3. What is vibrational Raman spectroscopy: a vibrational or an electronic spectroscopic technique or both?

#### 3.1 Non-resonance Raman spectroscopy

In most molecules, the energy differences between the electronic ground state and the excited electronic states are much larger that the mutual energy difference between the excited electronic states. Typically, there will therefore exist a relatively large spectral region between the energy of the final state in the Raman process and the energy of the first electronically excited state, where the wave number of the laser <sup>e</sup>ν<sup>p</sup> can be chosen without exciting an electronic absorption. Besides, <sup>e</sup>ν<sup>p</sup> will also typically be much larger than molecular vibrational frequencies, so that all the terms in the Raman tensor in Eq. (1) where r runs over vibrational quantum numbers in the electronic ground state can be neglected relative to the contributions which involves excited electronic states, i.e.,

<sup>Δ</sup>eν<sup>R</sup> <sup>¼</sup> <sup>e</sup>νvb, va <sup>≪</sup>eν<sup>p</sup> <sup>≪</sup>eνev, gva . The damping constant of the excited electronic states γ<sup>r</sup>¼<sup>e</sup> may also be neglected since they are of the order of magnitude as a vibrational quantum. With these assumptions and assuming that the electronic ground state is nondegenerate (which is typical), the expression for the Raman tensor in Eq. (1) can be approximated by

$$\left(a\_{\rho\sigma}^{v\_a \to v\_b}\right)\_{non-res} \cong \left< v\_b \middle| \hat{a}\_{\rho\sigma} \left(Q, \tilde{\nu}\_{v\_b, v\_a}, \tilde{\nu}\_p \right) \middle| v\_a \right>\tag{8}$$

where <sup>α</sup>^ρσ <sup>Q</sup>;eνvb, va ;eνp � � is the Cartesian component of the molecular transition polarizability tensor

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

$$\hat{a}\_{\rho\sigma}(\mathbf{Q}, \tilde{\nu}\_{\nu\_b, \nu\_a}, \tilde{\nu}\_p) = \sum\_{\epsilon \neq \mathbf{g}} \frac{\rho\_{\mathbf{g}\epsilon}(\mathbf{Q}) \sigma\_{\mathbf{g}}(\mathbf{Q})}{\tilde{\nu}\_{\epsilon, \mathbf{g}} - \tilde{\nu}\_p} + \frac{\sigma\_{\mathbf{g}\epsilon}(\mathbf{Q}) \rho\_{\mathbf{g}}(\mathbf{Q})}{\tilde{\nu}\_{\epsilon, \mathbf{g}} + \tilde{\nu}\_p} \tag{9}$$

To obtain Eq. (9) "the closure rule," i.e., 1 ^ <sup>¼</sup> <sup>∑</sup><sup>i</sup> j ii hij valid for any complete set of states j ii , has been applied to the vibrational sub-states of every electronic state, and the electronic transition moments defined above (expansion scheme 2) have been introduced. Because Eq. (9) involves a summation over all excited electronic states, a Taylor expansion of the electronic transition moments ρgeð Þ Q and σeg ð Þ Q would result in too many parameters, which cannot be determined. A direct Taylor expansion of the molecular transition polarizability tensor <sup>α</sup>^ρσ <sup>Q</sup>;eνvb, va ;eνp � � is therefore more convenient in this case:

$$\hat{a}\_{\rho\sigma}(\mathbf{Q}, \tilde{\nu}\_{v\_b, v\_a}, \tilde{\nu}\_p) \cong \hat{a}\_{\rho\sigma}^0 + \sum\_{k=1}^{3N-6} \left( \frac{\partial \hat{a}\_{\rho\sigma}}{\partial Q\_k} \right)\_0 Q\_k + \dots \tag{10}$$

where Qk is the kth normal coordinate. To calculate the Raman tensor, the expansion in Eq. (10) is inserted in Eq. (8), and the result is inserted in Eq. (1) in order to obtain the intensity of the Raman signal of the Raman transition j i a ! j i b .

Conclusion: It follows that the Raman signal only depends on variables related to the electronic ground state, i.e., the set of normal coordinates Q and the wave numbers of the Raman bands <sup>e</sup>νvb, va , besides the wave number of the laser <sup>e</sup>νp. However, the spectral distribution in the Raman spectra is independent of the laser wave number, which only has influence on the absolute intensity of the Raman signal. As seen from Eq. (9), all information about the individual electronically excited states is "washed out" in the calculation (and in the experimental signals), which means that we cannot obtain any information on individual excited electronic states by measuring the Raman signals under these conditions. Nonresonance Raman scattering (or RS for short) becomes therefore a vibrational spectroscopic technique just like IR and NIR. There are however two essential differences between RS and IR and NIR: first, the Raman signals are obtained by the scattering of laser light with wave numbers typically in the visible region and therefore much higher than the wave numbers of the IR and NIR photons, which are directly absorbed in IR and NIR spectroscopy. Second, since the Raman signal is controlled by a tensor instead of by a vector, the spectral selection rules of RS become different from those of IR and NIR. The selection rules for RS can easily be derived by considering Eq. (10) inserted in Eq. (8). Assuming room temperature and considering only the Stokes spectrum, it follows that Raman spectra only contain the fundamental transitions, i.e., j i va ¼ 0 ! j i vb ¼ 1 , where the intensity is provided by the first derivative of the polarizability tensor. Besides for smaller molecules (e.g., benzene) having a high symmetry, the polarization enables one to distinguish between totally symmetric and asymmetric vibrations in the Raman spectrum. The DPR for symmetric vibrations becomes 0 ≤ DPR < 0:75, which follows directly from Eq. (5) and the fact that the Raman tensor is symmetric in nonresonance (i.e., <sup>Σ</sup><sup>1</sup> <sup>¼</sup> 0). For asymmetric vibrations the DPR <sup>¼</sup> <sup>0</sup>:75, since the tensor, because of the symmetry, has no trace (i.e., <sup>Σ</sup><sup>0</sup> <sup>¼</sup> 0) for these modes. For molecules without symmetry, all vibrations are of course in principle totally symmetric. Due to the generally narrow bandwidths of the Raman bands, Raman spectra represent in most cases a very well-defined vibrational signature of the molecule. For this reason and since the Raman signal can be measured in a reflection geometry and because no sample preparation is really needed, Raman signals are

the formal integration over the electronic coordinates are Taylor expanded in the nuclear coordinates (e.g., see [11]). After the expansion of the state tensors in Eq. (6), using either scheme 1 or 2, the result is inserted in Eq. (1) in order to obtain the Raman intensity. Applying (2) to the first order (being in most cases sufficient),

ge

where all vibrational states are functions of the set of normal coordinate Q. The first and second lines are the state tensor in the Franck-Condon and the Herzberg-Teller approximations, respectively. Comparing Eq. (7) with the result in [27], it follows that the second expansion scheme leads to the simplest parametrization of the expression for the Raman intensity; although taken to infinite order, which is of course impossible in practice, the two expansion schemes

The vibronic models developed for RRS, which are found in the Raman literature, differ essentially from each other in two ways: (1) whether expansion scheme 1 or 2 has been applied and (2) the degree of approximation that has been

In most molecules, the energy differences between the electronic ground state and the excited electronic states are much larger that the mutual energy difference between the excited electronic states. Typically, there will therefore exist a relatively large spectral region between the energy of the final state in the Raman process and the energy of the first electronically excited state, where the wave number of the laser <sup>e</sup>ν<sup>p</sup> can be chosen without exciting an electronic absorption. Besides, <sup>e</sup>ν<sup>p</sup> will also typically be much larger than molecular vibrational frequencies, so that all the terms in the Raman tensor in Eq. (1) where r runs over vibrational quantum numbers in the electronic ground state can be neglected relative to the

may also be neglected since they are of the order of magnitude as a vibrational quantum. With these assumptions and assuming that the electronic ground state is nondegenerate (which is typical), the expression for the Raman tensor in Eq. (1)

non�res ffi vbjα^ρσ <sup>Q</sup>;eνvb, va

. The damping constant of the excited electronic states γ<sup>r</sup>¼<sup>e</sup>

;eνp � �

is the Cartesian component of the molecular transition

D E

jva

(8)

3. What is vibrational Raman spectroscopy: a vibrational or an electronic spectroscopic technique or both?

contributions which involves excited electronic states, i.e.,

αva!vb ρσ � �

;eνp � �

∂σeg ∂Q <sup>k</sup> � �

0

vbjQk h i jve ve h i jva

∂ρge ∂Q <sup>k</sup> � �

0 σ0 eg

(7)

geσ<sup>0</sup> eg þ ∑ 3N�6 k¼1

<sup>þ</sup> h i vbjve vej<sup>Q</sup> <sup>k</sup> h i <sup>j</sup>va <sup>ρ</sup><sup>0</sup>

the state tensor in Eq. (6) becomes

are equivalent.

introduced.

<sup>S</sup>j i<sup>e</sup> ve j i ρσ ¼h i vbjve ve h i <sup>j</sup>va <sup>ρ</sup><sup>0</sup>

Modern Spectroscopic Techniques and Applications

3.1 Non-resonance Raman spectroscopy

<sup>Δ</sup>eν<sup>R</sup> <sup>¼</sup> <sup>e</sup>νvb, va <sup>≪</sup>eν<sup>p</sup> <sup>≪</sup>eνev, gva

can be approximated by

where <sup>α</sup>^ρσ <sup>Q</sup>;eνvb, va

polarizability tensor

22

well suited for performing different kinds of multivariate analysis of solutions containing several molecular species.

being associated with an antisymmetric state tensor for a resonating electronic state

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

The resonance enhancement and the interference between the resonating state tensors are very sensitive to the magnitude of the damping constants γr¼ev of the resonating states. Since the damping is a measure of the lifetime of an excited electronic state, resonance Raman scattering may provide information about the dynamics of excited molecular states. However, to obtain this kind of information, measurement of the Raman signal at more than one laser wave number is required.

In RADIS the resonance Raman signal is monitored as a function of the excitation wave number. In practice a series of resonance Raman spectra are measured using either a number of discrete laser wave numbers or a tunable laser. RS, VRRS, and RADIS are illustrated in Figure 2. The figure illustrates, for a thought molecule, the changes in a series of Raman spectra obtained with increasing wave number toward resonance with an electronic state. The electronic resonance is illustrated by the UV/VIS absorption spectrum. One excitation spectrum, normally called an excitation profile (dotted curve), is obtained by plotting the intensity of a specific Raman band in the Raman spectrum versus the excitation wave number. It follows from the figure that there will be one excitation profile for each Raman band. It should be stressed that a complete RADIS experiment requires that two spectra are measured at each excitation wave number, namely, the parallel and perpendicular polarized spectra. From the polarized resolved spectra, the excitation profile can be calculated as the sum of these, but more importantly the DPR as a function of the excitation wave number can be determined. The DPR versus excitation wave num-

with Eu symmetry in the point group D4h.

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

3.3 Raman dispersion spectroscopy (RADIS)

ber is called the polarization dispersion curve.

Illustration of Raman, resonance Raman, and Raman dispersion (RADIS) spectroscopy.

Figure 2.

25

## 3.2 Vibrational resonance Raman spectroscopy

In resonance Raman scattering, the wave number of the laser is chosen within the UV or visible absorption band of the molecule. Since the assumption <sup>e</sup>νvb, va <sup>≪</sup>eν<sup>p</sup> <sup>≪</sup>eνev, gva is no longer valid, we have to go back to Eqs. (3) and (8) and insert approximations appropriate for the resonant scattering situation to be considered. First it should be noticed that resonance Raman scattering may form the basis of two different kinds of resonance Raman spectroscopy:


In VRRS the vibrational Raman spectrum is measured in the same way as in RS, i.e., the spectral distribution in the Raman scattered light is measured as a function of the Raman shift <sup>Δ</sup>eν<sup>R</sup> with a fixed value of the wave number of the laser, but, as said, <sup>e</sup>ν<sup>p</sup> is now chosen close to or within an electronic absorption of the molecule. The two most striking features of VRRS as compared to RS are that the intensity of the Raman signal is largely enhanced (typically with a factor of 10<sup>3</sup> � <sup>10</sup>6) and that the intensity distribution in the vibrational spectrum is different in general. It follows from Eq. (3) that the enhancement is due to the fact that the real part in the denominators of the Raman tensor becomes small or even zero for the states being close to or in resonance with the laser. Since the state tensors associated with the resonating states dominate in the Raman intensity, the selection rules will now be determined not only by the initial and final states as in the RS but also by these state tensors. It follows from Eq. (7) that this will change the intensity distribution of the Raman signal. A further consequence is that new vibrational modes may become Raman active, but more importantly not only the fundamentals are seen in VRRS spectra, but also overtones (i.e., multiple excitations of a single normal vibration) and combination bands (i.e., multiple excitations involving several vibrations) are frequently observed. The appearance of the overtones and combination bands enables one to estimate anharmonicity constants and thereby improve the modeling of the vibrational potential function in the electronic ground state.

Larger molecules, typically biomolecules, are often colored because they contain a chromophore with high symmetry. Important examples are the metal-porphyrins, where the ring structure has the ideal symmetry of D4<sup>h</sup>. When the wave number of the laser is chosen within the visible absorption band of the chromophore, essentially only the ring vibrations are resonance enhanced and show up in the Raman spectra with significant intensity. Besides making the total Raman spectrum simpler, it also enables one to study a small part of a very large molecule, e.g., a protein molecule. Finally, it should be noticed that under resonance conditions, the value of the DPR may take any number from zero to infinity. This is because the Raman tensor needs no longer be symmetric but in general may have all three tensor invariants Σ0, Σ<sup>1</sup>, and Σ<sup>2</sup> different from zero. As seen from Eq. (5), the value infinity is obtained when the tensor is purely antisymmetric, i.e., when only <sup>Σ</sup><sup>1</sup> 6¼ 0. In the 1970s very high values of the DPR of vibrational modes were first observed experimentally in the VRRS spectra of the heme-proteins [21]. The results were interpreted as being due to the scattering by a vibration with the symmetry a2<sup>g</sup>

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

being associated with an antisymmetric state tensor for a resonating electronic state with Eu symmetry in the point group D4h.

The resonance enhancement and the interference between the resonating state tensors are very sensitive to the magnitude of the damping constants γr¼ev of the resonating states. Since the damping is a measure of the lifetime of an excited electronic state, resonance Raman scattering may provide information about the dynamics of excited molecular states. However, to obtain this kind of information, measurement of the Raman signal at more than one laser wave number is required.

#### 3.3 Raman dispersion spectroscopy (RADIS)

well suited for performing different kinds of multivariate analysis of solutions

the UV or visible absorption band of the molecule. Since the assumption

basis of two different kinds of resonance Raman spectroscopy:

2. Raman dispersion spectroscopy (RADIS) (see Section 3.3.)

of the vibrational potential function in the electronic ground state.

24

Larger molecules, typically biomolecules, are often colored because they contain a chromophore with high symmetry. Important examples are the metal-porphyrins, where the ring structure has the ideal symmetry of D4<sup>h</sup>. When the wave number of the laser is chosen within the visible absorption band of the chromophore, essentially only the ring vibrations are resonance enhanced and show up in the Raman spectra with significant intensity. Besides making the total Raman spectrum simpler, it also enables one to study a small part of a very large molecule, e.g., a protein molecule. Finally, it should be noticed that under resonance conditions, the value of the DPR may take any number from zero to infinity. This is because the Raman tensor needs no longer be symmetric but in general may have all three tensor invariants Σ0, Σ<sup>1</sup>, and Σ<sup>2</sup> different from zero. As seen from Eq. (5), the value infinity is obtained when the tensor is purely antisymmetric, i.e., when only <sup>Σ</sup><sup>1</sup> 6¼ 0. In the 1970s very high values of the DPR of vibrational modes were first observed experimentally in the VRRS spectra of the heme-proteins [21]. The results were interpreted as being due to the scattering by a vibration with the symmetry a2<sup>g</sup>

1. Vibrational resonance Raman spectroscopy (VRRS)

In resonance Raman scattering, the wave number of the laser is chosen within

In VRRS the vibrational Raman spectrum is measured in the same way as in RS, i.e., the spectral distribution in the Raman scattered light is measured as a function of the Raman shift <sup>Δ</sup>eν<sup>R</sup> with a fixed value of the wave number of the laser, but, as said, <sup>e</sup>ν<sup>p</sup> is now chosen close to or within an electronic absorption of the molecule. The two most striking features of VRRS as compared to RS are that the intensity of the Raman signal is largely enhanced (typically with a factor of 10<sup>3</sup> � <sup>10</sup>6) and that the intensity distribution in the vibrational spectrum is different in general. It follows from Eq. (3) that the enhancement is due to the fact that the real part in the denominators of the Raman tensor becomes small or even zero for the states being close to or in resonance with the laser. Since the state tensors associated with the resonating states dominate in the Raman intensity, the selection rules will now be determined not only by the initial and final states as in the RS but also by these state tensors. It follows from Eq. (7) that this will change the intensity distribution of the Raman signal. A further consequence is that new vibrational modes may become Raman active, but more importantly not only the fundamentals are seen in VRRS spectra, but also overtones (i.e., multiple excitations of a single normal vibration) and combination bands (i.e., multiple excitations involving several vibrations) are frequently observed. The appearance of the overtones and combination bands enables one to estimate anharmonicity constants and thereby improve the modeling

<sup>e</sup>νvb, va <sup>≪</sup>eν<sup>p</sup> <sup>≪</sup>eνev, gva is no longer valid, we have to go back to Eqs. (3) and (8) and insert approximations appropriate for the resonant scattering situation to be considered. First it should be noticed that resonance Raman scattering may form the

containing several molecular species.

Modern Spectroscopic Techniques and Applications

3.2 Vibrational resonance Raman spectroscopy

In RADIS the resonance Raman signal is monitored as a function of the excitation wave number. In practice a series of resonance Raman spectra are measured using either a number of discrete laser wave numbers or a tunable laser. RS, VRRS, and RADIS are illustrated in Figure 2. The figure illustrates, for a thought molecule, the changes in a series of Raman spectra obtained with increasing wave number toward resonance with an electronic state. The electronic resonance is illustrated by the UV/VIS absorption spectrum. One excitation spectrum, normally called an excitation profile (dotted curve), is obtained by plotting the intensity of a specific Raman band in the Raman spectrum versus the excitation wave number. It follows from the figure that there will be one excitation profile for each Raman band. It should be stressed that a complete RADIS experiment requires that two spectra are measured at each excitation wave number, namely, the parallel and perpendicular polarized spectra. From the polarized resolved spectra, the excitation profile can be calculated as the sum of these, but more importantly the DPR as a function of the excitation wave number can be determined. The DPR versus excitation wave number is called the polarization dispersion curve.

Figure 2. Illustration of Raman, resonance Raman, and Raman dispersion (RADIS) spectroscopy.

In a typical UV/VIS absorption experiment performed on a solution, one measures the absorbance <sup>A</sup>, which is related to the molar extinction coefficient <sup>ε</sup>ð Þ <sup>e</sup><sup>ν</sup> via Lambert-Beers law, <sup>A</sup> <sup>¼</sup> <sup>ε</sup>ð Þ <sup>e</sup><sup>ν</sup> Cml, where Cm and <sup>l</sup> are the molar concentration and the path length, respectively. From the application of the quantum theory, the molar extinction coefficient is found to be proportional to the spatial average of the absolute square of the transition dipole moment:

$$\left| \varepsilon(\widetilde{\boldsymbol{\nu}}) \propto \sum\_{\boldsymbol{\nu}\_{\mathcal{I}}', \boldsymbol{\nu}\_{\mathcal{I}}', \boldsymbol{\nu}\_{\mathcal{I}}' \dots \boldsymbol{\nu}\_{\mathcal{N}-\delta}'} \left| \left< \boldsymbol{\nu}\_{1}^{\varepsilon}, \boldsymbol{\nu}\_{2}^{\varepsilon}, \boldsymbol{\nu}\_{3}^{\varepsilon} \dots \boldsymbol{\nu}\_{3N-\delta}^{\varepsilon} \middle| \boldsymbol{\rho}\_{\mathcal{K}}(\mathbf{Q}) | \mathbf{0}, \mathbf{0}, \mathbf{0}, \dots \right> \middle| \boldsymbol{\nu}\_{\mathcal{A}\boldsymbol{\nu}}^{\varepsilon} \middle| \boldsymbol{\nu} \right> \tag{11}$$

4. Examples

sive Raman literature [31–36].

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

For the a1<sup>g</sup> vibration, it follows that S

<sup>j</sup><sup>e</sup>0¼Eu,x<sup>i</sup> xx ¼ �<sup>S</sup>

j<sup>e</sup>0¼Eu, <sup>y</sup>i xy ¼ �S

DPR dispersion (red curve) for the b1<sup>g</sup>∧bg<sup>2</sup> vibrations (D2<sup>h</sup>).

j<sup>e</sup>0¼Eu, <sup>y</sup>i

we have that S

Figure 3.

27

antisymmetric, i.e., S

4.1 Example 1: perturbation of molecular symmetry

As has been shown, vibrational RS is exclusively a vibrational spectroscopic technique like IR and NIR. However, vibrational Raman spectroscopy performed under resonance conditions may be considered as either a vibrational spectroscopic technique or as an electronic spectroscopic technique, which of the two depends on the way the experiments are performed. Three examples are briefly discussed below. For more applications, the interested reader should consult the comprehen-

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

As demonstrated in [11], the non-commuting generator approach to molecular symmetry may be applied to calculate the structure of the state tensors. Figure 3 shows an example for two vibrations in point group D4<sup>h</sup>. The figure also demonstrates what happens when the symmetry is lowered so that the configuration is now described in point group D2<sup>h</sup>. Lowering of the symmetry may be a result of a chemical reaction or may be due to a perturbation of the configuration from the planar square to a planar rectangular shape. To the left in the figure, the state tensors in D4<sup>h</sup> for the Raman-active in-plane vibrations a1<sup>g</sup> , a2<sup>g</sup> , and b1<sup>g</sup> are shown, which are written in front of the tensors. The symmetries of the two components of the resonating, degenerate electronic state with Eu symmetry are written in the tensors on the positions, which correspond to the only nonvanishing elements. The plus and minus signs describe the numerical relations between the tensor elements.

<sup>j</sup><sup>e</sup>0¼Eu,x<sup>i</sup> xx <sup>¼</sup> <sup>S</sup>

values can be calculated by using Eq. (5) and the relations given in [11] or [3]. The DPR values are written to the right side of the tensors and are seen to be constants. By correlating the symmetries of the two point groups, the symmetries of the vibrations are changed as follows: a1<sup>g</sup> ! ag , b1<sup>g</sup> ! b2<sup>g</sup> and a2<sup>g</sup> ! bg1. The state tensors for the ag , b1<sup>g</sup> and bg<sup>2</sup> vibrations in D2<sup>h</sup> are also shown in Figure 3. As before, inside the tensors the symmetries which the intermediate states must have in order

(Left) Changes of the state tensors, induced by perturbation of a square planar (D4<sup>h</sup>) molecular configuration into a rectangular planar (D2<sup>h</sup>) configuration. (Right) Most notably is the change of the constant DPR into

j<sup>e</sup>0¼Eu,xi

j<sup>e</sup>0¼Eu, <sup>y</sup>i

yy . The state tensor for the a2<sup>g</sup> vibration is

yx . From the tensor structure, the DPR

yy , while for the b1<sup>g</sup> vibration,

where ve � <sup>v</sup><sup>e</sup> 1, v<sup>e</sup> 2, ve 3…ve <sup>3</sup>N�<sup>6</sup> are the vibrational quantum numbers referring to the excited electronic state and to the electronic ground state, respectively. Lið Þ <sup>e</sup><sup>ν</sup> is a normalized lineshape function for the i th transition j i <sup>g</sup><sup>0</sup> ! ev<sup>e</sup> i � � � with a typical FWHM width of the order of magnitude as a vibrational quantum. A calculation requires that the electronic transition moment is Taylor expanded in the nuclear coordinates. Due to the superposition of the intensity of the individual transitions in Eq. (11), it follows that the vibrational fine structure in UV/VIS absorption spectra of larger molecules is not well resolved as also experienced from experiments. This is different in the RADIS spectra, where the narrow Raman bands enable the excitation profiles to be well separated experimentally (as illustrated in Figure 2). Since each excitation profile only has contributions from a single Raman-active vibration, the vibrational fine structure in the UV/ VIS absorption can be resolved. Thus, the amount of available information about the excited electronic molecular states is much larger in RADIS than in UV/VIS absorption.

It follows that while VRRS is mainly a vibrational spectroscopic technique, RADIS has more in common with electronic spectroscopy. It follows that each Raman-active vibration just plays the role of a "sensor" used to monitor the vibrational fine structure in the UV/VIS absorption spectrum.

Since the late 1980s, a very large amount of systematic resonance Raman studies on different metal complexes [28, 29] and different metal-porphyrins [12] including heme-proteins have been performed with the goal of determining their structure, their bio-functionality, and the conditions for aggregation. In these studies both the VRRS and RADIS including both excitation profiles and polarization dispersion have been applied. Recently polarization-resolved VRRS has been combined with dynamic light scattering to study among other things the aggregation of Arenicola marina extracellular hemoglobin, which is a macromolecule with 144 heme groups instead of four as in human hemoglobin [30]. As already mentioned one great advantage of applying Raman scattering is that the technique can be performed as reflection measurements without much sample preparation. Besides, the Raman signals can be obtained through glass and other sheets of protection. Thus, Raman studies can be performed as in vivo or in situ studies. We refer to the comprehensive Raman literature on these matters for details (e.g., see [31–36]). A complete RADIS experiment may be time-consuming or in some cases impossible to carry out due to the lack of excitation lasers with the proper wave number. It may also be time-consuming to determine the correct intensity variation when the excitation wave number is changed due to changes in the scattering conditions (laser intensity and focus, laser-induced degradation of the molecule, change of the fluorescence, etc.). The application of internal standards and other means have to be introduced in order to ensure the correct experimental conditions. However, in some cases of practical interest, it is in fact possible to extract valuable information without completing a full RADIS experiment (see Sections 4.2 and 4.3).

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