Time-resolved X-ray Absorption Spectroscopy of aromatic Carbonyls using Spectral Domain Ghost Imaging

We studied UV-induced ultrafast photochemistry of aromatic carbonyls using time-resolved soft X-ray absorption spectroscopy. By utilizing the specific sensitivity of near edge X-ray absorption fine structure spectroscopy (NEXAFS) to the electronic character of the excited states (e.g., ππ* and nπ* states) we were able to elucidate the initial steps in the photodissociation mechanisms of Norrish Type-I reactions involving internal conversion and a subsequent interplay between the singlet and triplet states in one of the smallest aromatic carbonyls, Acetophenone (C6H5COCH3). We excited the gas phase sample with a 266 nm pump pulse and probed the reaction by scanning the X-ray photo energy across around the O K-edge (~520 to 540 eV). We monitored the X-ray absorbance through the measure of Auger-Meitner electron yield together with the photon spectrum. The experiment was carried out at the TMO instrument of the LCLS. The X-ray pulses generated by the free-electron laser (FEL) driven by self-amplified spontaneous emission (SASE) exhibit a strong shot to shot spectral jitter with a bandwidth of several eV. To overcome this spectral bandwidth limitation to resolve dynamics in the fine structure spectrum at an order of less than 1 eV we exploit the correlation present in the shot-to-shot fluctuations in the incoming X-ray pulses and the detected electron signal by employing spectral domain ghost imaging. Utilizing this technique we are able to resolve a pre-edge absorption feature, around 527 eV, which is attributed to the population of the nπ* state. The initial formation of this feature is observed with a rise time in the order of 100 fs. Subsequently a slow decay combined with spectral shift can be seen persisting for over 20ps, indicating dynamics to the triplet manifolds before dissociation. Our experimental results are corroborated by theoretical simulations.