Enhanced Predictability of Urea Crystallization by an Optimized Laser Repetition Rate

Laser-induced crystallization is a novel alternative to classical methods for crystallizing organic molecules but requires judicious choice of experimental parameters for the onset of crystallization to be predictable. This study investigated the impact of the laser repetition rate on the time delay from the start of the pulsed laser illumination to initiation of crystallization, the so-called induction time. A supersaturated urea solution was irradiated with near infrared ($\lambda = 1030$ nm) laser pulses of pulse duration $\tau = 5$ ps at a pulse energy of approximately $E = 340$ µJ while varying the repetition rate from 10 to 20~000 Hz. The optimal rate discovered ranged from 500 Hz to 1 kHz, quantified by the measured induction time (median 2-5 seconds) and the mean probability of inducing a successful crystallization event ($5 \times 10^{-2}$ \%). For higher repetition rates (5 kHz to 20 kHz), the mean probability dropped to $3 \times 10^{-3}$ \%. The reduced efficiency at high repetition rates is likely due to an interaction between an existing thermocavitation bubble and subsequent pulses. These results suggest that an optimized pulse repetition rate can be a means to gain further control over the laser-induced crystallization process.