Spectroscopic study on nonradiative transition and ionization of 5-methylpyrimidine at S1 probed by the slow-electron velocity-map imaging (SEVI) technique

Slow electron velocity-map imaging (SEVI) has been employed for the ionization spectroscopic study of the jet-cooled 5-methylpyrimidine in the first singlet 1(nπ∗) excited state. Resonant-enhanced two-color two-photon (1 + 1′) ionization spectrum gives well-resolved S1 vibrational structures up to ∼2400 cm−1 above the origin whereas spectral bands becomes completely broadened in the higher energy region as coupling of individual S1 bands to dark states becomes significant. Ionization of the S1 5-methylpyrimidine with different ionization wavelengths at several different delay times reveals the property of low-lying 3(ππ∗) triplet dark state which is prepared by fast nonadiabatic transition. SEVI spectra taken via various S1 intermediate states provide the detailed vibrational structures of the 5-methylpyrimidine cation (D0), also giving the adiabatic ionization energy of 9.1052 ± 0.0030 eV (73 438 ± 24 cm−1). Geometrical changes in the S0–S1–D0 transitions are discussed from Franck-Condon simulations based on calculations by (time-dependent) density functional theory.