Chiral asymmetry in the angle-resolved O and C $1s^{-1}$ core photoemissions of the $R$ enantiomer of glycidol

We present measurements of a photoelectron circular dichroism in photoionization from O and C $1s$ core levels, of the $R$ enantiomer of glycidol $\left({\mathrm{C}}_3{\mathrm{H}}_6{\mathrm{O}}_2\right)$ in the gas phase. This dichroism emerges from a forward-backward asymmetry in the angular distribution of electrons created on ionization with circularly polarized synchrotron radiation and is already fully present in the pure electric dipole approximation. Asymmetry factors obtained for the core levels in this study range up to a few percent, but it is likely that these values are limited by a failure to resolve photoemission from individual atomic sites. Theoretical modeling is provided to examine possible differences between these alternative atomic photoemission sites, and between different conformational structures of glycidol. The calculated chiral angular distribution parameters that support the circular dichroism display a much enhanced sensitivity to the molecular conformation compared to the conventional photoionization cross section and the $\beta$ parameter. Likely conformer structures can be suggested after comparison with the experiment.

Figure 1

O $1s$ XPS of $R$ glycidol. Spectra, $S\left(\mathrm{LCP}\right)$ and $S\left(\mathrm{RCP}\right)$, recorded with left and right circularly polarized radiation are shown, together with ${\Gamma }_{54.7}$, the experimental asymmetry between them.

Figure 2

Lowest-energy glycidol conformers. For greater clarity the oxygen atoms are drawn shaded.

Figure 3

(Color online) CMS $X\alpha$ O $1s$ photoionization calculations for glycidol conformers C1–C5. For each conformer the cross-section curves $\sigma$ represent the sum of the ${\mathrm{O}}_R$ and ${\mathrm{O}}_H$ atom $1s^{-1}$ ionization, while the $\beta$ anisotropy parameters are cross-section weighted averages for these two photoemission sites.

Figure 4

(Color online) Calculated O $1s$ PECD curves for conformers of $R$ glycidol with experimental data measured at $h\nu =543\mathrm{eV}$ for comparison. Each conformer curve is a cross-section weighted average of the ${\mathrm{O}}_R$ and ${\mathrm{O}}_H$ atom photoemission.

Figure 5

(Color online) Calculated O atom $1s^{-1}$ $b_1^{\{+1\}}$ parameters for the conformers C1–C5 of $R$ glycidol.

Figure 6

C $1s$ XPS of $R$-glycidol. Recorded spectra obtained using left and right circular polarization are shown, together with ${\Gamma }_{54.7}$, the experimental asymmetry between them.

Figure 7

(Color online) Experimental (●) and calculated C $1s$ PECD results for $R$ glycidol. Each calculated conformer curve C1–C5 is a cross section weighted average of the three C atom photoemission sites within the molecule.

Figure 8

(Color online) Calculated C atom $1s^{-1}$ $b_1^{\{+1\}}$ parameters for the conformers C1–C5 of $R$ glycidol: methanolic C (upper panel); stereogenic (chiral) C (middle panel); second oxirane ring C (lower panel).