Vibrational exciton coupling as a probe for phase transitions and shape changes of fluoroform aerosol particles.

Phase transitions and shape changes of aerosol particles play a fundamental role in atmospheric as well as technical processes involving aerosols. For the example of fluoroform particles, we demonstrate how information about both processes can be extracted from time-dependent infrared spectra by comparison with vibrational exciton calculations. We find volume crystallization rate constants for fluoroform particles in the range of J(v) = 10(8)-10(10) cm(-3) s(-1) at a temperature of T = 78 K. Furthermore, our investigation reveals that supercooled fluoroform droplets crystallize to the most stable monoclinic bulk crystal structure. Immediately after crystallization, the particles have a cube-like shape which evolves with increasing time to an elongated shape. The present results provide new data for a better understanding of the Rapid Expansion of Supercritical Solutions when fluoroform is used as a supercritical solvent under expansion conditions which lead to fluoroform aerosol formation.