Advances in ultrafast gas-phase x-ray scattering

Abstract

Recent developments of x-ray free electron lasers and pulsed electron sources have enabled ultrafast scattering to become an increasingly powerful tool for exploring molecular dynamics. This article describes our recent experimental and methodological advances in ultrafast gasphase x-ray scattering experiments at the LCLS. A re-designed short-pathlength windowless diffractometer is coupled with careful optimization of sample density and independent normalization of x-ray intensity fluctuations to provide gas-phase scattering patterns with exceptionally high signal-to-noise ratios. These advances, coupled with careful geometry optimization and data treatment, provide both ground-and excited-state signals in excellent agreement with high level ab-initio total scattering patterns.

Ultrafast X-ray scattering offers a structural view of excited-state charge transfer

“…The percent difference signal depends on the fraction of molecules that are optically excited, a scalar quantity that is determined during the experimental analysis. During the experiment, we keep the optical excitation laser intensity low to minimize multiphoton processes (40)(41)(42).…”

“…Experimental Details. The time-resolved gas-phase X-ray scattering setup has been introduced previously (40,52). The optical pump laser uses the fourth harmonic of a 120 Hz Ti:Sapphire laser operating at 800 nm, generating pulses at 200 nm with ∼80 fs pulse duration and ∼1 μJ/pulse energy on target.…”

Determination of excited state molecular structures from time-resolved gas-phase X-ray scattering

“…The experimental implementation has been described in detail previously. 28,29 The ensemble of free NMM molecules was excited by a 200 nm laser pulse and then probed with a 9.5 keV X-ray pulse generated by the LCLS. The scattering patterns were measured on a 2.3-megapixel Cornell-SLAC Pixel Array Detector (CSPAD) 30 at various delay times between the pump and probe pulses.…”

“…Expressing the time-resolved scattering signal as a percent difference has many advantages including accentuating the small changes in the laser-on scattering signal and canceling out various experimental artifacts that affect the laser-on and laser-off signals equally. 29 Since the reference scattering signal corresponds to the molecules in the ground state, the percent difference scattering patterns measure the excited state molecular structures in reference to the ground state. Considering that the change of the scattering patterns can be measured quite accurately 29 and that the ground-state structure is usually well known, 3,5 accurate excited state molecular structures can thus be derived.…”

“…29 Since the reference scattering signal corresponds to the molecules in the ground state, the percent difference scattering patterns measure the excited state molecular structures in reference to the ground state. Considering that the change of the scattering patterns can be measured quite accurately 29 and that the ground-state structure is usually well known, 3,5 accurate excited state molecular structures can thus be derived. In the present study, the ground-state structure of NMM (see equatorial structure in Fig.…”