Pump/probe Studies of Ultrafast Dynamics using X-ray Pulse Pairs

Leveraging the inherent time stability of x-ray free electron laser pulse pairs created from the same electron beam, together with the site-selectivity of x-ray wavelengths, we performed experiments at the LCLS XFEL facility tracking the dynamical evolution of molecular systems. X-ray pump/x-ray probe techniques provide sensitive probes of femtosecond molecular dynamics, and if attosecond x-ray pulses are employed, they can even be used to probe electron dynamics.  We present a time-resolved study of ultrafast dissociative ionization of N₂O using few-femtosecond x-ray pulse pairs.  We track the electron spectrum as a function of pump-probe delay and observe spectral shifts signaling dissociation of the cation after x-ray ionization and Auger-Meitner decay.  Using attosecond pulse-pairs, in a subsequent experiment, we track the electron dynamics of aminophenol following impulsive inner-valence ionization.  These experiments demonstrate the utility and feasibility of x-ray pump/x-ray probe experiments for studying gas phase molecular dynamics at XFEL facilities.