Linear Raman spectroscopy on aqueous aerosols: Influence of nonlinear effects on detection limits

The influence of stimulated Raman scattering on the characterization of aqueous aerosols by linear Raman spectroscopy was studied experimentally. Raman scattering from a chain of microdroplets was excited by an argon ion laser. Holographic Raman filters, a spectrograph and a cryogenically cooled CCD detector were used to record the spectra. A procedure to estimate the detection limits of Raman instrumentation is presented. For the set-up used here, the detection limit for nitrogen in a gas mixture was 3 ppm. The detection limit fbr aqueous aerosol particles was limited to 100 mg m(-3) by the occurrence of stimulated Raman scattering (SRS). The first measurement of SRS with continuous-wave excitation in water droplets is reported. The SRS thresholds for water droplets in the size range of 15-75 mu m were 0.3 to 1 MW cm(-2). For droplets, which consist of a 5 M NaNO3 solution, the threshold was 0.2 MW cm(-2). Continuous-wave excited four-wave mixing in microdroplets of NaNO3 solution is reported. (C) 1998 Elsevier Science Ltd. All rights reserved.