Molecular spectroscopy of a single aerosol particle.

The absorption spectrum of a single micrometer-sized aerosol particle is measured for the first reported time in the IR. Particle absorption is determined through the IR modulation of visible scattered light near a structure resonance. This technique, termed structure resonance modulation spectroscopy, is used to measure the IR absorption spectrum of an (NH(4))(2)SO(4) aerosol droplet of 5.4-microm diameter in the region from 970 to 1280 cm(-1). The resulting spectrum, when fitted by Mie theory, enables one to determine the molecular composition of the droplet.