Abstract

Heterogeneous reactions between cloud droplets and the gas phase are important in atmospheric chemistry. Consequently, there has been intense interest in developing diagnostics to measure physical and chemical aspects of droplets, such as differential evaporation rates and component concentration gradients. The realisation that microdroplets behave as optical resonators has led to consideration of droplet assisted techniques[ 1,2], Feedback is provided by whispering gallery waves that totally internally reflect at the droplet-air interface. Such modes are often called morphology dependent resonances (MDRs) and depending on their mode order, have distributions at varying depths below the liquid surface. In the present work, we employ a double cavity resonance MDR assisted spontaneous Raman process, called cavity enhanced Raman spectroscopy[3] (CERS). The CERS signal exhibited several orders of magnitude improved sensitivity over existing Raman probes including MDR assisted stimulated Raman scattering[2]. As little as 1 mM of nitrate in water was detected in effective MDR volumes of only 0.03 to 0.3 picoliters. CERS signals were linear with pump and differential Raman cross section and allowed multi-component mixtures with widely disparate concentrations to be measured.

© 1996 Optical Society of America