Nonphotochemical, Polarization-Dependent, Laser-Induced Nucleation in Supersaturated Aqueous Urea Solutions

We report a new photophysical phenomenon in which 1.06 μm pulses from a Q-switched Nd:YAG laser induce crystallization in supersaturated solutions of urea in water. Because the solutions are transparent at the incident wavelength, a photochemical mechanism is unlikely. The needle-shaped crystals that initially form tend to be aligned parallel to the electric field vector of the light, suggesting a Kerr-like field-induced alignment of urea molecules that aids in organizing prenucleating clusters. The effect has application to pump-probe nucleation studies and to clean nucleation in sealed systems.