Coherent anti-Stokes Raman spectroscopy

Despite the enormous intensities of stimulated Stokes and anti-Stokes waves, stimulated Raman spectroscopy has been of little use in molecular spectroscopy. The high threshold, which, according to (3.30), depends on the molecular density N, the incident intensity I a and the square of the small polarizability term (daij/dq) in (3.27), limits stimulated emission to only the strongest Raman lines in materials of high densities N. 

Because of the nonlinear interaction discussed in Sect. 3.3.1, new Stokes and anti-Stokes waves are generated (Fig. 3.16d). The waves co and C02 produce a large population density of vibrationally excited molecules by stimulated Raman scattering. These excited molecules act as the nonlinear medium for the generation of 

In a similar way, a Stokes wave with frequency cOg = 2co2 — co i is generated by the incident waves at coi and C02 (Fig- 3.16d). Since four waves are involved in the generation of the anti-Stokes wave, CARS is called 3. four-wave parametric mixing process (Fig. 3.17). 

It can be derived from (3.33) that the power S of the CARS signal (which is proportional to the square of the amplitude Ef) 

The most common pump system for pulsed CARS experiments are two dye lasers pumped by the same pump laser (N2 laser, excimer laser, or frequency-doubled Nd YAG laser). This system is very flexible because both frequencies co and C02 

In the collinear arrangement the anti-Stokes wave at cua = 2oo — 002 (cua 0 is detected through filters that reject both incident laser beams as 

Experimental setup of an early CARS experiment in gases using a ruby laser and a dye laser pumped by the ruby laser [8.57] 

The most common pump system for pulsed CARS experiments are two dye lasers pumped by the same pump laser (N2 laser, excimer laser or Nd YAG laser). This system is very flexible because both frequencies Wj and W2 can be varied over large spectral ranges. Since both the frequency and intensity flucutations of the dye lasers result in strong intensity fluctuations of the CARS signal, the stability of the dye lasers needs particular attention. With compact and stable systems the signal fluctuations can be reduced below 10% [8.52]. 

Meanwhile many CARS experiments have been performed with CW dye lasers. With liquid samples as well as with gaseous ones. An experimental setup for CW CARS of liquid nitrogen is shown in Fig.8.13 where the two incident, collinear pump waves are provided be the 514.5 nm argon laser line (wj) and a CW dye laser ( 2) pumped by the same argon laser [8.51]. 

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

Our first example of aP - signal is coherent anti-Stokes Raman spectroscopy, or CARS. Fomially, tire emission signal into direction k= - k + k. has 48 Feynman diagrams that contribute. Flowever, if the... 

Knopp G, Pinkas I and Prior Y 2000 Two-dimensional time-delayed coherent anti-Stokes Raman spectroscopy and wavepacket dynamics of high ground-state vibrations J. Raman Spectrosc. 31 51... 

Oudar J-L, Smith R W and Shen Y R 1979 Polarization-sensitive coherent anti-Stokes Raman spectroscopy Appi. Rhys. Lett. 34 758-60... 

Joe T and Albrecht A C 1993 Femtosecond time-resolved coherent anti-Stokes Raman spectroscopy of liquid benzene a Kubo relaxation function analysis J. Chem. Phys. 99 3244-51... 

Gross L P, Trump D D, MacDonald B G and Switzer G L 1983 10-Hz coherent anti-Stokes Raman spectroscopy apparatus for turbulent combustion studies Rev. Sc/. Instrum. 54 563-71... 

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... 

CARS. See Coherent anti-Stokes Raman spectroscopy. 

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... 

Koszykowski M. L., Farrow R. L., Palmer R. E. Calculation of collisionally narrowed coherent anti-Stokes Raman spectroscopy spectra, Opt. Lett. 10, 478-80 (1985). 

Early picosecond studies were carried out by Schneider et al, [63] on the parent spiro-oxazine (NOSH in Scheme 8) and similar derivatives. In a back-to-back work, they also described a complimentary CARS (coherent anti-Stokes Raman spectroscopy) investigation [69], Simply put, these authors found that the closed spiro-oxazine ring opened in 2-12 psec after laser excitation. The reaction was slower in more viscous solvents. An intermediate state formed within the excitation pulse and preceded the formation of merocyanine forms. This transient was named X in deference to the X transient named by Heiligman-Rim et al. for the spiropyran primary photoproduct [8], (See also the previous section.) The name X has since been adopted by other workers for the spiro-oxazines [26,65],... 

Dudovich, N., Oron, D., and Silberberg, Y. 2003. Single-pulse coherent anti-Stokes Raman spectroscopy in the fingerprint spectral region. J. Chem. Phys. 118 9208-15. 

Petrov, G. 1., Yakovlev, V. V., Sokolov, A., and Scully, M. O. 2005. Detection of Bacillus subti-lis spores in water by means of broadband coherent anti-Stokes Raman spectroscopy. Opt. Express 13 9537 2. 

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. 

As I have indicated, this presentation will be divided into two parts. In the first part we will discuss the development of Coherent Anti-Stokes Raman Spectroscopy, the problems inherent in applications to combustion sources, recent developments which address operational problems, and the state-of-the-art today. This will be followed by a similar discussion involving the use of saturated laser-induced fluorescence spectroscopy as a combustion diagnostic. 

The inelastic processes - spontaneous Raman scattering (usually simply called Raman scattering), nonlinear Raman processes, and fluorescence - permit determination of species densities as well as temperature, and also allow one, in principle, to determine the temperature for particular species whether or not in thermal equilibrium. In Table II, we categorize these inelastic processes by the type of the information that they yield, and indicate the types of combustion sources that can be probed as well as an estimate of the status of the method. The work that we concentrate upon here is that indicated in these first two categories, viz., temperature and major species densities determined from vibrational Raman scattering data. The other methods - fluorescence and nonlinear processes such as coherent anti-Stokes Raman spectroscopy - are discussed in detail elsewhere (5). 

Table II Space- and Time-Resolved Measurements from Inelastic Light Scattering. All methods are suitable for nonequilibrium conditions. Here, RS refers to Raman scattering, CARS to coherent anti-Stokes Raman spectroscopy, and RIKES to Raman-induced Kerr effect.

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. 

Druet, S., Taran, J.P.E., "Coherent anti-Stokes Raman Spectroscopy" in Chemical and Biochemical Applications of Lasers, ed. by C.B. Moore, Academic Press, 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. 

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