What Is Vibrational Raman Spectroscopy: A Vibrational or an Electronic Spectroscopic… DOI: http://dx.doi.org/10.5772/intechopen.86838

from the signature obtained from RS, and it depends on the specific wave number of the laser. Due to appearance of overtones and combination bands in the VRRS, the anharmonic corrections to the vibrational potential function in the electronic ground state can be estimated. It follows that VRRS is a vibrational spectroscopic technique, where the properties of the resonating states and the state tensors associated with these have an important influence on the spectral distribution.

Since the VRRS technique can be applied also as time-resolved spectroscopy, it is an attractive tool for the investigation of both the structure and dynamics of biomolecules. The main advantage of VRRS is the ability to investigate different parts of a large protein molecule by tuning the excitation wave number into the absorption band of the chromophore of interest. In a recent paper [42], the application of VRRS in the study of the structure and dynamics of various proteins is discussed. [42] gives an excellent review of this field covering both visible and UV resonance Raman as well as cw and time-resolved versions of VRRS.

The polarization properties of the resonance Raman signal are more important in resonance than in non-resonance. For example, as discussed in [43], the uniqueness of the polarization-resolved VRRS spectra combined with standard PCA chemometrics enables one to discriminate between closely related biomolecules with almost identical unpolarized VRRS spectra. The key point is that structural molecular change manifests itself through a change of the polarization of the Raman signal (DPR). The DPR defined in Eqs. (4) and (5) is an absolute quantity, which in combination with standard PCA renders the multivariate analysis insensitive to sample and experimental variations.

The discussion in Section 3.3 and the examples presented in Section 4 demonstrate that RADIS is closer to UV/visible absorption spectroscopy than it is to vibrational spectroscopy. Besides the spectral resolution is much higher enabling the vibrational fine structure of the absorption spectra to be resolved. In resonance, the interference between the state tensors, which is the origin of the sensitivity of the Raman signal with respect to changes of the molecular parameters, is restricted to those with energy denominators closest to the laser wave number. It was also demonstrated that the polarization properties of the Raman signal, expressed through the DPR, play a more important role than in non-resonance. As said already, the DPR is defined as the ratio between two Raman signals with different polarization. The interference, which can be both constructive and destructive, will in general be different in the two Raman signals depending on the wave number of the laser and the structure of the state tensors, which again is determined by the molecular symmetry and physical properties of the molecule. Section 4.1 and Figure 3 illustrate a simple example, where the molecular configuration in an electronically excited state is distorted, which, as seen, creates a significant polarization dispersion. To fully exploit the sensitivity of the DPR to changes in the molecular parameters, one must determine the polarization dispersion, i.e., one must monitor two resonance Raman spectra (the parallel and perpendicular polarized) at each laser wave number available. Traditionally the Raman spectra with different polarizations are measured in sequence. However, with CCD technology it is possible to measure the two Raman signals simultaneously, which will improve the accuracy of the DPR considerably. This requires a modification of the entrance and collection optics of a standard Raman spectrometer, so that the upper and lower halves of the CCD monitor the parallel and perpendicular polarized Raman signals, respectively [44, 45].

The examples discussed in Section 4.2 show that it is possible to detect a small change in color of a molecular sample by determining the change of the DPR of a dispersive Raman mode, applying only a single excitation wave number in the absorption spectrum. To be detected the color change must be due to a modification

automatically be born as three-way data and more importantly the spectral resolution is very high. Consequently, only few data points along the Raman shift axis <sup>e</sup>ν<sup>k</sup> and along the excitation axis <sup>e</sup>ν<sup>j</sup> (eν<sup>j</sup> is an excitation wave number) for small number of samples are really needed. Thus, the elements of the RADIS data matrix are given

application of a Tucker3 multivariate model, and various classification problems have been simulated and studied with the result that only few samples (<10) and few Raman lines (3– 4) and few excitation wave numbers (2–3) are needed to

What is vibrational Raman spectroscopy: a vibrational or an electronic spectro-

The Raman signal provides a highly resolved vibrational signature of the molecule. However, the signature depends on whether the molecular system (molecule or ion) is excited in non-resonance or in resonance with an electronic transition. In non-resonance it follows from Eqs. (9) to (10) that the Raman signal depends on the molecular polarizability tensor evaluated in the electronic ground state and not on the molecular dipole moment as in IR and NIR. In principle all fundamental

<sup>0</sup> 6¼ 0, may contribute to the vibrational

scopic technique or both? Although the Raman signal reflects the vibrational motions of the molecule in the electronic ground state, our discussion shows that the answer to the question is that the Raman technique can be applied both as a vibrational and as an electronic spectroscopic technique depending on the experi-

> ∂Qk � �

signature. The excited electronic states have no influence on the vibrational signature. The non-resonance Raman technique is therefore a genuine vibrational technique similar to IR and NIR. The main differences are that the Raman signals are measured in a different way than the IR and NIR signals and that the polarization may, for smaller and symmetric molecules, provide additional information, also

In resonance, where the laser wave number is chosen within an electronic absorption band of the molecule, Eq. (3) shows that the state tensors being closest to resonance with the laser will contribute most to the Raman tensor and to the Raman signal (resonance enhancement). Thus, the vibrations appearing in the resonance Raman spectra are mainly those associated with the electronic absorption. As discussed in Sections 3.2 and 3.3 and illustrated in Figure 2, resonance Raman scattering may form the basis of two different kinds of resonance Raman techniques termed VRRS and RADIS. In VRRS the focus is on the vibrational Raman spectra, just like in RS, but now obtained under resonance conditions, while in RADIS the focus is on the excitation profiles and the polarization dispersion curves. In RADIS the total Raman signal and the polarization-resolved Raman signals (giving the DPR defined in Eq. (5)) for a specific Raman-active vibration are monitored as a function of the excitation wave number. The vibronic expansion of the state tensor given in Eq. (7) shows that the Raman signal in resonance is determined by the electronic transition moment of the resonating state and its derivatives and by the relations between the vibrational sub-states associated with the electronic ground state (i.e., j i va and j i vb ) and the resonating electronic states (i.e., ve j i). Since this will change the selection rules as compared to non-resonance, the vibrational signature of a specific molecule obtained from VRRS is therefore generally different

. In [41] the RADIS data matrix has been analyzed by the

as xijk ¼ I

ð Þi Raman <sup>e</sup>ν<sup>j</sup>

5. Conclusions

for solutions.

32

mental conditions chosen.

vibrations in the molecule, where <sup>∂</sup>α^ρσ

;eνk<sup>Þ</sup>

Modern Spectroscopic Techniques and Applications

obtain reliable results. We refer to [41] for details.

�

of the chromophore, so that the same Raman modes are present before and after the color change. Also in these examples, the resonance Raman technique performs as a kind of electronic technique.

References

121:501

1264-1274

1958;112:1940

487-493

pp. 1235-1238

9890-9891

35

[1] Raman CV, Krishnan KS. A new type of secondary radiation. Nature. 1928;

DOI: http://dx.doi.org/10.5772/intechopen.86838

editors. Advances in Infrared and Raman Spectroscopy. Vol. 6. New York

[12] Schweitzer-Stenner R. Polarized

Phthalocyanines. 2001;5(3):198-224.

[13] Siebrand W, Zgierski MZ. In: Lim C, editor. Excited States. Vol. 4. New York: Academic Press Inc.; 1979. pp. 1-134

[14] Kramers HA, Heisenberg W. Über die Streuung von Strahlung durch Atome. Zeitschrift für Physik. 1925;

[15] Weisskopf V. Zur Theorie der Resonanzfluoreszenz. Annals of Physics.

[16] Goldberger KM, Watson KM. Collision Theory. New York, US: Dover

Books on Physics; 2004. p. 944. ISBN-13: 978-0486435077

[18] Dirac PAM. The Principles of Quantum Mechanics. 4th ed. Clarendon

[19] Mortensen OS. Structure and Bonding. Vol. 69. Berlin Heidelberg, Germany: Springer-Verlag; 1987.

[20] Hassing S. In: Khan M, editor. Raman Spectroscopy and Applications.

[21] Spiro TG, Strekas TC. Resonance Raman spectra of hemoglobin and cytochrome c: Inverse polarization and

Rijeka, Croatia: InTech; 2017.

Press: Oxford; 1967. 314p

[17] Davydov AS. Quantum Mechanics. 1st ed. Oxford: Pergamon Press; 1965.

resonance Raman dispersion spectroscopy on metalporphyrins.

Journal of Porphyrins and

DOI: 10.1002/jpp.307

31(1):681-708

1931;9:23

p. 680

pp. 1-38

pp. 143-162

(US): Wiley; 1980

What Is Vibrational Raman Spectroscopy: A Vibrational or an Electronic Spectroscopic…

[2] Raman CV. A new radiation. Indian Journal of Physics. 1928;2:387-398

[4] Gordon JP, Zeiger HJ, Townes CH. The maser-new type of microwave amplifier, frequency standard and spectrometer. Physics Review. 1955;99:

[5] Schawlow AL, Townes CH. Infrared and optical masers. Physics Review.

[6] Boyle WS, Smith GE. Chargecoupled semiconductor devices. Bell System Technical Journal. 1970;49:

[7] Smith BA. Astronomical imaging applications for CCDs. In: JPL

Conference on Charge-Coupled Device Technology and Applications; 1976.

[8] Xie L, Ling X, Fang Y, Zhang J, Liu Z. Graphene as a substrate to suppress fluorescence in resonance Raman spectroscopy. Journal of the American Chemical Society. 2009;131(29):

[9] Placzek G. In: Marx E, editor. Handbuch der Radiologie. Vol. 2.

Verlagsgesellschaft; 1934. pp. 209-374

[10] Long DA. Raman Spectroscopy. London, UK: McGraw Hill; 1977

Polarization and interference phenomena in resonance Raman scattering. In: Clarke RJH, Hester RE,

[11] Mortensen OS, Hassing S. Chapter 1:

Leipzig: Akademische

[3] Long DA. The Raman Effect. Chichester: John Wiley & Sons; 2002.

ISBN: 978-0-471-49028-9

Finally, it is shown in Section 4.3 that due to the coherent nature of the Raman process it generates automatically the so-called three-way multivariate data. This property is applied to solve chemical classification problems by using only a few (2– 3) excitation wave numbers in un-polarized RADIS in combination with a threeway multivariate model. As shown, only very few samples (<10) are needed, instead of the very large (500–600) number of samples required, when the visible absorption and fluorescence spectra are combined to produce the three-way data.
