D'Arrigo M C, Cruickshank F R, Pugh D, Sherwood J N, Wallis J D, Mackenzie C, Hayward D
Department of Pure & Applied Chemistry, University of Strathclyde, 295 Cathedral St., Glasgow, UK.
Phys Chem Chem Phys. 2006 Aug 28;8(32):3761-6. doi: 10.1039/b517388d. Epub 2006 Jul 21.
Hyper-Rayleigh scattering is revealed as a very sensitive monitor of cluster formation in solution, and as a means of studying the mechanism of crystal nucleation in molecular species. Two compounds are selected with particularly high second harmonic generation (SHG) powers in the crystalline state and experimental conditions are defined allowing the measurement of the beta value for one of these as 18+/-1x10(-30) esu. It is found to agree with current theoretical prediction of 20x10(-30) esu. In the more powerful of these, two photon induced fluorescence is found to be partly responsible for the SHG. The solubilities of both compounds in methanol are measured and it is observed that these differ by a factor of ten. When the solution concentration is increased beyond 45% of the saturation value, the quadratic coefficient exhibits non-linear behaviour with respect to concentration. Additionally, the widths of the distributions of the HRS signals increase initially with concentration as expected, but, beyond 45% saturation concentrations, these narrow again. These phenomena are interpreted as indicators of cluster formation in these solutions well below saturation concentrations. A future experimental design is proposed in which the coherent component will yield information on the organisation of the molecules in such clusters.
超瑞利散射被证明是溶液中团簇形成的非常灵敏的监测手段，也是研究分子物种中晶体成核机制的一种方法。选择了两种在晶体状态下具有特别高的二次谐波产生（SHG）功率的化合物，并确定了实验条件，使得能够测量其中一种化合物的β值为18±1×10⁻³⁰ esu。发现该值与当前20×10⁻³⁰ esu的理论预测相符。在其中更强的一种化合物中，发现双光子诱导荧光对二次谐波产生有部分贡献。测量了这两种化合物在甲醇中的溶解度，发现它们相差一个数量级。当溶液浓度增加到超过饱和值的45%时，二次系数相对于浓度呈现非线性行为。此外，超瑞利散射（HRS）信号分布的宽度最初如预期的那样随浓度增加，但在超过45%饱和浓度时，这些宽度又变窄了。这些现象被解释为在远低于饱和浓度的这些溶液中形成团簇的指标。提出了一种未来的实验设计，其中相干分量将提供有关此类团簇中分子组织的信息。
