Stokes radiation

This is not the case for stimulated anti-Stokes radiation. There are two sources of polarization for anti-Stokes radiation [17]. The first is analogous to that in figure B1.3.3(b) where the action of the blackbody (- 2) is replaced by the action of a previously produced anti-Stokes wave, with frequency 03. This radiation actually experiences an attenuation since the value of Im x o3 ) is positive (leading to a negative gam coefficient). This is known as the stimulated Raman loss (SRL) spectroscopy [76]. Flowever the second source of anti-Stokes polarization relies on the presence of Stokes radiation [F7]. This anti-Stokes radiation will emerge from the sample in a direction given by the wavevector algebra = 2k - kg. Since the Stokes radiation is... 

In the ordinary Raman effect, few molecules are found in their excited vibrational state. The strong pumping action of a laser beam changes this situation drastically, so that an appreciable fraction of all molecules in the laser beam are soon made available for anti-Stokes emission. Classically, the anti-Stokes radiation is generated by the interaction of the laser beam with molecular vibrations, but the phase of the latter is established by the still more intense Stokes radiation. As a consequence, an index-matching requirement... 

Similarly to the generation of coherent fundamental Stokes and anti-Stokes radiation, higher-order stimulated Stokes and anti-Stokes emission can be produced when high pump intensities are employed. 

The basis of the experimental femtosecond CARS apparatus developed by Okamoto and Yoshihara (1990) which is reproduced in Fig. 3.6-10 is essentially the same as that of Leonhardt et al. (1987) and Zinth et al. (1988) with the addition of the possibility to change the polarization of the laser radiation. The main parts of the system are two dye lasers with short pulses and high repetition rates, pumped by a cw mode-locked Nd YAG laser (1064 nm, repetition rate 81 MHz). The beam of the first dye-laser which produces light pulses with 75-100 fsec duration is divided into two parts of equal intensities and used as the pump and the probe beam. After fixed (for the pump beam) and variable (for the probe beam) optical delay lines, the radiation is focused onto the sample together with the Stokes radiation produced by the second laser (DL2), which is a standard synchronously pumped dye laser. The anti-Stokes signal generated in the sample is separated from the three input laser beams by an aperture, an interference filter, and a monochromator, and detected by a photomultiplier. For further details we refer to Okamoto and Yoshihara (1990). 

This is not the case for stimulated anti-Stokes radiation. There are two sources of polarization for anti-Stokes radiation [H]. The first is analogous to that in figure B1.3.3(bl where the action of the blackbody (-CO2) is replaced by the action of a previously produced anti-Stokes wave, with frequency co. This radiation actually... 

Raman effect - The inelastic scattering of light by a molecule, in which the incident photon either gives up to, or receives energy from, one of the internal vibrational modes of the molecule. The scattered light thus has either a lower frequency (Stokes radiation) or higher frequency (anti-Stokes radiation) than the incident light. These shifts provide a measure of the normal vibrational frequencies of the molecule. 

Vibrational excitations can be created, which causes a decrease in the frequency (i.e., in energy) of the scattered light, or they can be amiihilated, which causes an increase. The decrease in frequency is called Stokes scattering and the increase is anti-Stokes scattering. Stokes scattering is the normal Raman effect and Raman spectroscopy generally uses Stokes radiation. 

Here, Na and Nb are populations in the lower and upper states of the Raman transition, da/dq is the rate of change of polarizability with normal coordinate, no and ns are the refractive indices at o)o and 0)s5 r is the Raman linewidth and y the reduced mass. Eq. (1) describes a Stokes Raman laser producing coherent radiation at 0)3. In this brief and simplified description, we may consider that the strong pump and Stokes waves generate a coherent material excitation at 0)r. This oscillation causes variations in the refractive index which then modulate and scatter the incident laser radiation (o)o) thus producing sidebands or many orders of coherent Stokes and anti-Stokes radiation at frequen-... 

This superelastic photon scattering is called anti-Stokes radiation. 

In case of Stokes radiation the initial state of the molecules may be the vibrational ground state, while for the emission of anti-Stokes lines the molecules must have initial excitation energy. Because of the lower population density in these excited levels, the intensity of the anti-Stokes lines is lower by a factor exp(-hcov/kT). 

While the intensity of anti-Stokes radiation is very small in spontaneous Raman scattering due to the low thermal population density in excited molecular levels (Sect. 3.1), this is not necessarily true in stimulated Raman scattering. Because of the strong incident pump wave, a large fraction of all interacting molecules is excited... 

Fig. 3.13 Generation of stimulated anti-Stokes radiation (a) term diagram illustrating energy conservation (b) vector diagram of momentum conservation for the collinear and noncollinear case (c) radiation cone for different values of k showing red, yellow, and green rings of anti-Stokes radiation excited by a ruby laser at 694 nm...

While the intensity of spontaneous Raman lines is proportional to the incident pump intensity, but lower by several orders of magnitude compared with the pump intensity, the stimulated Stokes or anti-Stokes radiation depend in a nonlinear way on 7p but have intensities comparable to that of the pump wave. 

Fig. 3.16 (a) Level diagram of CARS (b) vector diagrams for phase matching in gases with negligible dispersion and (c) in liquids or solids with noticeable dispersion (d) generation of coherent Stokes radiation at cos = 2 >2 — (joi... 

With CARS the spatial resolution is greatly increased, in particular if BOX CARS is used. The focal volume from which the signal radiation is generated can be made smaller than 0.1 mm [344]. The local density profiles of reaction products formed in fiames or discharges can therefore be accurately probed without disturbing the sample conditions. The intensity of the stimulated anti-Stokes radiation is proportional to (3.3 la-3.3 lb). Figure 3.26 shows for illustration the H2 distribution in a horizontal Bunsen flame, measured from the CARS spectrum of the Q branch in H2. The H2 molecules are formed by the pyrolysis of hydrocarbon molecules [348]. Another example is the measurement of CARS spectra of water vapor in flames, which allowed one to probe the temperature in the postflame region of a premixed CH4 air flame [373]. 

A small molecular sample of 10 molecules in a volume of 5 mm x 1 mm is illuminated by 10 W of argon laser radiation at A. = 488 nm. The Raman cross section is <7 = 10 cm and the Stokes radiation is shifted by 1000 cm Calculate the heat energy dWn/dr generated per second in the sample, if the molecules do not absorb the laser radiation or the Stokes radiation. How much is dlTn/dr increased if the laser wavelength is close to resonance of an absorbing transition, causing an... 

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