Laser-Induced Transient Structural Changes in Ag(111) Studied by Time Resolved X-ray Diffraction
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Figure 1: Experimental setup for time resolved x-ray diffraction. This system was employed to study the transient lattice behavior of 150 nm Ag(111) crystals grown on mica substrate after excitation with 10 Hz, 100 fs, 267 nm THG pulses. The change in structure was probed with 0.6 ps, 6x106 x-ray photons per pulse of 8.04 KeV CuKα. The 100 fs, 267 nm, excitation pulse propagates through a delay line and is focused onto the sample. Zero time delay is defined as a time when the UV pulse impinges on the Ag crystal. This time was determined by two methods: melting of a Si crystal [10] and noncolinear sum frequency generation when the fundamental radiation (800 nm) was sent along the track of the X-ray beam and mixed with 400 nm in a KDP crystal position on the Ag crystal place. The accuracy of both methods was ~1 picosecond. RESULTS The skin depth for silver at 267 nm is ~15.9 nm, therefore, the X-ray rocking curves represent the structural lattice changes throughout the depth of the crystal owing to the fact that the CuKα X-ray beam penetrates and probes the entire thickness of the crystal, while the light pulse penetrates only through the skin-depth. The lattice vibration induced by the propagation of acoustic waves has been experimentally observed by electron diffraction for 20 nm thick Al film
[9, 11] and by time resolved XRD for 150 nm Au sample [6] and 90 nm Au sample [12]. The damping of coherent acoustic oscillation as a function of fluence in a germanium film heated by femtosecond pulses were measured by means of time resolved XRD and it was found that the central shift depends on the applied fluence [13]. The laser excitation energy density used in our experiment was either much lower than the melting threshold for the study contraction, expansion, and propagation of sound waves or close to the melting threshold for the study of atomic process of melting and recrystallization.
Figure 2: X-ray diffraction peak shift vs. time delay after irradiation of 23 mJ/cm2. The initial contraction followed expansion is depicted. Increasing the laser intensity result in higher lattice temperature changes and consequently to larger Bragg diffraction peak shifts. The peak shift of the rocking curve depends linearly on the laser intensity within the energy range used in the experiments. However, we have previously shown that fluence doesn’t have a significant effect on the period of coherent oscillations [6]. Figure 2 shows that peak shift under excitation fluence of 23 mJ/cm2. The period T could be calculated easily from the oscillation curves by using the equation for acoustic wave longitudinal velocity in solid silver (vl(s)=3650 m/s) for Ag(111) crystal. It should be noted that electronic and thermal energy losses into the mica substrate is negligible due to its large band gap energy and low thermal boundary conductance [12]. The period of coherent oscillations is found to be ~45 ps, which is 4 ps smaller than the measured period. This may be due to the delay between the blast and acoustic waves, which propagate with th
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