相位测量和频振动光谱(SFG)可以获得物质表面分子取向等信息,但在实验重复性、实验设计和界面分析等方面仍有一些关键问题没有解决。相位误差会引起光谱变化并误导界面结构分析,因此分析并准确控制误差是相位测量SFG的关键技术。使用z-切石英作为相位标准,测量了修饰在熔融石英基底上的十八烷基三氯硅烷(OTS)在C—H振动波段的和频振动光谱,对OTS的相位光谱进行了解析,结果表明OTS虚部光谱中, 2 878和2 936 cm~(-1)处的两个正峰分别是末端CH_3的对称振动(CH_3ss)和费米共振(CH_3FR), 2 960 cm~(-1)处的负峰为CH_3的反对称伸缩振动(CH_3as),这三个峰的光谱特征和指认与文献一致。2 910 cm~(-1)附近的负峰为CH_2反对称伸缩(CH_2as),与文献比较,约有20 cm~(-1)的偏移,且在2 850 cm~(-1)附近还观察到一个负峰,归属为CH_2对称伸缩(CH_2ss),分析认为与文献的差异可能是因为样品制备时间影响了OTS的分子排列结构。通过建立OTS虚部谱与CH_3取向角的关系,发现CH_3的三种振动模式的c轴与表面法线的夹角均小于90°,其H更多为向上取向且排列有序,表明相位测量相较于强度测量可以获得更丰富的表面信息。同时,讨论了待测样品和参考样品位置的非一致性对相位测量精度的影响。通过测量OTS在三个不同位置(12.1, 12.3和12.73 mm)的虚部谱,并与模拟相位误差的引入对虚部谱的影响对比,发现待测样品与参考样品测量位置间2.5μm的位移对应于1°的相位误差, 20°相位的偏移会导致零点位置移动约6 cm~(-1),从而引起振动峰位置和符号等的改变,导致对光谱的错误解析。为了获得界面分子稳定可靠的相位信息,需要严格控制两次样品测量位置一致。实验研究结果为提高和频振动光谱相位测量的精度与准确性提供了指导,为界面分子表面态的检测与分析、及微小信号的探测提供了有效手段。

Phase measurement sum-frequency vibration spectroscopy (SFG) can obtain molecular orientation information of the material surface, but there are still some key issues that remain unresolved, including experimental repeatability, experimental design, and interface analysis. Phase error can cause spectral changes and mislead interface structure analysis. Therefore, analyzing and accurately controlling errors is the key technique for phase measurement SFG. We used z-cut quartz as the phase standard, measured the sum frequency vibration spectrum of octadecyltrichlorosilane (OTS) modified on the fused silica substrate in the C-H vibration band, analyzed the phase spectrum of OTS. The results show that in the OTS imaginary spectrum, the two positive peaks at 2 878 and 2 936 cm(-1) are the symmetrical vibration (CH(3)ss) and Fermi resonance (CH3FR) of the terminal CH3, and the negative peak at 2 960 cm(-1) is anti-symmetric stretching vibration of CH3 (CH(3)as), and the spectral characteristics and designation of these three peaks are consistent with the literature. The negative peak near 2 910 cm is CH2 anti-symmetrical stretching (CH(2)as). Compared with the literature, there is an offset of about 20 cm(-1), and a negative peak is also observed near 2 850 cm(-1), which belongs to CH2 symmetrical stretching (CH(2)ss). We think that the difference from the literature may be due to the effect of sample preparation time on the molecular arrangement of OTS. By establishing the relationship between the imaginary part spectrum of OTS and the orientation angle of CH3, it is found that the angle between the c-axis and the surface normal of the three vibration modes of CH3 is less than 90 degrees, and its H is more oriented upward and arranged in order, indicating that the phase measurement can obtain richer surface information, compared with the intensity measurement. At the same time, the influence of the inconsistency of the position of the test sample and the reference sample on the phase measurement accuracy is discussed. By measuring the imaginary part spectrum of OTS at three different positions (12.1, 12.3, and 12.73 mm), and compared with simulated phase error on the imaginary part spectrum, indicate that the 2.5 mu m displacement between the measurement position of the test sample and the reference sample corresponds to 1 phase error the phase shift of 20 degrees will cause the zero position to move about 6 cm(-1), which causes changes in the position and sign of the vibration peaks, leading to incorrect interpretation of the spectrum. Therefore, in order to obtain stable and reliable phase information of the interfacial molecules, it is necessary to strictly control that the measurement positions of the two samples are consistent. The results of this experimental study provide guidance for improving the accuracy of phase measurements and provide an effective means for the detection and analysis of surface states of molecular molecules, including the detection of small signals.