Two-Photon Excited State Dynamics of Dark Rydberg and Bright Valence States in Furan

Two-photon absorption in systems with parity permits access to states that cannot be directly prepared by one-photon absorption. Here we investigate ultrafast internal conversion (IC) dynamics of furan by using this strategy in combination with femtosecond time-resolved photoelectron imaging. The dark Rydberg S_1 and bright valence S_2 states are simultaneously excited by two photons of 405 nm, and then ionized by two photons of 800 nm. The IC from S_2 to S_1 is clearly observed and extracted from the time dependence of the higher photoelectron kinetic energy (PKE) component. More importantly, the internal conversions to hot S_0 from directly-prepared S_1 and secondarily-populated S_1 are unambiguously identified by the time-dependence of the lower PKE component. The average lifetime of the S_2 and S_1 states is measured to be 29 fs. The internal conversions of S_2 to S_1, S_1 to hot S_0 occur on estimated timescales of 15.4 fs and 38 fs, respectively.