Hyper Raman spectroscopy

Technological advances, i. e. cw pumped acousto-optically Q-switched Nd YAG lasers with repetition rates of up to 5 kHz combined with multichannel detection systems have increased the ease of obtaining hyper Raman signals. By making use of this advanced technology, hyper Raman spectra of benzene and pyridine could be obtained by Ned-dersen et al. (1989). Spectra from benzene, deuterated benzene and carbon tetrachloride have been obtained with high signal-to-noise ratios by Acker et al. (1989). As example, we show in Fig. 6.1-17 the hyper-Raman spectra of benzene and deuterated benzene. 

Ziegler et al. (1989, 1990) have recently reviewed hyper Raman. spectroscopic studies including hyper Raman scattering in liquids and crystals, surface enhanced hyper Raman scattering (Golab et al., 1988) as well as the vibronic and rotational theory for resonance enhanced hyper Raman scattering. 

We have seen in Equation (9.11) how fhe dipole momenf induced in a maferial by radiation falling on if confains a small confribution which is proportional fo fhe square of fhe 

Hyper Raman scattering is at a wavenumber 2vq v r, where Vq is the wavenumber of the exciting radiation and —v r and +V[jr are the Stokes and anti-Stokes hyper Raman displacements, respectively. The hyper Raman scattering is well separated from the Raman scattering, which is centred on Vq, but is extremely weak, even with a 0-switched laser. 

The selection mles for molecular vibrations involved in hyper Raman scattering are summarized by 

The hyperpolarizability is a tensor with eighteen elements We shall not go further into 

Vibrations allowed in the infrared are also allowed in the hyper Raman effect. 

The hyperpolarizability is a tensor with eighteen elements Pijk. We shall not go further into their symmetry properties but important results of Equation (9.17) include  

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Ziegler L D 1990 Hyper-Raman spectroscopy J. Raman Spectrosc. 71 769-79... 

Similarly, the first-order expansion of the p° and a of Eq. (5.1) is, respectively, responsible for IR absorption and Raman scattering. According to the parity, one can easily understand that selection mles for hyper-Raman scattering are rather similar to those for IR [17,18]. Moreover, some of the silent modes, which are IR- and Raman-inactive vibrational modes, can be allowed in hyper-Raman scattering because of the nonlinearity. Incidentally, hyper-Raman-active modes and Raman-active modes are mutually exclusive in centrosymmetric molecules. Similar to Raman spectroscopy, hyper-Raman spectroscopy is feasible by visible excitation. Therefore, hyper-Raman spectroscopy can, in principle, be used as an alternative for IR spectroscopy, especially in IR-opaque media such as an aqueous solution [103]. Moreover, its spatial resolution, caused by the diffraction limit, is expected to be much better than IR microscopy. 

Hyper-Raman spectroscopy is not a surface-specific technique while SFG vibrational spectroscopy can selectively probe surfaces and interfaces, although both methods are based on the second-order nonlinear process. The vibrational SFG is a combination process of IR absorption and Raman scattering and, hence, only accessible to IR/Raman-active modes, which appear only in non-centrosymmetric molecules. Conversely, the hyper-Raman process does not require such broken centrosymmetry. Energy diagrams for IR, Raman, hyper-Raman, and vibrational SFG processes are summarized in Figure 5.17. 

Ina similarmarmerto surface-enhanced Raman scattering, surface-enhancement of hyper-Raman scattering is a promising method to study adsorbed molecules on metal surfaces [24]. Based on recent developments in plasmonics, design and fabrication of metal substrates with high enhancement activities is now becoming possible [21]. Combination of the surface enhancement with the electronic resonances would also be helpful for the practical use of hyper-Raman spectroscopy. Development of enhanced hyper-Raman spectroscopy is awaited for the study of solid/liquid interfaces. 

Leng, W., Woo, H. Y, Vak, D., Bazan, G. C. and Kelly, A. M. (2006) Surface-enhanced resonance Raman and hyper-Raman spectroscopy of water-soluble substituted stilbene and distyrylbenzene chromophores. J. Raman Spectrosc., 37, 132-141. 

The importance of the hyper Raman effect as a spectroscopic tool results from its symmetry selection rules. These are determined by products of three dipole moment matrix elements relating the four levels indicated in Fig. 3.6-1. It turns out that all infrared active modes of the scattering system are also hyper-Raman active. In addition, the hyper Raman effect allows the observation of silent modes, which are accessible neither by infrared nor by linear Raman spectroscopy. Hyper Raman spectra have been observed for the gaseous, liquid and solid state. A full description of theory and practice of hyper-Raman spectroscopy is given by Long (1977, 1982). 

Surface Enhanced Hyper-Raman Spectroscopy (SEHRS)... 

In a study of phenazine adsorbed on a silver electrode that employed both SERS and SEHRS, the electroreduction product of phenazine and the reduction intermediates could be identified [499]. In a comparative study with SERRS and resonantly enhanced hyper-Raman spectroscopy SERHRS, several dyes adsorbed on a roughened silver electrode were investigated [500]. According to the results, the efficiency... 

SEHRS Surface enhanced hyper-Raman spectroscopy scattering... 

SERHRS Surface enhanced resonance hyper-Raman spectroscopy... 

Surface Enhanced Hyper-Raman Spectroscopy (SEHRS) Hyper-Raman scattering is a nonlinear three-photon energy conversion process, that offers complementary informahon to Raman spectroscopy and has some advantages... 

The main interest in hyper-Raman spectroscopy is that certain vibrational modes that are inactive in both the infrared and in conventional Raman can be active in the hyper-Raman. A good example is thetorsional mode of ethylene. Unfortunately, hyper-Raman scattering is many orders of magnitude less intense than conventional Raman, so measurements take a long time and sensitivity can be low. Gas-phase studies are therefore unfavourable, and only a few gases have been studied so far, giving just vibrational spectra at modest resolution. 

The advantages of these nonlinear Raman techniques are the greatly increased signal-to-noise ratio and thus the enhanced sensitivity, the higher spectral and spatial resolution, and in the case of the hyper-Raman spectroscopy, the possibility of measuring higher-order contributions of molecules in the gaseous, liquid, or solid state to the susceptibility. 

S. Nie, L.A. Lipscomb, N.T. Yu, Surface-enhanced hyper-Raman spectroscopy. Appl. Spec-trosc. Rev. 26, 203 (1991)... 

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