Nahon Laurent, Nag Lipsa, Garcia Gustavo A, Myrgorodska Iuliia, Meierhenrich Uwe, Beaulieu Samuel, Wanie Vincent, Blanchet Valérie, Géneaux Romain, Powis Ivan
Synchrotron SOLEIL, L'Orme des Merisiers, St. Aubin BP 48, 91192 Gif sur Yvette, France.
Phys Chem Chem Phys. 2016 May 14;18(18):12696-706. doi: 10.1039/c6cp01293k. Epub 2016 Apr 20.
Photoelectron circular dichroism (PECD) manifests itself as an intense forward/backward asymmetry in the angular distribution of photoelectrons produced from randomly-oriented enantiomers by photoionization with circularly-polarized light (CPL). As a sensitive probe of both photoionization dynamics and of the chiral molecular potential, PECD attracts much interest especially with the recent performance of related experiments with visible and VUV laser sources. Here we report, by use of quasi-perfect CPL VUV synchrotron radiation and using a double imaging photoelectron/photoion coincidence (i(2)PEPICO) spectrometer, new and very accurate values of the corresponding asymmetries on showcase chiral isomers: camphor and fenchone. These data have additionally been normalized to the absolute enantiopurity of the sample as measured by a chromatographic technique. They can therefore be used as benchmarking data for new PECD experiments, as well as for theoretical models. In particular we found, especially for the outermost orbital of both molecules, a good agreement with CMS-Xα PECD modeling over the whole VUV range. We also report a spectacular sensitivity of PECD to isomerism for slow electrons, showing large and opposite asymmetries when comparing R-camphor to R-fenchone (respectively -10% and +16% around 10 eV). In the course of this study, we could also assess the analytical potential of PECD. Indeed, the accuracy of the data we provide are such that limited departure from perfect enantiopurity in the sample we purchased could be detected and estimated in excellent agreement with the analysis performed in parallel via a chromatographic technique, establishing a new standard of accuracy, in the ±1% range, for enantiomeric excess measurement via PECD. The i(2)PEPICO technique allows correlating PECD measurements to specific parent ion masses, which would allow its application to analysis of complex mixtures.
光电子圆二色性（PECD）表现为通过圆偏振光（CPL）光电离从随机取向的对映体产生的光电子角分布中强烈的向前/向后不对称性。作为光电离动力学和手性分子势的灵敏探针，PECD尤其受到关注，特别是随着近期可见和真空紫外激光源相关实验的开展。在此，我们报告，通过使用准完美CPL真空紫外同步辐射并使用双成像光电子/光离子符合（i(2)PEPICO）光谱仪，展示了手性异构体樟脑和葑酮相应不对称性的新的非常精确的值。这些数据还已根据通过色谱技术测量的样品绝对对映体纯度进行了归一化。因此，它们可作为新的PECD实验以及理论模型的基准数据。特别是我们发现，尤其是对于两个分子的最外层轨道，在整个真空紫外范围内与CMS-Xα PECD模型有很好的一致性。我们还报告了PECD对慢电子异构性的惊人灵敏度，在比较R-樟脑和R-葑酮时显示出大的且相反的不对称性（在10 eV左右分别为-10%和+16%）。在这项研究过程中，我们还可以评估PECD的分析潜力。实际上，我们提供的数据的准确性使得能够检测并估计所购样品中与完美对映体纯度的有限偏差，与通过色谱技术并行进行的分析结果高度一致，从而为通过PECD测量对映体过量建立了±1%范围内的新的准确度标准。i(2)PEPICO技术允许将PECD测量与特定母离子质量相关联，这将使其能够应用于复杂混合物的分析。
