pulse was set by an optical delay line ~not illustrated in the schematic!. Although a single laser pulse heated the entire surface exposed to x rays, an additional time delay at different spatial locations along the crystal's surface was introduced by reflecting the laser pulse from a diffraction grating prior to it being imaged onto the sample ~Fig. 1!. As such the reflected wave front became tilted relative to its direction of propagation, with the result that different rays traversed different optical path lengths before arriving at the semiconductor's surface. Since the maximum difference in the optical path lengths of different rays was 6 cm, a temporal window of 200 ps was sampled in a single x-ray topogram ~Fig. 1!, reflecting the finite time required for the tilted wave front to sweep across the crystal's surface exposed to x rays.16
laser pulse was altered by changing the optical delay line, all of the laser induced features of the x-ray topogram [Fig. 2b] were reproduced, but were translated along the x axis of the detector. This observation is quantified in Fig. 3, which plots the translation of the x-ray topogram versus the temporal change in the optical delay line. From this result the effective temporal resolution of the x-ray detector could be calibrated as 1.2 ps per pixel, which is in agreement with that determined from the optical configuration.
Although the acquisition time required to sample this 200 ps window was relatively short data shown in [Fig. 2b]derived from a single 60 min run the diffraction data were of sufficiently high quality that a good theoretical fit to the experimental results could be obtained. In [Fig. 2c] we show a simulated x-ray topogram for comparison. A detailed description of the theoretical model is given in Refs. 17 and 18
and it has proven to be applicable to a broad spectrum of experimental results.3,10 Intuitively, energy from the fs laser pump couples into the semiconductor by exciting electrons from the valence to the conduction band. After absorption, energy is initially dispersed as optical phonons, which are characterized by specific motions of atoms within the unit cell. On a slower time scale coherently excited acoustic phonons are generated,3,10 which are characterized by specific motions of the crystalline lattice and their propagation is determined by the optical, electronic and acoustic properties of the material.17 The propagation of these acoustic waves through the semiconductor perturbs the lattice spacing on a time scale of picoseconds, and results in the splitting and subsequent relaxation! of the Fe Ka line modeled in [Fig. 2c].
The authors thank Michael Wulff and Friedrich Schotte for assistance with preparatory experiments at ID09 of the European Synchrotron Radiation Facility [ESRF]. The authors acknowledge support from the European commission ''Improving the Human Potential program.'' J.L. and R.N. acknowledge support from the Swedish Science Research Council [V.R.]and the Swedish Strategic Research Foundation (S.S.F)
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