The lattice response of a Bi(111) surface upon impulsive femtosecond laser excitation is studied with time-resolved reflection high-energy electron diffraction. We employ a Debye-Waller analysis at large momentum transfer of 9.3 Å-1 ≤ Δ k ≤ 21.8 Å-1 in order to study the lattice excitation dynamics of the Bi surface under conditions of weak optical excitation up to 2 mJ/cm2 incident pump fluence. The observed time constants τ int of decay of diffraction spot intensity depend on the momentum transfer Δk and range from 5 to 12 ps. This large variation of τ int is caused by the nonlinearity of the exponential function in the Debye-Waller factor and has to be taken into account for an intensity drop ΔI > 0.2. An analysis of more than 20 diffraction spots with a large variation in Δk gave a consistent value for the time constant τT of vibrational excitation of the surface lattice of 12 ± 1 ps independent on the excitation density. We found no evidence for a deviation from an isotropic Debye-Waller effect and conclude that the primary laser excitation leads to thermal lattice excitation, i.e., heating of the Bi surface.

中文翻译：

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从Bi（111）表面超快速电子衍射：大动量传递下的脉冲晶格激发和Debye-Waller分析。

利用时间分辨反射高能电子衍射研究了Bi（111）表面在脉冲飞秒激光激发下的晶格响应。为了研究在高达2 mJ / cm2入射泵浦的弱光激发条件下Bi表面的晶格激发动力学，我们在9.3Å-1≤Δk≤21.8Å-1的大动量传递中采用了德拜-沃勒分析。通量。观察到的衍射光斑强度衰减的时间常数τint取决于动量传递Δk，范围为5到12 ps。τint的大变化是由Debye-Waller因子中的指数函数的非线性引起的，并且对于强度下降ΔI> 0.2必须予以考虑。对20多个Δk的较大变化的衍射点进行分析，得出了与激发密度无关的12±1 ps表面晶格振动激发的时间常数τT的恒定值。我们没有发现偏离各向同性Debye-Waller效应的证据，并得出结论，主要的激光激发导致热晶格激发，即Bi表面的加热。