Chapter 1
Introduction, Basic Theory and Principles

1.1 INTRODUCTION

The main spectroscopies employed to detect vibrations in molecules are based on the processes of infrared absorption and Raman scattering. They are widely used to provide information on chemical structures and physical forms, to identify substances from the characteristic spectral patterns (‘fingerprinting’) and to determine quantitatively or semiquantitatively the amount of a substance in a sample. Samples can be examined in a whole range of physical states, for example, as solids, liquids, vapours, hot and cold, in bulk, as microscopic particles or as surface layers. The techniques are very wide ranging and provide solutions to a host of interesting and challenging analytical problems. Raman scattering is less widely used than infrared absorption, largely due to problems with sample degradation and fluorescence. However, recent advances in instrument technology have simplified the equipment and reduced the problems substantially. These advances, together with the ability of Raman spectroscopy to examine aqueous solutions, samples inside glass containers and samples without any preparation, have led to a rapid growth in the application of the technique.

Practically, modern Raman spectrometers are simple to use. Variable instrument parameters are few, spectral manipulation is minimal and a simple interpretation of the data may be sufficient. This chapter and Chapter 2 aim to set out the basic principles and experimental methods to give the reader a firm understanding of the basic theory and practical considerations so that the technique can be applied at the level often required for current applications. However, with Raman scattering important information is sometimes not used or recognised. Later chapters will develop the minimum theory required to give a more in‐depth understanding of the data obtained and to enable comprehension of some of the many more advanced techniques, which have specific advantages for some applications.

1.2 HISTORY

The phenomenon of inelastic scattering of light was first postulated by A. Smekal in 1923 [1] and first observed experimentally in 1928 by C.V. Raman and K.S. Krishnan [2]. Since then, the phenomenon has been referred to as Raman scattering. In the original experiment, sunlight was focused by a telescope onto a sample, which was either a purified liquid or a dust‐free vapour. A second lens was placed by the sample to collect the scattered radiation. A system of optical filters was used to show the existence of scattered radiation with an altered frequency from the incident light – the basic characteristic of Raman scattering.

1.3 BASIC THEORY

When light interacts with matter, the photons which make up the light may be absorbed or scattered or may not interact with the material and may pass straight through it. If the energy of an incident photon corresponds to the energy gap between the ground state of a molecule and an excited state, the photon may be absorbed and the molecule promoted to the higher energy excited state. It is this change which is measured in absorption spectroscopy by detection of the loss of that energy of radiation. However, it is also possible for the photon to interact with the molecule and scatter from it. In this case there is no need for the photon to have an energy which matches the difference between two energy levels of the molecule. The scattered photons can be observed by collecting them at an angle to the incident light beam. If there is no absorption from any electronic transition, which has a similar energy to that of the incident light, the scattering efficiency increases as the fourth power of the frequency of the incident light.

Scattering is a commonly used technique. For example, it is widely used as a method for measuring particle size and size distribution down to sizes less than 1 μm. One everyday illustration of this is that the sky is blue because the higher energy blue light is scattered from molecules and particles in the atmosphere more efficiently than the lower energy red light. However, for molecular identification, a small component of the scattered light, Raman scattering, is particularly effective.

The word ‘light’ implies electromagnetic radiation within the wavelength range to which the eye is sensitive whereas in spectroscopy, the important point is whether the detector is sensitive to the radiation used and as a consequence much wider ranges of wavelengths can be used. As a result, the process of absorption is used in a wide range of spectroscopic techniques. For example, it is used in acoustic spectroscopy where there is a very small energy difference between the ground and excited states and in X‐ray absorption spectroscopy where there is a very large difference. In between these extremes, many of the common techniques such as NMR, EPR, infrared absorption, electronic absorption and fluorescence emission and vacuum ultraviolet spectroscopy are based on the absorption of radiation. Figure 1.1 indicates the wavelength range of some commonly used types of radiation.

Figure 1.1. The electromagnetic spectrum on the wavelength scale.

Radiation is often characterised by its wavelength (λ). However, in Raman spectroscopy, we are interested in information obtained from the scattered radiation on the vibrational states of the molecule being examined. These are usually more conveniently discussed in terms of energy and consequently it is usual to use frequency (ν) or wavenumber (ϖ) scales, which are linearly related to energy. The relationships between these scales are given in the equations below.

and

It is clear from Eq. (1.1) that the energy is proportional to the reciprocal of wavelength and therefore the highest energy region is shown on the left side in Figure 1.1.

The energy changes we detect in vibrational spectroscopy are those required to cause nuclear motion but the way in which radiation is employed in infrared and Raman spectroscopies is different. In infrared spectroscopy, infrared energy covering a range of frequencies is directed onto the sample. Absorption occurs where the frequency of the incident radiation matches that of a vibration so that the molecule is promoted to a vibrational excited state. The loss of this frequency of radiation from the beam after it passes through the sample is detected. In contrast, Raman spectroscopy uses a single frequency of radiation to irradiate the sample. It is the radiation scattered from the molecule, one vibrational unit of energy different from the incident beam, which is detected. Thus, unlike infrared adsorption, Raman scattering does not require matching of the incident radiation to the energy difference between the ground and excited states.

In a scattering process, the light interacts with the molecule and distorts (polarizes) the cloud of electrons round the nuclei to form a short‐lived state called a virtual state discussed in Chapter 3. This state is not stable and the photon is quickly reradiated. If only the electron cloud is distorted in the scattering process, when the electron cloud returns to the starting position the photons are scattered with the same frequency as the incident radiation. This scattering process is regarded as elastic scattering and is the dominant process. For molecules, it is called Rayleigh scattering. However, if nuclear motion is induced during the scattering process, energy will be transferred either from the incident photon to the molecule or from the molecule to the scattered photon. In these cases, the process is inelastic and the energy of the scattered photon is different from the incident photon by one vibrational unit. This is Raman scattering. It is inherently a weak process in that only one in every 106–108 photons which scatter is Raman scattered. In itself this does not make the process insensitive since with modern lasers and microscopes very high‐power densities can be delivered to very small samples but it does follow that other possible processes such as sample degradation and fluorescence can occur readily.

Figure 1.2 shows the basic processes which occur for one vibration. At room temperature, most molecules, but not all, are present in the lowest energy vibrational level. The virtual states are not real states of the molecule but are created when the laser interacts with the electrons and causes polarization and the energy of these states is determined by the frequency of the light source used. The Rayleigh process will be the most intense process since most photons scatter this way. Since it does not involve any energy change, the diagram shows the light returning to the same energy state. The Raman scattering process from the ground vibrational state m leads to absorption of energy by the molecule and its promotion to the higher energy excited vibrational state n. This is called Stokes scattering. However, due to thermal energy, some molecules may be present initially in an excited state as represented by n in Figure 1.2. Scattering from these states to the ground state m is called anti‐Stokes scattering and involves transfer of energy from the molecule to the scattered photon. The relative intensities of the two processes depend on the population of the various states of the molecule and also on symmetry selection rules. The...