Ultrafast 2.5-keV x-ray absorption probing of a chemical reaction with 3-ps time resolution

Abstract

We perform pump-probe measurements in which intense ultrashort optical pulses are the pump pulses that initiate a chemical reaction and ultrafast x-ray pulses are the probe pulses that monitor the response of the system. We present experimental results on the observation of a chemical reaction process, photoinduced dissociation of gas phase SF 6 molecules, detected by ultrafast x-ray absorption spectroscopy with 3 ps time resolution near the sulfur K edge at a photon energy of 2.48 keV (4.98 A). High contrast light pulses of 400 fs duration (500 mJ energy and 0.53 micrometers wavelength) from the INRS terawatt laser were focused on high atomic number targets at an intensity of 5 X 10 17 W/cm 2 in order to generate an x-ray continuum around the sulfur K edge. The SF 6 molecule exhibits intense near shape resonances at the sulfur K and L edges, due to the multiple scattering and interference of the emitted photoelectrons by the fluorine atoms that symmetrically surround the central sulfur atom. The shape resonance of the molecule is clearly resolved in the absence of any pump pulse, and the variation of the x-ray absorption spectrum was measured as a function of the delay between the optical pump and x-ray probe pulses. As expected from theory, the reaction process is faster than can be resolved with the 3 picosecond duration x-ray pulses used in this initial experiment. This fast response can, in principle, be used to measure the duration of ultrashort x-ray pulses.