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45023-AC10
Laser- and Electric-Field-Induced Nucleation in Supersaturated Solutions and Supercooled Melts
About ten years ago, we accidentally discovered that intense near-infrared laser pulses could induce supersaturated aqueous urea solutions to nucleate. We called this phenomenon nonphotochemical laser-induced nucleation (NPLIN) to distinguish it from the better-known, century-old field of ultraviolet and visible light-induced nucleation in supersaturated vapors, the mechanism of which typically involves the photochemical generation of a nonvolatile product that acts as a nucleus for the growth of the condensed phase.
NPLIN has also been demonstrated to control polymorphism through “polarization switching”, in which the polymorph formed depends on the polarization state of the laser light used. In those experiments, we showed that in aqueous glycine solutions with supersaturation (SS) in the range 1.45-1.54 (SS=c/c0, where c is the molal concentration of the solution and c0 is the molal concentration of a saturated solution), linearly polarized near-infrared light with a wavelength of 1064 nm produced gamma-glycine, while circularly polarized light produced alpha-glycine. Outside of this supersaturation window, different polarizations yield the same polymorph.
Because NPLIN appears to be independent of wavelength and occurs at wavelengths where neither the solvent nor the solute absorbs light, it is not likely to be a photochemical process. There is growing evidence that nucleation from liquid solutions is often a two-step process: the formation of liquid-like solute clusters followed by the organization of such clusters into a crystalline structure. We have hypothesized that NPLIN involves the electric-field induced alignment of molecules or groups of molecules in a prenucleating solute cluster, aiding the cluster to organize into a crystal-like entity, through optical Kerr alignment. Linear and circular polarizations induce linear and planar alignment respectively, and thus can induce the nucleation of different polymorphs. The fact that a simple change in polarization was enough to change the polymorph formed essentially rules out a photochemical mechanism for NPLIN.
In an effort to explain more quantitatively why linear, elliptical and circular polarization should induce different types of alignment for rod-like and disk-like polarizabilities, we have calculated order-parameter triangles based on optical Kerr alignment. We have recently demonstrated these types of alignments experimentally, in small-molecule liquids exhibiting large optical nonlinearities. Carbon disulfide represents a typical rod-like polarizability, while benzene represents a typical disk-like polarizability. When we measured the optical Kerr (OKE) effect in these two liquids, with pump and probe beams at right angles, we found that the OKE in carbon disulfide is greater with linear than with circular pump polarization, while the opposite is true for benzene.
We have recently observed “polarization switching” of polymorphs in a second substance: aqueous L-histidine. Like aqueous glycine, histidine also exhibits a supersaturation window, SS= 1.40-1.60 in which polarization switching occurs. Exposure of solutions to 0.24-GW/cm2, 7-ns pulses of 532-nm light induced the nucleation of different polymorphs of L-histidine depending on polarization state of the light. Circularly polarized laser pulses tended to nucleate the orthorhombic A polymorph, whereas linearly polarized pulses tended to nucleate a mixture of the orthorhombic A and monoclinic B polymorphs. Higher supersaturation also favors the formation of mixed polymorphs. These observations support the hypothesis that the laser is organizing hydrogen-bonded groups of histidine molecules through an optical Kerr alignment.
We have also studied a nonphotochemical laser-induced phase transition in a supercooled 4'-n-pentyl-4-cyanobiphenyl (5CB, also referred to as PCB and K15) liquid crystal, using linearly polarized 45-ps light pulses at a wavelength of 532 nm. The laser induced nucleation from the metastable supercooled isotropic phase to the nematic phase during slow cooling (0.001 deg C/min) and high light intensity (3.9 MW/cm2). The nematic phase director tended to align along the direction of the plane of polarization of the light. At the intensities used, there is no observable laser-induced reorientation of the director once the sample is in the nematic phase, nor any permanent laser-induced ordering when the sample is illuminated only in the stable isotropic phase during slow cooling. These experimental results are consistent with a mechanism based on optical Kerr alignment.
The bulk of the experimental work has been performed by graduate student Xiaoying Sun. We continue to collaborate with Prof Allan Myerson at the Illinois Institute of Technology, and the liquid crystal work represents a new collaboration with Prof. Peter Pallfy-Muhoray at Kent State University.
