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

Two-photon ionisation has been observed at similar to259 nm for single benzene molecules trapped in small helium droplets. Four spectral features have been identified corresponding to vibrational excitations coupled with the forbidden B-1(2u) <-- (1)A(1g) electronic transition. When compared with single-photon depletion signals, three of the REMPI signals exhibit a small (similar to1 cm(-1)) blue shift. The ion signals also exhibit a size-dependence, which maximises at droplets containing similar to1500 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. (C) 2004 Elsevier B.V. All rights reserved.