Excited state lifetimes and appearance potentials of benzene dimer and trimer

Abstract

Van der Waals clusters of benzene are formed in a supersonic jet, subjected to two-color resonance-enhanced two-photon ionization (2C-RE2PI) via the S2 state, and analyzed in a time-of-flight mass spectrometer. The lifetimes of the excited states of benzene dimer and trimer are obtained by the 2C-RE2PI technique. The observed lifetime data of the dimer and trimer show a biexponential decay, which corresponds to the lifetimes of the lowest singlet 1B1g state of a parallel spaced benzene excimer and an excited S1-van der Waals dimer state. The two excited states are likely to be separated by a shallow potential barrier. The observed lifetimes of the parallel stacked lowest singlet excimer and the vibrationally excited S1-van der Waals states of the benzene dimer are 122 and 24 ns, respectively. These excited states are produced as a result of the internal conversion from an optically excited S2-van der Waals dimer state. The appearance potentials of the benzene dimer and trimer are also measured. The photoionization curves of the dimer and trimer in the threshold region show very slow rise at 8.53±0.03 and 8.54±0.03 eV, respectively. The observed photoionization thresholds are much lower than those reported previously.