Local modes: their relaxation, polarization, and stereoselective excitation by lasers

In this paper, we present new results concerning the nature and relaxation of methyl and aromatic CH stretching of low-temperature crystalline durene (1,2,4,5-tetramethylbenzene). The experimental polarization ratios of the bands (Δv_(cH) = 5) are used to compare three limiting models (local modes, independent methyl modes, and fully delocalized modes) in light of new information on the conformation of durene obtained by low-temperature X-ray diffraction. Armed with these results, we provide a picture for the origin of the splittings observed for the Δv_(cH) = 5 spectra at 1.7 K and discuss the nature of the vibrational energy distribution of the single-photon excited states of the methyl and aromatic subsets; the aromatics are equivalent local modes by symmetry (line width > observed splitting) whereas the methyls are inequivalent local modes which are split (line width < splitting) by the molecular and crystal fields. The dynamics of the high-energy states (Δv_(cH) = 5) are inferred from the line widths. The methyl bands, which are much narrower than the aromatic band at low temperature, are broadened as the temperature is increased by a process which results in partial coalescence at room temperature. Consequently, specific inequivalent bond modes on nonrigid groups (e.g., methyl) can be selectively excited by optical pumping at low temperatures. In addition, crystal field splittings of the CH modes, particularly at low temperature, can be large enough to allow selective excitation of bond modes which would be equivalent in the isolated molecule. Thus we propose the use of low-temperature matrices or crystals as a molecular environment for (polarized) photoselection experiments on high-energy overtones.