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

Abstract

The S1–S0 electronic spectra, and the vibrational spectra of jet-cooled 2-pyridone (2PY) and its hydrogen bonded clusters, 2PY–H2O and 2PY dimer, have been investigated by population labeling and various double-resonant vibrational spectroscopies. For bare 2PY, the S1–S0 spectrum was measured by laser-induced fluorescence and population labeling spectroscopy. In addition, IR and Raman spectra of the NH stretching vibration were observed in S0 and S1. The results led to the conclusion that 2PY has two close lying electronic states in the S1 region, whose structures are slightly different with respect to the NH group. It was also found that the NH stretching frequency becomes smaller in S1 than in S0, indicating that the NH bond strength of 2PY becomes weaker in S1. The effect of the electronic excitation on the hydrogen bond strength has also been investigated by measuring the NH and OH stretching vibrations of the hydrogen bonded clusters in the two electronic states, and it was found that the hydrogen-bond strength is weaker in S1 than in S0. For 2PY dimer, the IR and the Raman spectra of the NH stretching bands showed a clear intensity alternation, confirming its C2h symmetric structure.