After the invention of laser light sources during the early 1960s, Raman spectroscopy was found to be a useful tool for qualitative analysis, functional group identification, and molecular structure and conformational elucidation.
A variety of fundamental and practical questions can be answered by Raman spectroscopy in a nondestructive way, namely on phases, material quality, composition, strain, effects of external perturbations (temperature, pressure, stress), and determination of thermodynamic and polarization properties.
Raman and Krishnan described the basics of Raman scattering in 1928. It was found that when a sample is irradiated by monochromatic light, some of the light scattered by molecules is wavelength-shifted relative to the incident radia-tion, and this frequency shift encodes information about the vibrational fre-quencies of the scattering molecules. The principle of the Raman effect [6–8] can be explained by a classic theory approach. When the electric field E of the exciting radiation with frequency v; E=Eosinð2pntÞ; impinges on the molecule, an oscillating dipole moment 3–modulated with the frequency of the incident wave is induced
m¼ aE ¼ aEosinð2pntÞ
where a is the polarizability of the molecule. It should be noted that the polarizability can be different for the three axis, leading to the formation of a
distorted ellipsoid, which has a dimension similar to that of the molecular volume. As a result, the molecule radiates after light absorption a wave with the same frequency as the exciting light, the so-called Rayleigh radiation. If, in addition, the molecule undergoes some internal motion, such as vibration or rotation, the oscillating dipole will have superimposed on it vibrational or rotational oscillation. Then the polarizability becomes
a¼ aoþ b sinð2pnvibtÞ
where aois the polarizability in equilibrium, and b is the ratio of polarizability change with the vibration. The dipole moment is
m¼ aE ¼ ðaoþ b sinð2pnvibtÞEosinð2pntÞÞ and we find
m¼ aoEosinð2pntÞ þ ½bEoðcos2pðn nvibÞt cos2pðn þ nvibÞtÞ where the first term corresponds to Rayleigh scattering, and the second term refers to Raman scattering, which contains some discrete number of frequencies with higher and lower values with respect to the incident radiation.
The scattered radiation analyzed by a spectrometer shows the central intense Rayleigh scattering peak and two sidebands for each vibration, shifted to higher and lower frequency values: the Raman lines. Ground-state molecules produce lines shifted to energies lower than the source, and lines at higher frequency are due to molecules in excited vibrational states. These lines, the result of the inelastic scattering of light by the sample, are called Stokes and anti-Stokes lines, respectively.
Raman vibrational spectra contain information about all aspects of the molecular structure. This information can be expressed as Raman frequencies, intensities, and depolarization factors. Complementary information can be obtained from IR spectroscopy, as Raman and IR spectra show many bands at practically the same frequency but with quite different intensities. As a general rule, a vibration is ‘‘IR active’’ when it modulates the dipole moment, and it is
‘‘Raman active’’ when the molecular polarizability is modulated. Symmetrical vibrations and bonds between identical atoms usually show strong Raman and weak IR bands. Raman spectroscopy has special advantages over IR spectro-scopy when solutions in water or coatings on glass are investigated because water and glass emit only a weak Raman signal, leading to good quality spectra.
3.3.1 Application Example
The Raman spectrum of HA single crystals is dominated by a sharp peak at 960 cm–1, which corresponds to the stretching mode (n1) of phosphate groups,
and three weak and broad bands around 1070 cm–1(n3, PO43–
stretching mode), 590 cm–1(n4, PO43–
bending mode), and 430 cm–1(n2, PO43–
stretching mode) [9]. These Raman emissions were also found for other CaP bulk materials and inorganic components of bone tissue but with different relative line intensi-ties [10, 11].
Figure 3.2 shows the Stokes spectrum of an HA coating that was obtained using the near-IR (NIR) Fourier transform technique with YAG laser excita-tion. The dominant features in the spectrum are formed by a broad doublet at 770 and 702 cm–1 and smooth bands at higher frequencies (1152 and 1515–1582 cm–1) [10, 11]. Because there were no corresponding bands in the anti-Stokes region, it is reasonable to assume that these bands may be produced by fluorescence emission, causing the band to emerge at 600 to 900 cm–1and the corresponding overtone-like bands (1152 and 1515–1582 cm–1) at higher fre-quency values [9]. These bands are artifacts of NIR Raman spectroscopy and should be disregarded in the normal Raman analysis. Nevertheless, useful information on the structure of multiphase CaP coatings [11–13], ranging from an amorphous CaP to a pure crystalline HA, can be obtained from careful
Fig. 3.2 Typical Raman spectrum of HA coating recorded with a Bruker RFS100 FT-Raman spectrometer equipped with an Nd:YAG laser (1064 nm) as an excitation source (4 cm–1 resolution, 512 scans, 1500 mW laser power). The HA film was grown on Ti substrate by electrophoretic deposition at room temperature. Inset Raman spectra of PLD coating depos-ited in a H2O atmosphere at various substrate temperatures: a, 208; b, 2008; c, 4008; d, 5008;
e, 6008C. These spectra were recorded using the excitation wavelength of an Ar-ion laser (488 nm). From [12], with permission
analyses of the dominant Raman signal in the range between 900 and 1000 cm–1 (Fig. 3.2, inset).
A sharp single line at 962 cm–1, attributed to the symmetrical phosphate stretch, is characteristic of crystalline HA; the presence of a relatively small, almost indistinct shoulder toward lower wave numbers is attributable to a not well crystallized apatite structure (Fig. 3.2e).
Additional bands at 947 and 970 cm–1indicate the existence of b-TCP and a-TCP phases (Fig. 3.2c).
A doublet at 941 and 947 cm–1confirms the presence of tetracalcium phos-phate (TTCP) phases in the coating.
A broad band centered at 950 cm–1is characteristic of an amorphous cap structure (Fig. 3.2a).
Although Raman spectroscopy is a powerful, nondestructive characteriza-tion procedure, the technique also has some disadvantages. The interpretacharacteriza-tion of Raman spectra of CaP coatings is a complicated task because the intensity of the Raman signals is weak compared to those of bulk materials. Additionally, precautions should be taken in the spectrum interpretation as fluorescence and resonance effects can be present. Normal Raman bands and fluorescence can be discriminated by observing the anti-Stokes and Stokes spectra [9].