Micro-Raman Spectroscopy

Theory and Application

Jürgen Popp, Thomas Mayerhöfer, Jürgen Popp, Thomas Mayerhöfer

• Publisher: De Gruyter
• Year: 2020
• ISBN:
  9783110514841
• Edition: 1
• Topic: Physical Sciences
• Subtopic: Chemistry
• Index: Chemistry

Micro-Raman Spectroscopy introduces readers to the theory and application of Raman microscopy. Raman microscopy is used to study the chemical signature of samples with little preperation in a non-destructive manner. An easy to use technique with ever increasing technological advances, Micro-Raman has significant application for researchers in the fields of materials science, medicine, pharmaceuticals, and chemistry.

1 Theoretical principles of Raman spectroscopy

Dana Cialla-May ,

Michael Schmitt and

Jürgen Popp

This article has previously been published in the journal Physical Sciences Reviews. Please cite as: Cialla-May, D., Schmitt, M., Popp, J. Theoretical principles of Raman spectroscopy Physical Sciences Reviews [Online] 2019, 4. DOI: 10.1515/psr-2017-0040.

Abstract

This contribution reports on the theoretical foundations of Raman spectroscopy. Since the discovery of the Raman effect in 1928, Raman spectroscopy with its linear and nonlinear variants has established itself as a powerful analytical tool in almost all scientific fields (chemistry, physics, material sciences, pharmacy, biology, (bio)medicine, geology, mineralogy, environmental sciences, etc.). First, a short introduction to linear Raman spectroscopy is given, followed by two approaches to increase the intrinsically weak Raman signal, namely resonance Raman and surface enhanced Raman spectroscopy. The last part of this contribution briefly introduces nonlinear Raman processes observed using pulsed lasers as excitation sources.

Keywords: Raman scattering, resonance Raman spectroscopy, surface enhanced Raman spectroscopy, non-linear Raman spectroscopy,

The Raman effect was discovered in 1928 by the Indian physicist C.V. Raman. This method describes the inelastic scattering of photons on a quantized molecular system [1, 2]. In most cases, the vibrational states of molecules are utilized as scattering system and that is why Raman spectroscopy is often referred to as a vibrational spectroscopic technique. Vibrational Raman spectroscopy¹ is a complementary method to IR absorption spectroscopy. The two approaches differ in their physical origin: while IR absorption describes the direct absorption of an IR photon to excite a vibrational quantum (i.e. one photon absorption process), in Raman spectroscopy as mentioned above the vibrational excitation takes place via a two-photon scattering process. In IR absorption spectroscopy vibrational modes leading to a change in the dipole moment during the vibration can be excited, while for Raman active vibrations the polarizability has to change. Since molecular vibrations are distinct for every molecule, vibrational spectra can therefore be interpreted as a type of characteristic “molecular fingerprint” of an examined inorganic, organic or biological molecule or more complex systems like e.g. biological cells and tissue. For the latter vibrational spectral contributions are assigned to proteins, lipids, nucleic acids and carbohydrates. Thus, vibrational spectroscopy is applied for the qualitative and quantitative analysis in chemistry, biology, material and life sciences and biomedicine. However, since water exhibits a large dipole moment its vibrations show a large IR absorption cross-section strongly limiting in vivo studies. This is in stark contrast to Raman spectroscopy where the water vibrations show low scattering cross-sections allowing to record Raman spectra in aqueous solution very easily, thus making Raman spectroscopy a perfect candidate for labelfree in vivo investigations on a molecular level [3]. As pathologic anomalies are accompanied by changes in the biochemical composition and structure of biomolecules, the Raman spectrum provides a sensitive and specific fingerprint of the type and state of the specimen. During the last years, Raman spectroscopy has therefore been recognized as an extremely powerful tool for bioanalytical and biomedical applications [4]. Advantages of the technique for biomedical problems include the following: (i) it is label-free and (ii) non-destructive. However, while the molecular selectivity of Raman spectroscopy is very high its sensitivity is very low, i.e. the Raman scattering process is characterized by small Raman cross-sections. In general, only one photon out of 10⁸ photons is scattered inelastically. Several Raman signal-enhancing techniques increasing the intrinsically weak Raman scattering cross-sections by several orders of magnitude are known. The two most prominent approaches are resonance Raman spectroscopy and surface enhanced Raman spectroscopy (SERS). In addition to these two linear approaches, non-linear coherent Raman scattering methods, e.g. coherent anti-Stokes Raman scattering (CARS), are also known to providing signal enhancement due to the coherent excitation of vibrational modes. In this paper, a brief theoretical introduction into the most prominent linear and nonlinear Raman techniques will be provided.

1.1 Inelastic light scattering – a brief introduction into linear Raman spectroscopy

The underlying theory of inelastic Raman scattering is comprehensively discussed within the literature [5] to which the interested reader is referred to. The aim of this paper is to give a brief introduction into this topic [6].

1.1.1 Basic principles of Raman spectroscopy

Due to the interaction of light with matter, an electric dipole within molecules is induced since the atoms become polarized, i.e. the center of negative charge is displaced from the atomic nuclei upon the interaction with an electromagnetic field. The induced electric dipole moment is directly proportional to the electric fi...