Polarization-dependent sum frequency generation (SFG) vibrational spectroscopy was employed to examine CO overlayers on Pt(111) and Pd(111) single crystal surfaces at room temperature. Utilizing different polarization combinations (SSP and PPP) of the visible and SFG light allows to determine the molecular orientation (tilt angle) of interface molecules but the analysis of the measured Ippp/Issp is involved and requires a proper optical interface model. For CO/Pt(111), the hyperpolarizability ratio R=βaac/βccc=βbbc/βccc is not exactly known and varying R in the range 0.1-0.5 yields tilt angles of 40°-0°, respectively. Based on the known perpendicular adsorption of CO on Pt, an exact R-value of 0.49 was determined. Polarization-dependent SFG spectra in the pressure range 10-4 to 36 mbar did not indicate any change of the tilt angle of adsorbed CO. Modeling also indicated a strong dependence of Ippp/Issp on the incidence angles of visible and IR laser beams. Complementing previous low temperature/low pressure data, room temperature CO adsorption on Pd(111) was examined from 10-6 to 250 mbar. The absolute PPP and SSP spectral intensities on Pt and Pd were simulated, as well as the expected Ippp/Issp ratios. Although CO on Pt and Pd should exhibit similar intensities (at high CO coverage), the higher Ippp/Issp ratio for Pd (48 vs. 27 on Pt) renders the detection of adsorbed CO in SSP spectra difficult. The presence or absence of CO species in SSP spectra can thus not simply be correlated to tilted or perpendicular CO molecules, respectively. Careful modeling, including not only molecular and interface properties, but also the experimental configuration (incidence angles), is certainly required even for seemingly simple adsorbate-substrate systems.

中文翻译：

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极化依赖SFG光谱的近环境压力CO吸附Pt（111）和Pd（111）上。

偏振相关和频率产生（SFG）振动光谱用于在室温下检查Pt（111）和Pd（111）单晶表面上的CO覆盖层。利用可见光和SFG光的不同偏振组合（SSP和PPP）可以确定界面分子的分子取向（倾斜角），但是涉及到对测得的Ippp / Issp的分析，需要正确的光学界面模型。对于CO / Pt（111），超极化率R =βaac/βccc=βbbc/βccc尚不完全清楚，在0.1-0.5范围内改变R分别会产生40°-0°的倾斜角。基于已知的CO在Pt上的垂直吸附，确定的精确R值为0.49。在10-4至36 mbar的压力范围内，与偏振有关的SFG光谱未表明吸附的CO的倾斜角有任何变化。建模还表明，Ippp / Issp与可见光和IR激光束的入射角密切相关。作为对先前的低温/低压数据的补充，研究了室温CO在Pd（111）上的吸附从10-6到250 mbar。模拟了Pt和Pd上的绝对PPP和SSP光谱强度，以及预期的Ippp / Issp比。尽管Pt和Pd上的CO表现出相似的强度（在高CO覆盖率下），但Pd的Ippp / Issp比值较高（Pt上为48 vs. 27），这使得在SSP光谱中检测吸附的CO变得困难。因此，不能简单地将SSP光谱中是否存在CO物种分别与倾斜或垂直的CO分子相关。仔细的建模，不仅包括分子和界面特性，还包括实验配置（入射角），