School of Electronics and Computer Science, University of Southampton, Highfield, Southampton SO17 1BJ, UK.
Anal Bioanal Chem. 2010 Apr;396(8):2813-23. doi: 10.1007/s00216-009-3272-0. Epub 2009 Nov 15.
Until 2006 the performance of confocal Raman spectroscopy depth profiling was typically described and modeled through the application of geometrical optics, including refraction at the surface, to explain the degree of resolution and the precise form of the depth profile obtained from transparent and semicrystalline materials. Consequently a range of techniques, physical and analytical, was suggested to avoid the errors thus encountered in order to improve the practice of Raman spectroscopy, if not the understanding of the underlying mechanisms. These approaches were completely unsuccessful in accounting for the precise form of the depth profile, the fact that spectra obtained from laminated samples always contain characteristic peaks from all materials present both well above and below the focal point and that spectra can be obtained when focused some 40 mum above the sample surface. This paper provides further evidence that the physical processes underlying Raman spectroscopy are better modeled and explained through the concept of an extended illuminated volume contributing to the final Raman spectrum and modeled through a photon scattering approach rather than a point focus ray optics approach. The power of this numerical model lies in its ability to incorporate, simultaneously, the effects of degree of refraction at the surface (whether using a dry or oil objective lens), the degree of attenuation due to scatter by the bulk of the material, the Raman scattering efficiency of the material, and surface roughness effects. Through this we are now able to explain why even removing surface aberration and refraction effects through the use of oil immersion objective lenses cannot reliably ensure that the material sampled is only that at or close to the point of focus of the laser. Furthermore we show that the precise form of the depth profile is affected by the degree of flatness of the surface of the sample. Perhaps surprisingly, we show that the degree of flatness of the material surface is, in fact, more important than obtaining a precise refractive index match between the immersion oil and the material when seeking a high-quality depth profile or Raman spectrum from within a transparent or semicrystalline material, contrary to accepted norms that samples for interrogation by Raman spectroscopy require little preparation.
直到 2006 年，共聚焦拉曼光谱深度剖析的性能通常通过应用几何光学来描述和建模，包括表面的折射，以解释从透明和半晶材料获得的分辨率和深度剖析的确切形式。因此，为了避免在提高拉曼光谱实践中遇到的误差，提出了一系列物理和分析技术，如果不是为了理解潜在机制的话。这些方法完全不能解释深度剖析的确切形式，即从层压样品获得的光谱总是包含聚焦点上下所有材料的特征峰，并且当聚焦在样品表面上方约 40 微米处时可以获得光谱。本文进一步证明，拉曼光谱背后的物理过程通过扩展的照明体积的概念来更好地建模和解释，该体积有助于最终的拉曼光谱，并通过光子散射方法而不是点聚焦射线光学方法进行建模。该数值模型的强大之处在于它能够同时包含表面折射程度（无论是使用干镜还是油镜）、材料主体散射引起的衰减程度、材料的拉曼散射效率以及表面粗糙度效应的影响。通过这种方式，我们现在能够解释为什么即使通过使用油浸物镜消除表面像差和折射效应，也不能可靠地确保所采样的材料仅处于或接近激光的焦点。此外，我们还表明，深度剖析的确切形式受样品表面平整度的影响。也许令人惊讶的是，我们表明，实际上，当从透明或半晶材料中获得高质量的深度剖析或拉曼光谱时，材料表面的平整度比获得浸油和材料之间的精确折射率匹配更为重要，这与公认的规范相矛盾，即用于拉曼光谱检测的样品几乎不需要准备。
