Vibrational linewidth

A. V. Rao, F. Ozanam, and J. N. Chazalviel, In situ fourier-transform electromodulated infrared study of porous silicon formation Evidence for solvent effects on the vibrational linewidths, J. Electrochem. Soc. 138 153, 1991. 

R.G. Tobin, Vibrational linewidths of adsorbed molecules Experimental considerations and results. Surf Sci 1987,183, 226. 

Light sources can either be broadband, such as a Globar, a Nemst glower, an incandescent wire or mercury arc lamp or they can be tunable, such as a laser or optical parametric oscillator (OPO). In the fomier case, a monocln-omator is needed to achieve spectral resolution. In the case of a tunable light source, the spectral resolution is detemiined by the linewidth of the source itself In either case, the spectral coverage of the light source imposes limits on the vibrational frequencies that can be measured. Of course, limitations on the dispersing element and detector also affect the overall spectral response of the spectrometer. 

Due to the rather stringent requirements placed on the monochromator, a double or triple monocln-omator is typically employed. Because the vibrational frequencies are only several hundred to several thousand cm and the linewidths are only tens of cm it is necessary to use a monochromator with reasonably high resolution. In addition to linewidth issues, it is necessary to suppress the very intense Rayleigh scattering. If a high resolution spectrum is not needed, however, then it is possible to use narrow-band interference filters to block the excitation line, and a low resolution monocln-omator to collect the spectrum. In fact, this is the approach taken with Fourier transfonn Raman spectrometers. 

Therefore, the absorjDtion line is massively inlromogeneously broadened at low temperature. An inliomogeneous lineshape can be used to detennine the static or quasistatic frequency spread of oscillators due to a distribution of environments, but it provides no dynamical infonnation whatsoever [94, 95]. As T is increased to 300 K, the absorjDtion linewidth decreases and increases. At 300 K, the lineshape is nearly homogeneously broadened and dominated by vibrational dephasing, because fast dephasing wipes out effects of inliomogeneous environments, a well known phenomenon tenned motional narrowing [951. 

Brueck S. R. J. Vibrational two-photon resonance linewidth in liquid media, Chem. Phys. Lett. 50, 516-20 (1977). 

Figures 4 and 5 show the Raman and IR spectra of ce-Ss in the range up to about 100 cm A comparison of these spectra with those presented in Figs. 2 and 3 reveals that the linewidths are much smaller at low temperatures (ca. 0.02-0.2 cm ). The wavenumbers and assignments of the external and torsional modes as reported by Gautier and Debeau [106] and Becucci et al. [107] are listed in Table 3. The spectra in Figs. 4 and 5 clearly demonstrate that there is no gap between the external vibrations and the crystal components of the lowest internal vibration Vg. Moreover, at about 76 cm an IR active lattice mode appears between two components of the fundamental Vg at 74 cm and 79 cm respectively.

The last mentioned dynamical capability of lasers is in its infancy. It is possible in principle to make optical measurements which are analogous to coherent NMR measurements, and thereby to observe homogeneous linewidths in inhomogeneously broadened systems, to measure optical or vibrational Ti and T2 relaxation times directly, and to observe quantum recurrences. 

Reynolds S, Oxley DP (1996) Measurement and modeling of vibrational-mode lineshape and linewidth in inelastic electron tunneling spectroscopy. Surf Sci 368 324-329... 

On the basis of the observations mentioned above, we conclude that the vibration at 1875 cm-1 is a Si—H vibration. This follow from (1) the sharpness of the feature, linewidth less than 50 cm-1, and (2) the frequency relative to the range of measured B—H and Si—H frequencies. Figure 15 presents a model for the local bonding configuration involving the H atom. This model accounts for the frequency of vibration as well... 

Table 4.2. The basic parameters of the models under discussion for some adsorption systems. The dephasing (2f 4)) and the relaxation (2T 3)) contributions to the full spectral linewidth for local vibrations as well as the...

Resolution The energy resolution of EELS is nowadays around 15 cm-1 at best, but many instruments operate at lower resolution. The resolution of RAIRS is not limited by the spectrometer (as low as 0.5 cm-1) but by the linewidth of the vibration under study. 

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