5 Vibrational Spectroscopies
5.1 General Features
In the appropriate frequency ranges electromagnetic radiations can interact with the vibrational modes of a condensed phase whose energy depends on interatomic forces and on the geometry of the structural units involved. Specific structural units such as Q n species thus have a characteristic vibrational signature whose changes as a function of chemical composition or temperature and pressure yields valuable structural information. With infrared (IR) and Raman spectroscopy, one probes in this way the interaction between atomic entities undergoing vibrational motion and an incident electromagnetic radiation whose energy is in the IR or visible regions of the spectrum. The difference between these techniques is that the interaction of the atoms and entities undergoing vibrational motion involves absorption and scattering of photons in IR spectroscopy and Raman spectroscopies, respectively, the energy of the incident photons varying by an amount (or a multiple of it) E = hν = hc/λ, where c is the speed of light, h Planck’s constant, ν and λ the frequency and wavelength of the vibrational mode probed. When recorded against the frequency of the light source, a spectrum thus shows a series of bands or peaks that are the fingerprints of specific vibrational modes. In contrast, Brillouin spectroscopy does not bring direct structural information because it relies on the interaction of photons with acoustic waves, which probes the bulk and not some specific units of the substance. But it yields elastic coefficients, which themselves strongly depend on structure.
5.2 Infrared Spectroscopy
In IR spectroscopy the sample is illuminated by radiation from an IR source [11, 12]. The incident photons are absorbed if there is a change in the induced dipole moment of the bonds undergoing the vibration. This is due to the nonuniform distribution of charge along the bond. The IR radiation is measured as it is passed through (absorbed) or reflected by the sample. In general, molecules or molecular groups that have strong changes in their dipole moments (polar molecules and asymmetric vibrations) usually have strong IR spectra. Some vibrations can be observed by both IR and Raman spectroscopy whereas others may only be observed by one or the other and hence which technique is employed depends somewhat upon the types of vibrations that will be investigated.
IR spectroscopy is widely used to investigate water [13] and carbon dioxide in glasses [14], boron (B) coordination in borate glasses [15], and the structure of sol‐gel derived glasses [16] and glass thin films. Kamitsos [12] has recently reviewed the application of IR spectroscopy to studies of glasses. The technique investigates the absorption or reflectance of IR radiation from the far (~10–400 cm−1) to mid‐IR regions (~400–5000 cm−1). The samples can be powders (mg), glass chips (mm), or preferably glass chips where the surface of the chip has been polished. Older transmission studies often used glass powders mixed with some sort of matrix material (usually an alkali halide).
Studies on volatiles are carried out with IR absorbance techniques whereas IR reflectance methods are the most common for structural studies of glasses (especially borates) because in this case one does not need to correct for a number of spectral aberrations caused by a variety of sources such as, for example, variations in sample thickness. Reflectance spectra are transformed with the Kramers–Krönig (KK) transformation or via dispersion analysis to provide the optical and dielectric properties of the glass (cf. [12]). As in Raman spectroscopy, the observed peaks are generally characteristic of specific vibrational motions and molecular groups. In borate glasses (Figure 7) peaks at 800–1150 cm−1 are, for instance, due to B─O stretching vibrations of BO4 tetrahedra whereas the bands at 1150–1550 cm−1 are indicative of stretching vibrations of B─O bonds in triangular borate units. The progressive changes in the 800–1550 cm−1 range observed in alkaline earth borates thus indicate a change in the B coordination from three to four when bond lengths and bond strengths change.
Figure 7 Band assignments in infrared absorption spectra of borate glasses obtained by Kramers–Krönig analyses of reflectance data, and effect on the absorption of the nature and concentration of (a) 33 mol % and (b) 45 mol % alkaline earth cation.
Source: After [12].
5.3 Raman Spectroscopy
Raman spectroscopy involves illuminating the sample with monochromatic light from a laser and observation of the photons that are scattered from the sample (see [17, 18] for a comprehensive discussion). The incident laser photons interact with the molecular vibrations or phonons in the sample and some of the incident photons gain or lose energy as a consequence of the interaction. This gain or loss in energy, observed as a change in frequency of the scattered photons, is called the Raman shift and results from inelastic interactions between the incident photons and the electron cloud around the vibrating atoms in the sample. When the electron cloud is deformed, a Raman shift will occur. This deformation is termed the polarizability of the molecule or bond. The Raman shift (Δν) is reported in terms of cm−1, where Δν = (ν0 – νm), λ0 = 1/ν0 is the laser wavelength, and νm the frequency of the Raman band.
Raman spectroscopy is widely used to investigate the structure of glasses and amorphous materials because it is sensitive to subtle structural changes that may occur in the glass network. It can thus be used to observe a number of short‐ and intermediate‐range features. Its primary use is in investigating changes in the connectivity and ring statistics of the glass network and the Q n speciation in silicate glasses.
The Raman spectrum is characteristic of the material and can be used as a spectral signature to discriminate different types of glasses, and vibrations associated with specific structural features such as small and large rings, intermediate‐range structure, BO and modifier vibrations, and NBO vibrations associated with differing Q species. The relative intensity of different vibrational bands is related to the concentration and nature of the vibrational source contributing to the band and can be used to obtain the relative concentrations of different molecular groups or structural entities. In some cases it is possible to obtain quantitative information through curve fitting of the Raman spectrum, although such measurements remain somewhat controversial. The samples are usually glass chips (mm) or polished glass surfaces similar to IR.
As examples, the Raman spectrum of silica glass and a series of sodium silicate glasses are shown in Figure 8. A number of Raman bands or peaks are observed whose positions depend on the type of atoms undergoing vibration and the nature of the vibration. Their intensities depend upon the degree of polarizability of the bonds and molecular groups involved. Heavier atoms exhibit bands at lower Raman shifts because the vibrational frequency in the classic harmonic approximation depends on both the bond force constant and masses of the atoms involved as exemplified by a diatomic molecule for which