Angle-dependent study of a direct optical transition in the sp bands of Ag(111) by one- and two-photon photoemission
Angle-dependent study of a direct optical transition in the sp bands of Ag(111) by one- and two-photon photoemission
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通过单光子和双光子光电发射对 Ag(111) sp 带中的直接光学跃迁进行角度相关研究
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发表时间：
2007
期刊：
影响因子：
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通讯作者：
H. Petek
中科院分区：
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作者：
A. Winkelmann;V. Sametoglu;Jin Zhao;A. Kubo;H. Petek
我们测量了Ag（111）单光子和双光子激发的角相关光电子谱。Ag（111）的$sp$带跃迁的色散可以用一个近似自由电子模型来再现。所观察到的色散与已知的能带结构一致。我们说明了如何低能量电子的强折射成为一个限制因素，以获得定量的能带结构信息。相反，低能电子的一个明确的直接光学带间跃迁可以提供一个灵敏的探针的内部潜力。我们观察到不对称的双光子光电子强度分布相对于检测沿着表面正常。这些强度分布可以很好地描述了一个唯象模型，该模型采用菲涅耳方程来计算入射辐射的样品内的电场分量。非常好的协议被发现使用表列的光学常数和动量矩阵元素，这是面向沿着表面法线。相反，观察到的Ag（111）单光子光发射的强度分布不符合简单的菲涅耳模型。我们解释为表面光电效应的影响。通过与Cu（001）的比较，我们发现，如果可以忽略表面光致发射等附加效应的影响，那么对于动量矩阵元沿着表面法线的取向，菲涅耳模型中单光子光致发射和双光子光致发射的预期强度分布是有效的。
We have measured angle-dependent photoemission spectra for one-photon and two-photon excitation from Ag(111). The observed dispersion of the $sp$-band transition of Ag(111) can be reproduced using a nearly-free-electron model for the initial and final states involved. The observed dispersion agrees with the known band structure. We illustrate how the strong refraction of low-energy electrons becomes a limiting factor to obtain quantitative band-structure information. Conversely, low-energy electrons of a well-defined direct optical interband transition can provide a sensitive probe of the inner potential. We observe asymmetric two-photon photoelectron intensity distributions with respect to detection along the surface normal. These intensity distributions can be well described by a phenomenological model which employs the Fresnel equations to calculate the electric field components of the incident radiation inside the sample. Very good agreement is found using tabulated optical constants and a momentum matrix element, which is oriented along the surface normal. In contrast, the observed intensity distribution for one-photon photoemission from Ag(111) does not fit the simple Fresnel model. We interpret this as the influence of surface photoemission. By comparison to Cu(001), we show that the expected intensity distributions of the Fresnel model for one-photon photoemission and two-photon photoemission are valid for an orientation of the momentum matrix element along the surface normal if the influence of additional effects like surface photoemission can be neglected.