Polarization Switching of Crystal Structure in the Nonphotochemical Light-Induced Nucleation of Supersaturated Aqueous Glycine Solutions

By switching between linear and circular polarization in the irradiation of supersaturated solutions of the amino acid glycine in water with intense nanosecond pulses of near-infrared laser light, we have obtained the $\gamma$ and $\alpha$ phases, respectively, through nonphotochemical light-induced nucleation (NPLIN). This is the first report of light polarization controlling crystal structure. The intensity dependence of NPLIN in aqueous urea is also reported.

Figure 1

(a) Two views of the packing of molecules in $\alpha$ glycine [13]. The bottom view shows the packing and hydrogen bonding in one layer of glycine molecules (viewed along the $b$ axis). The top view (along the $c$ axis) shows how cyclic dimers pack together to form hydrogen-bonded double layers. (b) Two views of the packing of molecules in $\gamma$ glycine [11]. The top view shows a helical chain (viewed perpendicular to the $c$ axis). The bottom view (along the $c$ axis) shows the threefold symmetry of a helical chain. These helical chains pack together hexagonally through lateral hydrogen bonds.

Figure 2

Intensity dependence of NPLIN in aqueous urea, showing the number of solutions that nucleated at or below a given laser intensity. The total number of solutions illuminated was 18.

Figure 3

Different types of alignment for disklike and rodlike polarizabilities in the presence of intense linearly or circularly polarized light. This figure shows the complete alignment achieved in the limit of infinite electric field. The alignments achieved experimentally with the laser field are on the order of $1°$, so that the orientation distributions produced are only slightly different from an isotropic distribution.