Photon Raman-shifted

A Raman spectrum consists of scattered intensity plotted versus energy each peak corresponds to a given Raman shift from the incident light energy. If the molecule happens to be in an excited vibrational state when an incident photon is scattered,... 

The complex quantity, y6br = e (y(3)r) + i Im (x r), represents the nuclear response of the molecules. The induced polarization is resonantly enhanced when the Raman shift wp — ws matches the frequency Qr of a Raman-active molecular vibration (Fig. 6.1A). Therefore, y(3)r provides the intrinsic vibrational contrast mechanism in CRS-based microscopies. The nonresonant term y6bnr represents the electronic response of both the one-photon and the two-photon electronic transitions [30]. Typically, near-infrared laser pulses are used to prevent the effect of two-photon electronic resonances. With input laser pulse frequencies away from electronic resonances, y(3)nr is independent of frequency and is a real quantity. It is important to realize that the nonresonant contribution to the total nonlinear polarization is simply a source for an unspecific background signal, which provides no chemical contrast in some of the CRS microscopies. While CARS detection can be significantly effected by the nonresonant contribution y6bnr [30], SRS detection is inherently insensitive to it [27, 29]. As will be discussed in detail in Sects. 6.3 and 6.4, this has major consequences for the image contrast mechanism of CARS and SRS microscopy, respectively. 

Thus, in the Raman spectra, it is a frequency shift, a Av, that is observed. The value of such shifts in the frequency of nonelasticaUy scattered light is not dependent on the exciting frequency, but on the structure of the molecule with which the photon interacts. Thus, the Raman shifts reflect vibrational transitions, as do the IR spectra. 

Intensity of melastically (Raman) scattered photons (i) as a function of the wavenumber separation (Raman shift) from the elastically (Rayleigh) scattered photons and (ii) as a ftmction of the exciting radiation wavelengths (three-dimensional spectrum). 

The absorption of IR light by a vibrating molecule follows dipolar selection rules. This means that, to interact with the electric field of an incident photon so as to excite a normal mode of a molecule, absorb the photon, and be observed in the IR spectrum, a given normal mode must distort the molecule such that it alters the molecule s dipole moment. On the other hand Raman scattering, which is a two-photon process (the two being the incident and scattered photons), follows quadrupolar selection rules. This means that, to interact with an incident photon so as to result in the excitation of the normal mode of a molecule and the scattering of a Raman-shifted photon, the mode must distort the molecule such that it alters the molecule s polarizability. 

The essential point here is that the signal magnitude for a given Raman shift value, as plotted in the spectrum after Fourier transformation, is proportional to the photons observed for that Raman shift only, while the noise has contributions from all Raman shift values. If we define (f>s as the rate of electron electron generation from photons reaching the detector, averaged over all Nr resolution elements ... 

Figure 5.1. Schematic of the operation of a dispersive multichannel Raman spectrometer. Each detector element detects photons of a different Raman shift, and the spectrum is read out directly in terms of intensity (number of photons) vs. detector position (Raman shift).

Figure 5.2. Schematic of a nondispersive, FT-Raman spectrometer. A single detector monitors photons with all Raman shifts, after each has been modulated by a multiplexer such as an interferometer. Raman spectrum is obtained by Fourier transformation of the detector output (interferogram).

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