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Published June 1, 1987 | Published
Journal Article Open

Fourier transform infrared spectroscopy of a single aerosol particle


A method is developed for obtaining the molecular composition of a single suspended microparticle by Fourier transform infrared spectroscopy. The particle is held in an electrodynamic balance and irradiated simultaneously by the infrared output from a Michelson interferometer and the visible light from a dye laser. The laser is tuned to an edge of an optical resonance of the particle while the IR beam is chopped. Through evaporation and condensation the chopped IR beam causes a size modulation of the droplet, which in turn induces a fluctuation in the laser light scattered from the particle. The scattered light is detected at 90° with a photomultiplier, and the amplitude of the light fluctuation is measured with a lock-in amplifier. The lock-in signal is then inverted by a discrete fast Fourier transform routine (FFT), to yield the particle absorption spectrum. Spectra of (NH₄)₂SO₄ droplets at different solute concentrations are presented. The data shown include the first infrared spectrum of a highly supersaturated solution.

Additional Information

© 1987 American Institute of Physics. Received 28 November 1986; accepted 17 February 1987. This research was supported by U.S. Environmental Protection Agency Grant No. R-81-857 and by gifts from General Motors Corporation and Mobil Oil Corporation. S. Arnold was supported by a Chevron Visiting Professorship at the California Institute of Technology and by National Science Foundation Grant No. ATM-8413574. The authors thank Anthony B. Pluchino of the Aerospace Corporation and Ralph Waniak of Advanced Kinetics for helpful advice during this work.

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