The X-ray Pump-Probe instrument at the Linac Coherent Light Source

Abstract

The X-ray Pump-Probe instrument achieves femtosecond time-resolution with hard X-ray methods using a free-electron laser source. It covers a photon energy range of 4-24 keV. A femtosecond optical laser system is available across a broad spectrum of wavelengths for generating transient states of matter. The instrument is designed to emphasize versatility and the scientific goals encompass ultrafast physical, chemical and biological processes involved in the transformation of matter and transfer of energy at the atomic scale.

Keywords: FEL; X-ray; pump–probe; time-resolved.

Figure 1

Overview of the XPP instrument layout. Distances are indicated in meters from the center of the diffractometer. S&D are slits and non-destructive intensity diagnostics, DCM is a large-offset double-crystal monochromator, CCM is a channel-cut monochromator, TT is a timetool measuring the arrival time of the optical laser in reference to the X-rays, M1 and M2 are silicon mirrors that can be used to deflect the beam in the vertical direction and can also provide harmonic rejection, L-IN is the laser in-coupling for the optical laser, D is a diagnostic. Components located downstream of the dashed line can be translated into the main LCLS line and allow the XPP instrument to take advantage of the full power and properties of the fundamental. The gray line is the LCLS main line, whereas the black one is offset 600 mm horizontally by the DCM. The sample at the XPP instrument is located approximately 200 m downstream of the undulators.

Figure 2

Time-resolved normalized diffracted X-ray intensity for three superlattice reflections. The (−2 1/2 0) reflection is measured off resonance at 6.53 keV. The (0 −5/2 0) reflection is measured near resonance at 6.553 keV, and the (0 −3 0) reflection is measured on resonance at 6.555 keV. [Reprinted by permission from Macmillan Publishers Ltd: Beaudet al. (2014 ▶). Nat. Mater. 13, 923–927, copyright (2014).]

Figure 3

Spectroscopic pump–probe results from the photoexcited spin transition of [Fe^II(bpy)₃]²⁺ in solution. After excitation into the MLCT state, transition into the HS ⁵T₂ state [inset (c)] causes an increase of the Fe-to-ligand distance [inset (a)]. This structural change is tracked by the XANES trace at 7125 eV [(a), black symbols, vertically offset by 0.5]. The structural HS rise has been fitted by an exponential rise of 162 ± 6 fs (Lemke et al., 2013 ▶). (b) Time-dependent Kβ emission difference spectra. The transient traces at 7061 eV and 7054 eV have been extracted [(a) green diamond symbols and (c) black symbols, respectively]. The data have been overlaid with results from global fits including (blue lines) and excluding (green dashed lines) population of and intermediate ³T triplet state (Zhang et al., 2014 ▶). [Reprinted by permission from Macmillan Publishers Ltd: Zhang et al. (2014). Nature (London), 509, 345–348, copyright (2014).]