Atom-Specific Identification of Adsorbed Chiral Molecules by Photoemission

The study of chiral adsorbed molecules is important for an analysis of enantioselectivity in heterogeneous catalysis. Here we show that such molecules can be identified through circular dichroism in core-level photoemission arising from the chiral carbon atoms in stereoisomers of 2,3-butanediol molecules adsorbed on Si(100), using circularly polarized x rays. The asymmetry in the carbon $1s$ intensity excited by right and left circularly polarized light is readily observed, and changes sign with the helicity of the radiation or handedness of the enantiomers; it is absent in the achiral form of the molecule. This observation demonstrates the possibility of determining molecular chirality in the adsorbed phase.

Figure 1

Structural models before and after the adsorption. (a) Molecular models of enantiomers of the $S,S$ (left) and $R,R$ (right) forms of 2,3-butanediol. (b) Adsorption geometry for ($S,S$) 2,3-butanediol on a Si(100) surface as inferred from core- and valence-level photoemission (Ref. 10)

Figure 2

(a) Schematic view of the experimental geometry; $\mathbf{n}$ is the surface normal, $\mathbf{q}$ the direction of light incidence, and $\mathbf{k}$ the direction of electron emission. (b) Photoelectron spectra of the carbon $1s$ core levels for the adsorbed stereoisomers of 2,3-butanediol excited by right (dashed line, blue online) and left circularly polarized (solid line, red online) light. Dichroism is observed on the higher-binding energy component attributed to emission from the carbon atoms in the chiral center from the $(R,R)$ and $(S,S)$ forms. The dichroic effect changes sign either with photon polarization or molecular handedness. No dichroism is observed in the achiral $(R,S)$ form. (c) Difference signal for the three stereoisomers.

Figure 3

(a) CD effect in the C $1s$ level of the enantiomers of 2,3-butanediol measured for emission angles ($\theta$) in-plane, i.e., within the plane of incidence and (b) out of plane, i.e., in the plane perpendicular to the plane of incidence ($\varphi$). Note the large asymmetry and its reversal upon switching the molecular handedness in (a). Error bars mean statistical deviation in the fitting of each peak area.