Optothermal Spectroscopy of Polynuclear Aromatics in a Molecular Beam: Structure and Dynamics.

Optothermal spectroscopy has been used to examine molecules on both the ground electronic state and the electronic excited state potential energy surface. Molecules which were studied in the ground electronic state using high resolution infrared spectroscopy include pyrazine rm (C_4N_2H_4), naphthalene rm (C_{10}H_8), chlorobenzene rm (C_6H_5Cl), formamide (HCONH_2), deuterated formamide (DCONH_2), and 1,4-benzoquinone rm (C_6H_4O_2). An examination of the vibrational mode coupling exhibited by each molecule correlated the presence of large amplitude, low frequency motions with extensive mode coupling. Formamide, deuterated formamide, and 1,4-benzoquinone each possessed such a motion and the spectra of these molecules exhibited extensive perturbations in their vibrational spectra. The remaining molecules did not possess a large amplitude, low frequency motion and they exhibited few perturbations. In addition to the structural features of these molecules, the density of states also plays a role. The rigid molecules pyrazine, naphthalene, and chlorobenzene demonstrated that sporadic, local perturbations increased as the state density increased. The molecular structure was shown to play a large role while the state density plays only a minor role in determining the extent of vibrational mode coupling in molecules. Ultraviolet spectroscopy was used to probe non-radiative relaxation channels of trans-stilbene rm (C_{14}H_{12}) and p-bromoaniline rm (C_6H _4BrNH_2). The spectra were used to demonstrate that trans-stilbene undergoes photoisomerization with a barrier of ~1250 cm ^{-1} in the electronic excited state. The relative quantum yields of fluorescence were also determined for trans-stilbene. p-Bromoaniline was shown to undergo a different non-radiative relaxation process. The presence of the bromine atom induces a fast intersystem crossing to the triplet state.