Hicks J M, Petralli-Mallow T, Byers J D
Department of Chemistry, Georgetown University, Washington DC 20057, USA.
Faraday Discuss. 1994(99):341-57. doi: 10.1039/fd9949900341.
Surface second-harmonic generation (SHG) is recognized as a second-order non-linear laser-based spectroscopy offering good surface sensitivity and selectivity. With the goal of developing optical methods to study chiral surfaces, a way to detect dichroic behaviour in SHG in analogy to circular dichroism (CD) spectroscopy has been elucidated. The SHG efficiency from a monolayer of oriented chiral molecules [(R)- or (S)-2,2'-dihydroxy-1,1'-binaphthyl (BN)] has a strong dependence upon whether the excitation beam has left- or right-handed circular polarization. The relative differential SHG signal is much larger than in ordinary CD spectroscopy. The large chiroptical effects in SHG are attributed not to R, the rotational strength responsible for traditional CD (R = Im mu.m), but to the electric dipole-allowed second-order non-linear tensor, chi (2). There is a chiral element of chi (2), chi (xyz), that must be included in descriptions of the SHG response from chiral surfaces. The ability to measure chiral properties of an adsorbed molecule separately, with no background from its achiral properties, provides another form of chiral spectroscopy, different from CD and having no analogue in linear spectroscopy, but related in a well understood manner.
表面二次谐波产生（SHG）被认为是一种基于二阶非线性激光的光谱技术，具有良好的表面灵敏度和选择性。为了开发研究手性表面的光学方法，人们已经阐明了一种类似于圆二色性（CD）光谱法来检测SHG中双色行为的方法。单层取向手性分子[（R）-或（S）-2,2'-二羟基-1,1'-联萘（BN）]的SHG效率强烈依赖于激发光束是左旋还是右旋圆偏振。相对差分SHG信号比普通CD光谱法中的要大得多。SHG中的大旋光效应并非归因于传统CD中负责的旋转强度R（R = Im μ.m），而是归因于电偶极允许的二阶非线性张量χ(2)。存在一个手性元素χ(2)的χ(xyz)，它必须包含在手性表面SHG响应的描述中。能够单独测量吸附分子的手性性质，而不受其非手性性质的背景干扰，提供了另一种手性光谱形式，它不同于CD，在线性光谱中没有类似物，但以一种易于理解的方式相关。
