Ultrafast Decay Dynamics of the 2ππ* Electronic State of -Methyl-2-pyridone.

The ultrafast decay dynamics of -methyl-2-pyridone upon excitation in the near-ultraviolet range of 261.5-227.9 nm is investigated using the femtosecond time-resolved photoelectron spectroscopy method. Irradiation at 261.5 nm prepares -methyl-2-pyridone molecules with high vibrational levels in the 1ππ* state. The radiation-less decay to the ground state via internal conversion is suggested to be the dominant channel for the 1ππ* state with large vibrational excess energy, which is revealed by a lifetime of 1.6 ± 0.2 ps. As the pump wavelength decreases, we found that irradiation at 238.5 and 227.9 nm results in the population of the 2ππ* state. This is in agreement with the assignment of the vapor-phase UV absorption bands of -methyl-2-pyridone. On the basis of the detailed analysis of our measured time-resolved photoelectron spectra at all pump wavelengths, we conclude that the 2ππ* state has an ultrashort lifetime of 50 ± 10 fs. In addition, the S(1ππ*) state is subsequently populated via internal conversion and decays over a lifetime of 680-620 fs. The most probable whole deactivation pathway of the 2ππ* state is discussed. This experimental study provides new insights into the excitation energy-dependent decay dynamics of electronically excited -methyl-2-pyridone.