Observation of resonant two-photon photodetachment of water cluster anions via femtosecond photoelectron spectroscopy

Photoexcitation of the (H2O)−n (n=20–100) clusters with 100 fs pulses at 800 nm results in an increasing propensity for two-photon electron photoejection with increasing cluster size. This increase correlates with the size range (n≈30) where the first excited electronic state drops below the electron continuum, and the electronic absorption band approaches the energy of the 800 nm pump photon. No above-threshold, two-photon detachment is observed for n=20. Differences in the shape of the resonant two-photon photoelectron spectrum compared to that arising from direct (high energy) photodetachment are interpreted in terms of the vibrational state selection created in the resonant step.