Enantioselective fragmentation of an achiral molecule in a strong laser field

Abstract

Chirality is omnipresent in living nature. On the single molecule level, the response of a chiral species to a chiral probe depends on their respective handedness. A prominent example is the difference in the interaction of a chiral molecule with left or right circularly polarized light. In the present study, we show by Coulomb explosion imaging that circularly polarized light can also induce a chiral fragmentation of a planar and thus achiral molecule. The observed enantiomer strongly depends on the orientation of the molecule with respect to the light propagation direction and the helicity of the ionizing light. This finding might trigger new approaches to improve laser-driven enantioselective chemical synthesis.

Fig. 1. Enantioselective fragmentation of formic acid.

Center: Achiral formic acid in syn-conformation. The mean value of cos(α) is color-coded in the enveloping sphere for the corresponding propagation direction of the laser in the molecular system. The colored arrows and helices indicate the directions of incidence for LCP. (A to E) For selected directions, the surrounding panels show the distribution of cos(α) in blue and the photoion circular dichroism (PICD) as the normalized difference between distributions for right- and left-handed circularly polarized light (RCP and LCP) in red.

Fig. 2. Chiral and achiral structures of formic acid.

The selected different molecular orientations with respect to the laser propagation direction are indicated in Fig. 1. The ball-and-stick model indicates the direction of the linear momentum vectors, and the transparent lobes represent the measured data; the distance from the C atom and color shows the count rate in the respective direction. The O–C=O plane is highlighted in turquoise. The letters connect the structures with the panels in Fig. 1. (A) R enantiomer. (B) Achiral structure. (C) S enantiomer.

Fig. 3. Differential PICD of formic acid.

(A) PICD for the R enantiomer. (B) PICD for the S enantiomer. The normalized difference was generated from the histogram for the (A) R enantiomer [with the S enantiomer in (B)] with LCP with the direction of the laser beam in the molecular system and those with RCP. (C to F) PICD with gating on cos(α). (C) 0 < │cos(α)│ < 0.25. (D) 0.25 < │cos(α)│ < 0.5. (E) 0.5 < │cos(α)│ < 0.75. (F) 0.75 < │cos(α)│ < 1. (G) Mean of the absolute value of the PICD versus cos(α) with $<|\text{PICD}|>={\iint }_{-1,-180}^{1,180}|\text{PICD}(\text{cos}(\mathrm{\theta }),\mathrm{\phi },\text{cos}(\mathrm{\alpha }))|\mathit{d}\text{cos}(\mathrm{\theta })\mathit{d}\mathrm{\phi }$.

Fig. 4. Assignment of the fragments of the Coulomb explosion.

The linear momenta of the H atoms in the molecular frame are plotted for the assignment of the fragments. The linear momentum of the C ion defines the x direction; together with the linear momentum of the O ions, it defines the xy plane. The angle in the coordinate system indicates the direction of the linear momentum vector, while the distance and the color scale indicate the number of counts along this direction.