Single droplet displacement infrared action spectroscopy

Aqueous microdroplets have fascinated scientists with unique properties that deviate from bulk solutions. Understanding microdroplet chemistry has been limited by few spectroscopic measurements on single well-defined microdroplets. Here we present a new technique to record an IR spectrum of a single droplet called Single Droplet Displacement Infrared Action Spectroscopy (SiDDIRAS) with potential broad applications in physical, analytical, and environmental chemistry. The droplet was trapped in a linear quadrupole electrodynamic balance. When IR radiation at resonant frequency was introduced, mass loss from IR-induced evaporation caused the droplet to displace upwards. Mapping the displacement as a function of IR frequency yields an IR spectrum of a single microdroplet. As a demonstration, the SiDDIRA spectrum of an ~8 µm droplet containing NaCl and NaN₃ shows that the azide asymmetric stretch blueshifts by 5 cm^-1 relative to bulk with significant peak broadening, evidence that this droplet is supersaturated with salts compared to bulk solutions.