和频生成光谱
和频产生(SFG)光谱用于以单分子层灵敏度评估表面和界面的振动特性。与对体相性质敏感的拉曼光谱不同,SFG 光谱对反演对称性被破坏的表面和界面处分子层的物理及化学性质极为敏感。
在 SFG 光谱中,一束中红外光和一束可见光在样品表面或界面处重叠。所产生的和频信号的特定性质(如偏振和强度)可提供表面偶极子取向及振动光谱的信息。SFG 是一种二阶非线性过程,仅在反演对称性被破坏时才会发生,这使其具有特定的表面敏感性。在这类光谱中,为获得高光谱分辨率并进而区分所研究分子的振动指纹,其中一束脉冲需要具有足够窄的光谱带宽。
SFG 光谱的主要科学目标之一是通过观察普通水的 OH 伸缩振动(位于 2.6–3.5 μm)和重水的 OD 伸缩振动(位于 3.5–4.5 μm)来研究界面水的动力学。其他受关注的光谱范围包括 4.5–5.5 μm(金属羰基化合物和腈类的振动在此发生)以及 5.5–6.5 μm(界面生物分子的振动在此发生)。此外,由于较低频率的生物分子振动,进一步拓展至中红外区域也极具研究价值。
以 CARBIDE 或 PHAROS 飞秒激光器为泵浦源的宽带中红外光源(如 ORPHEUS-MIR),可覆盖上述感兴趣的光谱范围,并能同时作用于多个振动能级,为和频产生光谱的开展提供了便利。通过使用 SHBC 对激光频率进行倍频并窄化光谱带宽,可满足所需的技术指标。在这种配置下,和频信号产生于可见光谱范围,从而无需复杂的红外探测设备。
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和频生成光谱
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