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J. Php. Chem. 1982, 86,2336-2339
Harvard University for use of their microwave spectrometers. The Harvard spectrometer is supported by NSF grant GP-37066x, and the Kansas spectrometer by NSF grant MPS-74-22178. We are also grateful to Dr. Paul Turner for communicating his results before publication and for many stimulating discussions. We thank one of the referees for unusually perceptive, thorough, and constructive criticism. Calculations were carried out at the University of Connecticut computer center and at Lawrence Berkeley Laboratory. This research was supported in part by the National Resource for Computation in Chemistry under a grant from the National Science Foundation (Grant No. CHE-7721305) and by the Basic
Energy Science division of the US Department of Energy (Contract No. W-7405-ENG-48). Supplementary Material Available: Tables of observed band frequencies, J 1 values, and B C values for neopentyl, n-propyl, neohexyl, isobutyl, n-butyl, isopentyl, isopropyl, 2-butyl, tert-butyl, and tert-amyl nitrites. Also, figures of B + C vs. v for torsionally excited neopentyl nitrite, intensity of the AFr band series vs. Stark field for neopentyl nitrite, and calculated B + C values for isopropyl, n-propyl, neohexyl, isobutyl, 2-butyl, tert-amyl, n-butyl, and isopentyl nitrites (22 pages). Ordering information is given on any current masthead page.
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Dephasing of Optical Phonons in a Substitutionally Disordered Organic Solid. The Lowest-Frequency Raman-Active Phonon of a Binary Solid Solution between p -6romochiorobenzene and p -Dichlorobenzene Larry A. Hess and Paras N. Piasad'+ Department of Chemistry, State Universlty of New York at Butfalo, Buffalo, New York 14214 (Received: June 9, 1981; I n Final Form: December 2, 198 1)
The temperature dependence of the line width is investigated for the lowest-frequency Raman-active phonon (28 cm-') in a 1:l molar solid solution of p-dichlorobenzene (DCB) and p-bromochlorobenzene (BCB). It is found that, at 2 K, the line width is dominated by the contribution due to substitutional disorder, yet the line shape is a Lorentzian. This result is in agreement with the prediction of a theory based on a configurationally averaged Green's function. The temperature-dependence study is successfully used to determine the mechanism of dephasing due to anharmonic phonon-phonon interactions. The anharmonic contribution of the line width in the solid solution fits the mechanism of dephasing due to a TI relaxation in which the optical phonon decays into two acoustic phonons of half its frequency by cubic anharmonic interactions. The same mechanism has been found to explain the temperature dependence of line widths of the corresponding phonons in the p-dichlorobenzene and p-bromochlorobenzene neat crystals.
Introduction Dephasing of an optical phonon can be studied by either a coherent transient technique (time-domain measurements such as time-resolved CARS) or a study of the line width (frequency-domain measurements) of the optical tran~ition.'-~The T2relaxation, responsible for dephasing, has been found3" to be in subnanoseconds for low-frequency optical phonons in organic crystals at
