Resonant multiphoton ionization photoelectron spectroscopic study of benzene. Evidence for fast intramolecular vibrational relaxation within the 1E1u state

Resonant multiphoton ionization photoelectron measurements were carried out for benzene under collision-free conditions in such a way that the benzene molecule is ionized by four photons through various two-photon allowed vibronic levels of the 1B2u excited state. As a result, single prominent, somewhat broad photoelectron bands were observed in the 0.14–0.36 eV region, arising from Δv=0 ionization transitions. The prominent peak is shifted with increasing photon energy by K=hνl−C, where K is the photoelectron energy, hνl the photon energy, and C a constant. This relationship strongly indicates that the third photon is resonant with a real intermediate excited state. Therefore, the overall ionization process is expressed by (2+1+1). It is concluded that fast intramolecular vibrational relaxation occurs at the third-photon states within the 1Elu (ππ*) state and subsequent ionization takes place from vibrationally relaxed levels. The relaxation within the 1Elu state is supported from the 0–0 energy which was evaluated here for the ionized states from the photoelectron data. It is also suggested that the lifetime of the vibrationally relaxed 1Elu states is the order of 10−11–10−12 s or longer.