title: Towards efficient time-resolved X-ray absorption studies of electron dynamics at photocatalytic interfaces.
abstract: We present a picosecond time-resolved X-ray absorption spectroscopy (tr-XAS) setup designed for synchrotron-based studies of interfacial photochemical dynamics. The apparatus combines a high power, variable repetition rate picosecond laser system with a time-resolved X-ray fluorescence yield detection technique. Time-tagging of the detected fluorescence signals enables the parallel acquisition of X-ray absorption spectra at a variety of pump-probe delays employing the well-defined time structure of the X-ray pulse trains. The viability of the setup is demonstrated by resolving dynamic changes in the fine structure near the O1s X-ray absorption edge of cuprous oxide (Cu2O) after photo-excitation with a 355 nm laser pulse. Two distinct responses are detected. A pronounced, quasi-static, reversible change of the Cu2O O1s X-ray absorption spectrum by up to ∼30% compared to its static line shape corresponds to a redshift of the absorption edge by ∼1 eV. This value is small compared to the 2.2 eV band gap of Cu2O but in agreement with previously published results. The lifetime of this effect exceeds the laser pulse-to-pulse period of 8 μs, resulting in a quasi-static spectral change that persists as long as the sample is exposed to the laser light, and completely vanishes once the laser is blocked. Additionally, a short-lived response corresponding to a laser-induced shift of the main absorption line by ∼2 eV to lower energies appears within <200 ps and decays with a characteristic timescale of 43 ± 5 ns. Both the picosecond rise and nanosecond decay of this X-ray response are simultaneously captured by making use of a time-tagging approach - highlighting the prospects of the experimental setup for efficient probing of the electronic and structural dynamics in photocatalytic systems on multiple timescales.
authors.affiliation: Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA. ogessner@lbl.gov sneppl@lbl.gov.
authors.forename: Stefan
authors.initials: S
authors.lastname: Neppl
authors.affiliation: Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA. ogessner@lbl.gov sneppl@lbl.gov.
authors.forename: Johannes
authors.initials: J
authors.lastname: Mahl
authors.affiliation: Space Sciences Laboratory, University of California, Berkeley, California, USA.
authors.forename: Anton S
authors.initials: AS
authors.lastname: Tremsin
authors.affiliation: Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, USA.
authors.forename: Bruce
authors.initials: B
authors.lastname: Rude
authors.affiliation: Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, USA.
authors.forename: Ruimin
authors.initials: R
authors.lastname: Qiao
authors.affiliation: Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, USA.
authors.forename: Wanli
authors.initials: W
authors.lastname: Yang
authors.affiliation: Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, California, USA.
authors.forename: Jinghua
authors.initials: J
authors.lastname: Guo
authors.affiliation: Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California, USA. ogessner@lbl.gov sneppl@lbl.gov.
authors.forename: Oliver
authors.initials: O
authors.lastname: Gessner