A simple model is presented as an aid in understanding, first, the relative noise performance and, second, the noise reduction achievable by referencing, in different experimental approaches to single shot broadband coherent anti-Stokes Raman scattering (CARS). Qualitative agreement is obtained with previous experimental investigations of CARS noise. The broadband dye laser radiation is described as the sum of independent modes with random phases. The dye laser contribution to the CARS noise is then approximately inversely proportional to the square root of the number of dye laser modes generating the detected signal. A fundamental idea is that in Raman resonant spectra only the number of Stokes modes actually participating in driving the Raman resonance should be counted. This means, e.g., that for narrow Raman resonances, as in an atmospheric flame, the noise generated by the dye laser will be higher for a single-mode pump laser than for a multimode pump laser with the experimental CARS configuration normally employed. The implications of the model for the dual broadband type CARS techniques are also discussed.
Articles you may be interested inLaser microbe sampling and laser-enhanced ionization spectrometry in flames for surface analysis AIP Conf. Proc. 329, 105 (1995) An improved theory for laser enhanced ionization in flames has been developed for one-and twostep laser excitations. The model gives an analytical expression for the sensitivity of the method for a given transition of any element. The theoretical expression is compared with experimentally measured signals for a number of elements and the agreement is found to be generally good.
A Laser Enhanced Ionization (LEI) detector has been connected on-line with a high pressure liquid chromatography (HPLC) system equipped with a UV absorbance detector. The feasibility of this combined detector system has been demonstrated for metal analysis in the HPLC separated compounds. In the situation where the compounds are not separated in the HPLC column the LEI detector can be used for selective detection of the compounds containing the metal. The detection limits of the system for metal determination were in these experiments approximately 1 ng for Fe, but can easily be improved orders of magnitude.
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