Stokes laser 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... 

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],... 

In the case of electron transfer reactions, besides data on the dynamic Stokes shift and ultrafast laser spectroscopy, data on the dielectric dispersion (w) of the solvent can provide invaluable supplementary information. In the case of other reactions, such as isomerizations, it appears that the analogous data, for example, on a solvent viscosity frequency dependence 17 ( ), or on a dynamic Stokes fluorescence shift may presently be absent. Its absence probably provides one main source of the differences in opinion [5, 40-43] on solvent dynamics treatments of isomerization. 

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. 

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-... 

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. 

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. 

Eckbreth, A. G., and Schreiber, P. W. "Coherent Anti-Stokes Raman Spectroscopy (CARS) Application to Combustion and Gas-Phase Diagnostics." Chemical Applications of Nonlinear Raman Spectroscopy in Laser Applications, Vol. 1. New York Academic Press, 1981. 

Greenhalgh, D. A. "Laser Diagnostics of Combustion Devices and Chemical Reactors Using Coherent Anti-Stokes Raman Spectroscopy." 183-94. From the Proceedings of the 2nd International Conference on Lasers in Manufacturing, March 26-28,1985. 

Kataoka, H., Maeda, S., and Hirose, C. "Effects of Laser Line Width on the Coherent Anti-Stokes CARS Spectroscopy Rrofiles." Applied Spectroscopy 36 (1982) 565. 

Barton, S., and Garneau, J. M. "Effect of Pump-Laser Linewidth on Noise in Single-Pulse Coherent Anti-Stokes Raman Spectroscopy Temperature Measurements." Optics Letters 12, no. 7 (1987) 486. 

Rahn, L. A., Farrow, R. L, and Lucht, R. P. "Effect of Laser Field Statistics on Coherent Anti-Stokes Raman Spectroscopy Intensities." Optics Letters 9 (1984) 223. 

Michele Marrocco, PhD, is a researcher in laser spectroscopy at ENEA (Rome, Italy) (1999 to present). He received his degree in physics from the University of Rome in 1994. He was employed as a postdoctorate at the Max-Planck Institute for Quantum Optics (Munich, Germany), as a researcher at the Quantum Optics Labs at the University of Rome (Rome, Italy), and as an optics researcher by the army. His research activities include traditional and innovative spectroscopic techniques for diagnosis of combustion and nanoscopic systems studied by means of optical microscopy. The techniques used include adsorption, laser induced fluorescence, spontaneous Raman, stimulated Raman gain, stimulated Raman loss, coherent anti-Stokes Raman, degenerate four wave mixing, polarization spectroscopy, laser induced breakdown, laser induced incandescence, laser induced thermal gratings. He has over 30 technical publications. 

Figure B2.3.8. Energy-level schemes describing various optical methods for state-selectively detecting chemical reaction products left-hand side, laser-induced fluorescence (LIF) centre, resonance-enhanced multiphoton ionization (REMPI) and right-hand side, coherent anti-Stokes Raman spectroscopy (CARS). The ionization continuum is denoted by a shaded area. The dashed lines indicate virtual electronic states. Straight arrows indicate coherent radiation, while a wavy arrow denotes spontaneous emission.

Raman spectroscopy, which concerns the spectral analysis of radiation scattered by an atom or molecule. Recent developments are coherent anti-Stokes Raman spectroscopy (CARS) and surface enhanced Raman spectroscopy (SERS), both of which are sensitive laser-based techniques, and examples of laser spectroscopy. 

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