Vibrational dephasing in polar molecular liquids: An application to a fundamental and its overtone in phenyl acetylene

Abstract

Based upon the results reported here for phenyl acetylene and the results of other investigators' studies, a working model of phase relaxation in polar molecular liquids is proposed. The model rests upon the established concept of a phase annihilation operator' and a postulated phase conduction band structure for polar molecular liquids. Three cases are discussed; briefly, they are (I) no effect upon the signal of interest by dilution, (11) narrowing of the signal of interest by dilution, and (111) broadening of the signal of interest by dilution. The latest effect has not been elucidated previously in the literature. Actual examples of each of these cases are discussed. Phenyl acetylene's vcc and 2vcc modes are shown to display Case I1 behavior. These two modes are found in the Raman spectrum at 2113 Em-' (vcc) and 4205 Em-' (2vcc). They have been examined by varying the concentration of this molecule in two dipolar solvents, a quadrupolar solvent and an octopolar solvent. In addition, temperature studies on the neat sample were carried out at 10-30 K intervals for vcc and at 275 K and 90 K for 2vcc. From the results of our investigation, a relaxation mechanism consistent with a quadrupolequadrupole dephasing type mechanism is postulated for both vcc and 2vcc.