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October 21, 2002
Volume 80, Number 42
CENEAR 80 42 p. 16
ISSN 0009-2347


CHEMISTRY

CRYSTAL CONTROL
Choice of polarized laser light directs form of crystal polymorph

8042NOTW7.Alpha4
8042NOTW7.Gamma1
YOU CHOOSE Glycine crystals can be grown in the form (top) or form (bottom), depending on the type of polarized light used to irradiate glycine solutions.
COURTESY OF JELENA MATIC, POLYTECHNIC UNIVERSITY
U
sing short pulses of light to probe crystallization in solutions, scientists have demonstrated that polarized laser radiation can steer crystallization toward one polymorphic form over another.

The new procedure for controlling crystallization may provide researchers with a means for selecting among the forms in which a compound crystallizes. The study broadens understanding of crystallization mechanisms, which are essential to industrial chemical processes such as separation and purification.

Specifically, chemistry professor Bruce A. Garetz and graduate student Jelena Matic of Polytechnic University, Brooklyn, and Allan S. Myerson, a chemical engineering professor at Illinois Institute of Technology in Chicago, showed that irradiating supersaturated solutions of glycine with circularly polarized near-infrared light causes the compound to crystallize in the a phase. In contrast, linearly polarized light leads to the form of the crystal [Phys. Rev. Lett., 89, 175501 (2002)].

The work builds on earlier studies in which the team showed that linearly polarized light causes urea to nucleate in a few nanoseconds instead of a few days, and in such a way that the needle-shaped crystals tend to align with their needle axes parallel to the laser light's electric field direction. The group proposes that nucleation occurs through electric-field-induced alignment of the molecules.

Unlike much of the previous work in the field, in which crystal nucleation is induced via photochemical processes, "the present experiment shows conclusively that an entirely different mechanism is possible," says University of Chicago chemistry professor David W. Oxtoby. "This could be of importance in materials science and pharmaceutical chemistry where various crystalline forms of a substance can have entirely different physical, chemical, and biological properties."



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