation about the symmetry axis appears to be consistent with a slightly damped free rotation model of diffusion whereas the rotational motion perpendicular to the C 6 axis can best be interpreted in terms of a small-step brownian rotation model.

Molecular reorientational motion in liquid acetonitrile: Raman band shapes, diffusion constants and activation energy of reorientation

Isotropic I α(ω) and anisotropic I β(ω) Raman band shape analyses of the ν1(a1) fundamentals in liquid CH3CN and CD3CN are reported. The ν2 and ν4 fundamentals are complicated by secondary structure and therefore were not studied in detail. Values of the perpendicular rotational diffusion constants D⊥, derived from the ν1(a1) fundamentals of both isotopic species are in good agreement with those obtained using NMR relaxation and dielectric methods and in addition agree closely with values calculated from microviscosity theory. The latter is based upon a model which involves a connection between the rotational and translational motions in liquids. In this model, reorientations perpendicular to the C3 axis proceed by small step Brownian diffusion and D⊥ (Raman) for CD3CN yields an activation energy for liquid state molecular reorientations perpendicular to the C3 symmetry axis of 2.0 ± 0.3 kcal/mole, in good agreement with existing NMR and microviscosity values. Comparison of Raman data and results reported here with available infrared results reveals that the D⊥(ir) values are too high by a factor of 2. The source of the infrared errors is discussed.Isotropic I α(ω) and anisotropic I β(ω) Raman band shape analyses of the ν1(a1) fundamentals in liquid CH3CN and CD3CN are reported. The ν2 and ν4 fundamentals are complicated by secondary structure and therefore were not studied in detail. Values of the perpendicular rotational diffusion constants D⊥, derived from the ν1(a1) fundamentals of both isotopic species are in good agreement with those obtained using NMR relaxation and dielectric methods and in addition agree closely with values calculated from microviscosity theory. The latter is based upon a model which involves a connection between the rotational and translational motions in liquids. In this model, reorientations perpendicular to the C3 axis proceed by small step Brownian diffusion and D⊥ (Raman) for CD3CN yields an activation energy for liquid state molecular reorientations perpendicular to the C3 symmetry axis of 2.0 ± 0.3 kcal/mole, in good agreement with existing NMR and microviscosity values. Comparison of Raman data and results reported...

Raman and depolarized Rayleigh scattering in the liquid state: Reorientational motions and correlations in orientation for symmetric top molecules

Reorientational motions perpendicular to the major axis of symmetric top molecules in the liquid state have been examined as a function of temperature by investigating isotropic and anisotropic Raman scattering vibrational band contours. These studies yield information about single molecule reorientation for the systems C6H6, C6D6, CH3CN, CD3CN, CH3I, DCCl3, and HCBr3. Depolarized Rayleigh scattering measurements on the same systems, also as a function of temperature, are combined with data on single particle reorientations to yield information about correlations in orientations between pairs of molecules. These correlations between molecules are found to be important for most symmetric top molecules in the liquid state and the temperature dependence of the Raman and Rayleigh band shapes and bandwidths suggest that hard core forces dominate the cooperative motions in the systems studied.

Molecular Structures of PCl4F, PCl3F2, PCl2F3, and PF5: Infrared and Low‐Temperature Raman Vibrational Spectra

Molecular structures of PCl4F, PCl3F2, PCl2F3, and PF5 are derived from vapor‐state infrared spectra (2000–250 cm—1) and low‐temperature liquid‐state Raman displacements (Δν=50–1200 cm—1) together with qualitative polarization measurements. All of the compounds appear to have a basic trigonal bipyramidal framework.The spectra of PCl4F are best interpreted in terms of a C3v structure in which the fluorine atom occupies an axial site; in PCl3F2 (D3h point group) the fluorine atoms also assume axial positions. For PCl2F3, the symmetrical D3h structure is shown to be incorrect. Available evidence supports the C2v structure in which the fluorine atoms appear in one equatorial and two axial sites. Phosphorus pentafluoride is found to have a regular trigonal bipyramidal structure.Complete vibrational assignments are made in terms of the normal modes and thermodynamic functions are evaluated for PCl5, PCl3F2, and PF5.

Relative and Absolute Raman Scattering Cross Sections in Liquids

Peak and total differential Raman scattering cross sections for several liquids, CH3OH, C2H5OH, i‐C3H7OH, (CH3)2CO, CH3CCl3, CH3I, c‐C6H12, and C6H5Br, were determined relative to the v2 (a1g)=944 cm−1 line of C6D6 as an internal standard. Using an absolute value for the peak differential cross section of this line and measured values of the radiant intensities and depolarization ratios of selected Raman lines in the above liquids, we have obtained absolute values for ∂σ and ∂σ/∂Ω. Results are expected to be accurate to ± 10 % unless specified otherwise. Measurements were made using a medium power cw argon ion laser operating at 4880 A, a double monochromator and a photomultiplier (S‐20 and S‐11) detector.

Intermolecular energy transfer in liquid benzene: Raman spectra, linewidth measurements, picosecond spectroscopy, and vibrational relaxation times

A collision-induced intermolecular vibrational energy transfer process in liquid C6H6 has been studied using detailed measurements of Raman line shapes and linewidths and picosecond spectroscopy. Lifetimes of molecules in the upper vibratonal energy level of the Raman transition ν2(ν =0→1) in C6H6 and C6D6 have been evaluated using both frequency and time measurements. By varying the absolute number of C6H6–C6H6 collisions over a wide range by dilution of C6H6 with C6D6, the lifetime of the ν′2 state can be altered from 4.7−7.0×10−12 sec. The higher value corresponds to an intrinsic lifetime of ν′2 in the liquid state while the other is a lower value because of a near resonance collision induced intermolecular vibration to vibration energy transfer process which occurs in pure C6H6. Direct measurement of the ν2(a1g) vibrational lifetimes by picosecond spectroscopy gives values of 5×10−12 sec for pure liquid C6H6 and 8×10−12 sec for pure liquid C6D6.

Raman scattering from anodic oxide‐GaAs interfaces

The nature of the interface between anodically grown oxide films and gallium arsenide substrates was studied using Raman backscattering. Room‐temperature spectra of GaAs covered with as‐grown anodic films as well as anodized samples dried under nitrogen at 250°C showed only the first‐order longitudinal (LO) and transverse (TO) optical modes and the less‐intense two‐phonon features of the GaAs substrate. Heating the films at 450°C and above results in the appearance of intense LO (257 cm−1) and TO (198 cm−1) bands due to crystalline arsenic and the structureless Raman scattering near 200–250 cm−1 due to amorphous arsenic. Polarized Raman spectra indicate that elemental arsenic is not an intrinsic oxidation product of the room‐temperature anodization. We suggest that the thermally induced solid‐state interfacial reaction, As2O3+2GaAs→Ga2O3+4As, is responsible for the presence of arsenic at the oxide‐semiconductor interface following annealing.

Stokes/Anti-Stokes Raman Vibrational Temperatures: Reference Materials, Standard Lamps, and Spectrophotometric Calibrations

The problem of measuring equilibrium vibrational temperatures in materials by using Raman spectroscopy has been analyzed theoretically and experimentally. One major problem is the determination of the corrections, due to the Raman instrument response, that must be applied to raw data to obtain the temperature. Normally a “standard” lamp is used for this purpose but the problems that can arise are legion. Methods for determining spectral response and temperature response function corrections that must be applied to raw Raman data, for ultimately deriving the equilibrium temperatures of various materials, have been developed such that standard lamps are not required. Instead pure vitreous silica and liquid cyclohexane, which are both readily available in all spectroscopy laboratories, are selected as reference materials of choice for determining the appropriate spectral corrections applicable to the spectrophotometric data. The results were tested against data obtained from other transparent materials, namely CCl4, CHCl3, CH3PCl2, GeCl4, GeBr4, and CS2. GeBr4 and CS2 were poor choices because of the proximity of the melting point of the former to room temperature and laser-induced photolysis of the latter. Detailed statistical analyses of the results indicate a marked lowering of systematic errors over those yielded by standard lamp calibrations. Shortcomings of both methods are also discussed, including several lesser known spectral abnormalities arising from the use of a standard lamp.

Infrared Spectra of Methylgermane, Methyl‐d3‐Germane, and Methylgermane‐d3

The infrared spectra of gaseous CH3GeH3, CD3GeH3, and CH3GeD3 were observed in the range 4000–400 cm—1. The 12 fundamental frequencies for each molecule are assigned to normal modes and the spectra are discussed in some detail. Teller—Redlich product‐rule ratios support the assignments. The height of the barrier hindering internal rotation was found from measurements on twelve combination bands involving the torsional modes to have an average value of 1270 cal/mole. The torsional force constant was estimated to be 0.31×10—12 erg/rad. The corresponding barrier height and torsional force constant for methylsilane were estimated to be 1715 cal/mole and 0.44×10—12 erg/rad, respectively, from published data.Analyses of the rotational structures of ν7–10 and of ν12 and 2ν12 in the spectrum of CH3GeH3 and of ν7–10 and ν12 in the spectrum of CD3GeH3 afforded values for the band origins, certain rotational constants and ζ7–12 for both molecules.

Spectroscopic Study of Radiofrequency Oxygen Plasma Stripping of Negative Photoresists. I. Ultraviolet Spectrum

The stripping of photoresists from a silicon wafer using an rf oxygen plasma has been monitored using the optical emission from electronically excited OH and CO species in the ultraviolet region of the spectrum. The band systems at 283.0 nm (CO*, OH*), 297.7 nm (CO*), and 308.9 nm (OH*) are intense and spectrally isolated from other systems and arise from plasma-induced oxidation of the polymeric photoresist material. The endpoint of plasma stripping and the amount of stripped material is easily determined quantitatively. In addition, variations in stripping rate of photoresist as a function of wafer position in the reaction chamber can be detected.