Population labeling spectroscopy for the electronic and the vibrational transitions of 2-pyridone and its hydrogen-bonded clusters

The S-1-S-0 electronic spectra, and the vibrational spectra of jet-cooled 2 -pyridone (2PY) and its hydrogen bonded clusters, 2PY-H2O and 2PY dimer, ha ve been investigated by population labeling and various double-resonant vib rational spectroscopies. For bare 2PY, the S-1-S-0 spectrum was measured by laser-induced fluorescence and population labeling spectroscopy. In additi on, IR and Raman spectra of the NH stretching vibration were observed in S- 0 and S-1. The results led to the conclusion that 2PY has two close lying e lectronic states in the S-1 region, whose structures are slightly different with respect to the NH group. It was also found that the NH stretching fre quency becomes smaller in S-1 than in S-0, indicating that the NH bond stre ngth of 2PY becomes weaker in S-1. The effect of the electronic excitation on the hydrogen bond strength has also been investigated by measuring the N H and OH stretching vibrations of the hydrogen bonded clusters in the two e lectronic states, and it was found that the hydrogen-bond strength is weake r in S-1 than in S-0. For 2PY dimer, the IR and the Raman spectra of the NH stretching bands showed a clear intensity alternation, confirming its C-2h symmetric structure. (C) 2000 American Institute of Physics. [S0021-9606(0 0)00626-7].