Coherent anti-Stokes Raman spectroscopy CARS

2 Coherent anti-Stokes Raman spectroscopy (CARS) 

Section 6.1.4.2.1 is contributed by W. Kiefer, Wurzburg and A. Leipertz, Erlangen 

For the case that there are not too many constituents in the gas under investigation the use of the pure rotational CARS technique (Zheng et al., 1984 Alden et al., 1986) may be superior to vibrational CARS thermometry since the spectra are easily resovable (for N2 the adjacent rotational peaks have a spacing of appr. 8 cm ) compared with the 

In addition to temperature measurements, the gas-phase CARS technique also provides information on the fluctuating properties occuring for instance in turbulent combustion systems. However, concentration measurements are more difficult to perform than temperature ones because the absolute intensity is required, while temperature measurements are only based on the shape of the spectrum. Simultaneous information on the relative concentrations between several species are easier to obtain. For such investigations the technique called dual broadband CARS has been introduced by Eckbreth and Anderson (1985) which allowed the researchers to simultaneously generate CARS spectra of CO2, N2 and H2O in the postflame zone of a premixed C2H4-air flame. 

Besides the initial object of CARS as a tool for the determination of molecular energy levels and its application to combustion studies and related areas, there are several other scientific applications of academic interests. These include spectral lineshape studies, collisional effects, Dicke narrowing, nonadditivity effects, and motional narrowing. CARS turned out to be an excellent method for these studies (Berger et al., 1992 and references therein). 

3 Coherent Anti-Stokes Raman Spectroscopy (CARS) 

In the previous section we discussed the fact that a sufficiently strong incident pump wave at a frequency w can generate an intense Stokes wave at w = wj - wy. Under the combined action of both waves a nonlinear polariza-tion P of the medium is generated which contains contributions at the frequencies wy = w - w, wy, and These contributions 

Despite the enormous intensities of stimulated Stokes and anti-Stokes 

If the incident waves are at optical frequencies, the difference frequency (Dp = (1)2 - (D2 is small compared with (d in case of rotational-vibrational frequencies (Dp. In gaseous Raman samples the dispersion is generally negligible over small ranges A(d = (d - (D2 and satisfactory phase matching is obtained for oolUnear beams. The Stokes wave at (d = 2u 2 anti-Stokes 

1) The signal levels in CARS may exceed that obtained in spontaneous Raman 

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COHERENT ANTI-STOKES RAMAN SPECTROSCOPY (CARS)... 

Coherent anti-Stokes Raman spectroscopy (CARS) [59] has also found utility in the detemiination of the internal state distributions of products of chemical reactions. This is one of several coherent Raman spectroscopies based on the... 

An interesting variation of Raman spectroscopy is coherent anti-Stokes Raman spectroscopy (CARS) (99). If two laser beams, with angular frequencies CO and CO2 are combined in a material, and if cjj — is close to a Raman active frequency of the material, then radiation at a new frequency CJ3 = 2cJ2 — may be produced. Detection of this radiation can be used to characterize the material. Often one input frequency is fixed and the other frequency, from a tunable laser, varied until matches the Raman frequency. CARS has the capabiHty for measurements in flames, plasmas, and... 

Several types of time-resolved Raman spectroscopies have been reported and reviewed by Hamaguchi and co-workers and Hamaguchi and Gustafson. These include pump-probe spontaneous and time-resolved coherent Raman spectroscopy of the anti-Stokes and Stokes varieties [coherent anti-Stokes Raman spectroscopy (CARS) and coherent Stokes Raman spectroscopy (CSRS)], respectively). Here we will focus on pump-probe time-resolved spontaneous Raman spectroscopy. 

Two techniques, which appear well suited to the diagnostic probing of practical flames with good spatial and temporal resolution, are coherent anti-Stokes Raman spectroscopy (CARS) and saturated laser fluorescence. The two techniques are complementary in regard to their measurement capabilities. CARS appears most appropriate for thermometry and major species concentration measurements, saturated laser fluorescence to trace radical concentrations. With electronic resonant enhancement (6), CARS may be potentially useful for the latter as well. Fluorescence thermometry is also possible (7, 8) but generally, is more tedious to use than CARS. In this paper, recent research investi-... 

Eckbreth, A. C. Hall, R. J. and Shirley, J. A. "Investigations of Coherent Anti-Stokes Raman Spectroscopy (CARS) for Combustion Diagnostics," Paper 79-0083, Presented at the 17th AIAA Conference on Aerospace Sciences, New Orleans, LA, June 15-17, 1979. 

XN.R., the non-resonant susceptibility, gives rise to the background interference in Coherent Anti-Stokes Raman Spectroscopy (CARS) (J5). This interference which arises from solvents or closely spaced lines is responsible for the CARS band shape distortion observed under certain conditions. 

Other methods include tip-enhanced Raman using 20- to 30-nm diameter Au or Ag tips, polarized Raman, stimulated Raman, micro-Raman spectroscopy, and coherent anti-Stokes Raman spectroscopy (CARS), where two laser beams are combined to generate an anti-Stokes beam, and so on. 

Resonance-enhanced coherent anti-Stokes Raman spectroscopy (CARS) has proven to be a useful technique for investigating the molecular structures of transient species.33 In nonpolar and polar solvents, CARS spectra of spirooxazine derivatives indicated the existence of two similar isomeric species. 

However, the total quantum efficiency for O3 destruction following photolysis is not observed to depend upon the wavelength of actinic radiation in this region (Castellano and Schumacher, 1962). Rather, the value of O-Og is measured as 2 both above and below the 611 nm threshold. In addition, lovely observations of the O2 product of O3 photolysis in the Chappuis bands using Coherent Anti-Stokes Raman Spectroscopy (CARS) have confirmed (Valentini, 1983) that it is only 02(X S g) which is formed in... 

Coherent anti-Stokes Raman spectroscopy (CARS) Unlike spontaneous Raman emission, the magnitude of signal in CARS is number of orders stronger as multiple photons are employed to address the molecular vibrations, resulting in production of a signal in which the emitted waves are coherent with one... 

Coherent Raman spectroscopy Coherent Raman spectroscopy is a term that refers to a series of closely related nonlinear Raman techniques in which the scattered Raman radiation emerges from the sample as a coherent beam -coherent meaning that the photons are all in phase with one another. The coherent techniques include Stimulated Raman Scattering (SRS), Coherent anti-Stokes Raman Spectroscopy (CARS), Coharent Stokes Raman Spectroscopy (CSRS), and Stimulated Raman Gain Spectroscopy (SRGS). Although most of the nonlinear Raman techniques are also coherent techniques, there is one incoherent nonlinear Raman process called Hyper Raman. 

The designs of the previously mentioned selectivity schemes ignore the possibility of control of the evolution of excitation energy via exploitation of the coherence properties of the coupled matter-electromagnetic field system. Several schemes that do exploit the coherence of the time evolution of a wavepacket excitation have recently been proposed. This chapter is concerned with one of these schemes, namely, the use of coherent pulse sequences to control product formation in chemical reactions. We shall see that this scheme follows naturally from an understanding of the characteristics of time-delayed coherent anti-Stokes Raman spectroscopy (CARS) and of photon echo spectroscopy. 

Another noncontact technique for measuring high temperatures involves Raman spectroscopy, in particular the nonlinear process known as coherent anti-Stokes Raman spectroscopy (CARS) (Radiation Thermometry, 1982). This technique is finding practical applications in measurements of temperatures of flames (in internal combustion engines, in jet engines) and of hot gases. The imprecision of such temperature measurements is generally a few percent. 

New possibility in studies of biphonons and Fermi resonance has appeared due to the development of coherent anti-Stokes Raman spectroscopy (CARS). As was shown by Flytzanis and his group (68)-(70) this technique for noncen-... 

Experiments have also been started that use the inelastic light scattering and include the methods of coherent anti-Stokes Raman spectroscopy (CARS), as well as electron energy loss spectroscopy (EELS). The methods (see, for instance, (4)(5)) are based on the application of various physical processes, as can be seen from their names. Accordingly they complement one another and enable us to study the elementary excitations of a surface over a wide range of energies and wave vectors. 

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