D-丙氨酸单晶低温变温光学性质的研究

Temperature-Dependent Optical Activity and Birefringence Study of D-Alanine Single Crystal

用PEM-90光弹调制器和透射偏振光的方法，研究D-丙氨酸单晶结构低温变温光学性质的变化。D-丙氨酸单晶是双轴晶体，晶体各向异性，先测定劳埃像，结合D-丙氨酸晶体特征，确定晶面为[010]面。由于电弱力宇称不守恒，D-和L-丙氨基酸分子间存在字称破缺能差，Salam预言在某临界温度（-250K）下，D-氨基酸分子会发生二级相变。在218-290K，通过原位测量D-丙氨酸晶体的旋光性质（I2f／Idc）随温度的变化，发现D-丙氨酸晶体在250K左右有明显的旋光相变，与前期D-缬氨酸晶体低温相变结果相类似，从而为Salam预言的二级相变提供了佐证。

The measurement of the anisotropy of optical adtivity and birefringenee is one of the most important clues to studying physical properties of a biaxial crystal of D-alanine. In order to investigate a second-order phase transition predicted by A. Salam between two states of D-alanine, the behavior of birefringence and optical activity is useful for the phenomenological approach to the transition mechanism. The optical activity as a peculiar quantity can respond to the modulation of the crystal lattice and to the change in the bonding nature of constituent atoms. In the present paper, the authors use the PEM-90 photoelastic modulator to study the conformation change of D-alanine at the temperature ranging from 220 to 290 K. The temperature dependence of I21/Idc showed that the conformation of D-alanine molecule in single crystal changed around 250 K. The obtained results provide an obvious evidence of optical rotation phase transition predicted by Salam.