In situ aerosol absorption spectroscopy was performed using two novel photothermal detection schemes. The first, based on a photorefractive effect and coherent detection, called 'phase fluctuation optical hetero-dyne'(PFLOH) spectroscopy, could, depending on the geometry employed, yield particle specific or particle and gas absorption data. Single particles of graphite as small as 1 μm were detected in the particle specific mode. In another geometrical configuration, the total absorption (both gas and particle) of submicron sized aerosols of ammonium sulfate particles in equilibrium with gaseous ammonia and water vapor were mea-sured at varying CO2 laser frequencies. The specific absorption coefficient for the sulfate ion was measured to be 1 be 0.5 m2/g at 1087 cm -1. The absorption coefficient sensitivity of this scheme was less than or equal to 10 cm -1. The second scheme is a hybrid visible Mie scattering scheme incorporating photothermal modulation. Particle specific data on ammonium sulfate droplets were obtained. For chemically identical species, the relative absorption spectrum versus laser frequency can be obtained for polydisperse aerosol distributions directly from the data without the need for complex inverse scattering calculations.
Anthony J. Campillo,
"Photothermal Spectroscopy Of Aerosols", Proc. SPIE 0286, Laser Spectroscopy for Sensitive Detection, (8 September 1981); doi: 10.1117/12.965811; https://doi.org/10.1117/12.965811