School of Science, Tianjin University of Technology and Education, Hexi, Tianjin 300222, People's Republic of China.
Noyes Laboratory of Chemical Physics, California Institute of Technology, Pasadena, CA 91125.
Proc Natl Acad Sci U S A. 2020 Feb 11;117(6):2805-2814. doi: 10.1073/pnas.1906243117. Epub 2020 Jan 29.
The experimental sum frequency generation (SFG) spectrum is the response to an infrared pulse and a visible pulse and is a highly surface-sensitive technique. We treat the surface dangling OH bonds at the air/water interface and focus on the absolute SFG intensities for the resonant terms, a focus that permits insight into the consequences of some approximations. For the polarization combinations, the calculated linewidths for the water interface dangling OH SFG band at 3,700 [Formula: see text] are, as usual, too large, because of the customary neglect of motional narrowing. The integrated spectrum is used to circumvent this problem and justified here using a Kubo-like formalism and theoretical integrated band intensities rather than peak intensities. Only relative SFG intensities are usually reported. The absolute integrated SFG intensities for three polarization combinations for sum frequency, visible, and infrared beams are computed. We use molecular dynamics and the dipole and the polarizability matrix elements obtained from infrared and Raman studies of [Formula: see text]O vapor. The theoretical expressions for two of the absolute susceptibilities contain only a single term and agree with experiment to about a factor of 1.3, with no adjustable parameters. The Fresnel factors are included in that comparison. One of the susceptibilities contains instead four positive and negative terms and agrees less well. The expression for the SFG correlation function is normally derived from a statistical mechanical formulation using a time-evolving density matrix. We show how a derivation based on a two-field relaxation leads to the same final result.
实验和频产生（SFG）光谱是对红外脉冲和可见脉冲的响应，是一种高度表面敏感的技术。我们处理空气/水界面处的悬空 OH 键，并关注共振项的绝对 SFG 强度，这一关注点使我们能够深入了解某些近似的后果。对于偏振组合，在 3700[Formula: see text]处，水界面悬空 OH SFG 带的计算线宽通常太大，这是由于习惯性地忽略了运动变窄。积分光谱用于解决这个问题，并在这里使用类似于 Kubo 的形式主义和理论积分带强度而不是峰值强度来证明其合理性。通常只报告相对 SFG 强度。计算了三个偏振组合的和频、可见和红外光束的绝对积分 SFG 强度。我们使用分子动力学和从[Formula: see text]O 蒸汽的红外和拉曼研究中获得的偶极矩和极化率矩阵元素。两个绝对磁化率的理论表达式仅包含一个项，与实验值的吻合度约为 1.3 倍，没有可调参数。在该比较中包含菲涅耳因子。其中一个磁化率包含四个正项和负项，吻合度较差。SFG 相关函数的表达式通常是从使用时变密度矩阵的统计力学公式推导出来的。我们展示了基于双场弛豫的推导如何得出相同的最终结果。
