Bordenyuk Andrey N, Jayathilake Himali, Benderskii Alexander V
Department of Chemistry, Wayne State University, Detroit, Michigan 48202, USA.
J Phys Chem B. 2005 Aug 25;109(33):15941-9. doi: 10.1021/jp051632g.
We combine frequency- and femtosecond time-domain measurements of vibrational coherences for spectroscopic characterization of surface monolayer films, utilizing 3-wave mixing as the surface-selective technique. Frequency-domain spectra in the CH-stretch region are obtained by infrared + visible sum frequency generation (SFG). Time-domain coherences are measured using SFG free induction decay (SFG-FID), where a 75 fs IR pulse excites several vibrational modes and a delayed 40 fs visible pulse probes the oscillating surface polarization. A unified framework based on optical Bloch equations is used to simultaneously analyze the time- and frequency-domain data. We compare molecular organization of monolayers in different two-dimensional phases. Highly ordered films transferred at high surface pressure are dominated by two transitions in the frequency domain, CH3 symmetric stretch (2875 cm(-1)) and CH3-Fermi resonance with bend overtone (2935 cm(-1)), and a coherent quantum beat in the time-domain at the difference frequency (approximately 540 fs period). At lower surface pressure, relative amplitudes change and additional transitions emerge (CH3 asymmetric stretch and CH2 modes), indicating changes in molecular orientation and onset of disorder. Information redundancy in the combined frequency- and time-domain data allows more accurate determination of the spectral parameters than purely frequency- or time-domain techniques.
我们利用三波混频作为表面选择性技术，结合频率和飞秒时域测量振动相干性，用于表面单层膜的光谱表征。通过红外+可见和频产生（SFG）获得CH伸缩区域的频域光谱。使用SFG自由感应衰减（SFG-FID）测量时域相干性，其中一个75飞秒的红外脉冲激发多种振动模式，一个延迟的40飞秒可见脉冲探测振荡的表面极化。基于光学布洛赫方程的统一框架用于同时分析时域和频域数据。我们比较了不同二维相中单层膜的分子组织。在高表面压力下转移的高度有序薄膜在频域中由两个跃迁主导，即CH3对称伸缩（2875 cm(-1)）和CH3与弯曲泛音的费米共振（2935 cm(-1)），以及时域中差频处的相干量子拍（周期约540飞秒）。在较低表面压力下，相对振幅发生变化并出现额外的跃迁（CH3不对称伸缩和CH2模式），表明分子取向的变化和无序的开始。与纯频域或时域技术相比，频域和时域组合数据中的信息冗余允许更准确地确定光谱参数。
