부산대학교 도서관

Laser-induced fluorescence, resonant two-photon ionization, UV-UV hole burning, UV depletion, and single vibronic level fluorescence (SVLF) spectra of jet-cooled diphenylmethane (DPM) have been recorded over the 37 300–38 400 cm-1 region that encompasses the S1←S0 and S2←S0 transitions. All transitions in the laser-induced fluorescence excitation spectrum are due to a single conformational isomer of DPM with C2 symmetry. The S1←S0 origin transition occurs at 37 322 cm-1, supporting a short progression in the symmetric torsion T with spacing of 28 cm-1. The S2←S0 origin transition occurs 123 cm-1 above the S1 origin and possesses very weak torsional structure, observable only under saturating laser power conditions. A combination of SVLF spectroscopy and hot band studies is used to assign the frequencies of the symmetric torsion (T), antisymmetric torsion (T), and butterfly (β) vibrations in the S0, S1, and S2 states. The emission from the S2 origin is composed of two components, a set of sharp transitions ascribable to the S2 state and a dense “clump” of transitions ending in ground-state levels 81, 88, and 93 cm-1 above the S0 zero-point level ascribable to S1(v) emission. Assignment of the transitions in the clump leads to the conclusion that the single vibronic level responsible for the emission has mixed S2/S1 character. The mixing involves several torsional vibronic levels in the S1 manifold close in energy to the S2 origin, with the correct symmetry to couple the two states. These levels involve significant torsional excitation. The close energetic proximity of these levels leads to a breakdown of typical vibronic coupling selection rules. [ABSTRACT FROM AUTHOR]