Ultrafast reflection high-energy electron diffraction is employed to follow the lattice excitation of a Bi(111) surface upon irradiation with a femtosecond laser pulse. The thermal motion of the atoms is analyzed through the Debye-Waller effect. While the Bi bulk is heated on time scales of 2 to 4 ps, we observe that the excitation of vibrational motion of the surface atoms occurs much slower with a time constant of 12 ps. This transient nonequilibrium situation is attributed to the weak coupling between bulk and surface phonon modes which hampers the energy flow between the two subsystems. From the absence of a fast component in the transient diffraction intensity, it is in addition concluded that truncated bulk phonon modes are absent at the surface.

中文翻译：

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减速的晶格激发和表面不存在体声子模：fs-激光激发时从Bi（111）表面超快电子衍射。

飞秒激光脉冲照射后，采用超快反射高能电子衍射跟踪Bi（111）表面的晶格激发。通过Debye-Waller效应分析原子的热运动。当Bi块以2到4 ps的时间尺度加热时，我们观察到表面原子的振动运动的激发发生得慢得多，时间常数为12 ps。这种瞬态不平衡的情况归因于体声子模式与表面声子模式之间的弱耦合，这阻碍了两个子系统之间的能量流。由于瞬态衍射强度中没有快速成分，因此可以得出结论，在表面上不存在截短的体声子模。