Evidence of a shift between one- and two-photon processes associated with benzene trapped in helium nanodroplets

Two-photon ionisation has been observed at ∼259 nm for single benzene molecules trapped in small helium droplets. Four spectral features have been identified corresponding to vibrational excitations coupled with the forbidden 1B2u ← 1A1g electronic transition. When compared with single-photon depletion signals, three of the REMPI signals exhibit a small (∼1 cm−1) blue shift. The ion signals also exhibit a size-dependence, which maximises at droplets containing ∼1500 atoms, and is interpreted in terms of a critical size needed to either pick up molecule, or to evaporate completely all of the remaining helium atoms prior to detection of the benzene ion.