Excitation Energy-Dependent Decay Dynamics of the S 1 State of N -Methyl-2-pyridone.

The UV-induced decay dynamics of N -methyl-2-pyridone is investigated using a femtosecond time-resolved photoelectron spectroscopy method. Irradiation in the wavelength range of 339.3-258.9 nm prepares N -methyl-2-pyridone molecules with very different vibrational levels of the S 1 (1 1 ππ*) state. For v ' = 0 (origin) and a few low-energy vibrational levels slightly above the S 1 state origin, the radiative decay channel is in operation for some specific vibrations. This is revealed by the excited-state lifetime of ≫1 ns. In addition, some other nearby S 1 vibronic states have a much shorter lifetime in the range of several picoseconds to a few tens of picoseconds, indicating that the radiation-less decay to the ground state (S 0 ) via internal conversion is the dominant channel for them. As the pump wavelength slightly decreases, the radiative decay is suddenly not important at all, and the deactivation rate of the S 1 state becomes faster. At shorter pump wavelengths, the lifetime of highly excited vibrational states of the S 1 state further decreases with the increase in the vibrational excess energy. This study provides quantitative information about the excitation energy-dependent decay dynamics of the S 1 state of N -methyl-2-pyridone. Methyl substitution effects on the excited-state dynamics of 2-pyridone are also discussed.