Ultrafast Decay Dynamics of the 21ππ* Electronic State of N-Methyl-2-pyridone

The ultrafast decay dynamics of N-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 N-methyl-2-pyridone molecules with high vibrational levels in the 11 pi pi* state. The radiation-less decay to the ground state via internal conversion is suggested to be the dominant channel for the 11 pi pi* 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 21 pi pi* state. This is in agreement with the assignment of the vapor-phase UV absorption bands of N-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 21 pi pi* state has an ultrashort lifetime of 50 +/- 10 fs. In addition, the S1(11 pi pi*) state is subsequently populated via internal conversion and decays over a lifetime of 680-620 fs. The most probable whole deactivation pathway of the 21 pi pi* state is discussed. This experimental study provides new insights into the excitation energy-dependent decay dynamics of electronically excited N-methyl-2-pyridone.