Inverse Raman scattering

Inverse Raman scattering Inverse Raman scattering (IRS) is a coherent process involving stimulated loss at an anti-Stokes-shifted frequency. The term inverse Raman refers to the fact that, at resonance, the probe radiation is attenuated. In spontaneous Raman spectroscopy, on the other hand radiation at Raman-active frequencies would he generated in the course of the experiment. Inverse Raman scattering (IRS) and stimulated Raman gain (SRG) are closely related. While one involves stimulated gain at an anti-Stokes-shifted frequency, the other involves stimulated loss at a Stokes-shifted frequency. 

The high power densities available from lasers were found to be capable of inducing a number of strange (nonlinear) Raman effects, though many were of limited applicability to problems in chemistry. Typical examples included SIRS (stimulated inverse Raman scattering), RIKES (Raman induced Kerr effect spectroscopy) and the hyper Raman effect. 

Cable J R and Albrecht A C 1986 The inverse transform in resonance Raman scattering Conf. sponsored by the University of Oregon ed W L Peticolas and B Hudson... 

Cable J R and Albrecht A C 1986 A direct inverse transform for resonance Raman scattering J. Chem. Phys. 84 4745-54... 

Joo T and Albrecht A C 1993 Inverse transform in resonance Raman scattering an iterative approach J. Phys. Chem. 97 1262-4... 

Using Raman scattering, Steigmeier of RCA Zurich has found in one of our samples a weak structure at 1875 cm-1. This finding, which implies a lack of inversion symmetry, is consistent with our Si—H model. 

High-power pulsed lasers offer the possibility of studying nonlinear phenomena such as stimulated Raman scattering, the inverse Raman effect and the hyper-Raman effect. These investigations have contributed much to our knowledge of the solid-state and liquid stucture of matter and its higher order constants. 

When the power of the exciting radiation is raised into the megawatt range, nonlinear Raman effects are observed, namely the stimulated Raman effect, the inverse Raman effect (Stoicheff absorption), and the hyper-Raman effect. The results of such experiments with single crystals will be discussed in the last chapter, with special emphasis on stimulated Raman scattering from polaritons. 

In resonance Raman scattering (ga 0), it is possible to have pp >. For example, if ctxy — —ctyx and the remaining off-diagonal elements are zero, g° = gs = 0 and ga 0. Then, (1-49) gives pp —> oo. This is called anomalous (or inverse) polarization (abbreviated as ap or ip). As will be shown in Section 1.15, resonance Raman spectra of metallopophyrins exhibit polarized (Aig) and depolarized (B g and B ) vibrations as well as those of anomalous (or inverse) polarization () ... 

Here, E is the strength of the applied electric field (laser beam), a the polarizability and / and y the first and second hyper-polarizabilities, respectively. In the case of conventional Raman spectroscopy with CW lasers (E, 104 V cm-1), the contributions of the / and y terms to P are insignificant since a fi y. Their contributions become significant, however, when the sample is irradiated with extremely strong laser pulses ( 109 V cm-1) created by Q-switched ruby or Nd-YAG lasers (10-100 MW peak power). These giant pulses lead to novel spectroscopic phenomena such as the hyper-Raman effect, stimulated Raman effect, inverse Raman effect, coherent anti-Stokes Raman scattering (CARS), and photoacoustic Raman spectroscopy (PARS). Figure 3-40 shows transition schemes involved in each type of nonlinear Raman spectroscopy. (See Refs. 104-110.)... 

Figure 4.6.1 depicts another major difference between Raman scattering and infrared processes. To be active in the infrared spectra, transitions must have a change in the molecular dipole associated with them. For Raman activity, in contrast, the change has to be in the polarizability of the molecule. These two molecular characteristics are qualitatively inversely related. 

Up to now/ the dimer laser system has been described alone in terms of population inversion between suitable energy levels/ and for this description the condition S2 > A 2 is indeed the only necessary condition for cw laser oscillation/ as long as the thermal population density in the lower laser level remains negligibly low. However/ as this optically pumped laser system is a coherently excited three level system/ the coherent emission can also be described as stimulated Raman scattering/ which is resonantly enhanced by the common level 3 of the pump and laser transitions. This coupled two photon or Raman process does not require a population inversion between levels 3 and 2 and introduces qualitatively new aspects which appreciably influence and change the normal laser behaviour. For a detailed and deeper description of the coherently excited three level dimer... 

The methods of nonlinear Raman spectroscopy, i. e. spontaneous hyper Raman scattering (based on the hyperpolarizability) and coherent nonlinear Raman scattering (based on the third-order-nonlinear susceptibilities) are discussed in detail in Sec. 3.6.1. In Sec. 3.6.2 the instrumentation needed for these types of nonlinear spectroscopy is described. In this section we present some selected, typical examples of hyper Raman scattering (Sec. 6.1.4.1), coherent anti-Stokes Raman. scattering (Sec. 6.1.4.2), stimulated Raman gain and inverse Raman spectroscopy (Sec. 6.1.4.3), photoacoustic Raman spectroscopy (Sec. 6.1.4.4) and ionization detected stimulated Raman spectroscopy (Sec. 6.1.4.5). 

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