Fuel vapor measurements by linear Raman spectroscopy using spectral discrimination from droplet interferences.

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Abstract
Vapor-phase measurements by linear Raman spectroscopy are performed in the vicinity of methanol droplets. Several types of interference by these droplets are identified and removed by appropriate filtering. This procedure, together with the phase-dependent spectral shift of the OH stretching vibration frequency, is proved to permit single-pulse linear Raman measurements of methanol vapor and nitrogen on a line with coexisting droplets. Laser-induced droplet breakdown is found to limit the applicable laser irradiance to approximately 2 GW/cm2 and is avoided by use of a flash-lamp-pumped dye laser with high energy (1–7 J) and long pulses (1.5 µs).