Non-photochemical light-induced nucleation and control of polymorphism through polarization-switching

This dissertation examines the effect of polarization, intensity and wavelength on crystallization from supersaturated solutions using non-photochemical light-induced nucleation (NPLIN). Using NPLIN crystal structure can be controlled. Intense pulses of linearly-polarized laser light induce the nucleation of the gamma-glycine polymorph, which otherwise does not form under the same conditions. Moreover, intense pulses of circularly-polarized light induce the alpha-glycine polymorph to crystallize from solutions prepared using the same procedure. The observation that polymorphism could be controlled by changing between linear and circular polarization was named polarization-switching. It represents the strongest evidence to date that the mechanism involved in NPLIN is indeed non-photochemical. The interaction of light and matter responsible for the phenomenon is discussed, as well as the implication of the results of NPLIN experiments on the current understanding of the structure of supersaturated solutions and nucleation. The success rate of NPLIN shows a non-linear dependence on intensity, with a threshold value of 0.02--0.03 GW/cm2. Nucleation could successfully be induced at two different wavelengths, lending further support to the non-photochemical mechanism hypothesis. Green light was shown to be more effective than near-IR light at inducing nucleation. The difference was attributed to the lower absorption of water at the visible wavelength. The potential for use of NPLIN in fundamental studies of nucleation through pump-probe experiments is explored. In addition, evidence is presented that NPLIN has the potential to create unknown polymorphs. The powder x-ray diffraction pattern of a new polymorph of L-alanine is presented and discussed.