Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, USA.
J Chem Phys. 2011 Feb 28;134(8):084701. doi: 10.1063/1.3518457.
Time and frequency domain sum-frequency generation (SFG) were combined to study the dynamics and structure of self-assembled monolayers (SAMs) on a fused silica surface. SFG-free induction decay (SFG-FID) of octadecylsilane SAM in the CH stretching region shows a relatively long time scale oscillation that reveals that six vibrational modes are involved in the response of the system. Five of the modes have commonly been used for the fitting of SFG spectra in the CH stretching region, namely the symmetric stretch and Fermi resonance of the methyl group, the antisymmetric stretch of the methyl, as well as the symmetric and antisymmetric stretches of the methylene group. The assignment of the sixth mode to the terminal CH(2) group was confirmed by performing a density function theory calculation. The SFG-FID measures the vibrational dephasing time (T(2)) of each of the modes, including a specific CH(2) group within the SAM, the terminal CH(2), which had never been measured before. The relatively long (∼1.3 ps) dephasing of the terminal CH(2) suggests that alkyl monolayer structure is close to that of the liquid condensed phase of Langmuir Blodgett films.
时间和频率域和频产生（SFG）被结合起来研究自组装单层（SAM）在熔融石英表面的动力学和结构。十八烷基硅烷 SAM 在 CH 伸缩区域的 SFG-自由感应衰减（SFG-FID）显示出相对较长的时间尺度振荡，表明该系统的响应涉及六个振动模式。五个模式通常用于 CH 伸缩区域的 SFG 光谱拟合，即甲基的对称伸缩和费米共振、甲基的反对称伸缩，以及亚甲基的对称和反对称伸缩。第六个模式到末端 CH(2)基团的分配通过进行密度泛函理论计算得到了证实。SFG-FID 测量了每个模式的振动退相时间（T(2))，包括 SAM 内的特定 CH(2)基团和末端 CH(2)基团，这是以前从未测量过的。末端 CH(2)的相对较长（∼1.3 ps）退相表明，烷基单层结构接近于 Langmuir Blodgett 薄膜的液体凝聚相。
