Strong Differential Photoion Circular Dichroism in Strong-Field Ionization of Chiral Molecules

We investigate the differential ionization probability of chiral molecules in the strong-field regime as a function of the helicity of the incident light. To this end, we analyze the fourfold ionization of bromochlorofluoromethane (CHBrClF) with subsequent fragmentation into four charged fragments and different dissociation channels of the singly ionized methyloxirane. By resolving for the molecular orientation, we show that the photoion circular dichroism signal strength is increased by 2 orders of magnitude.

Figure 1

Differential PICD as normalized difference between the enantiomers for CHBrClF as function of the angle between the bromine ion momentum and the direction of light propagation. The red curve reflects the measurement results for LCP, the blue for RCP. The error bars indicate the statistical error.

Figure 2

Differential ionization probability for $R$ CHBrClF and LCP. (a) Each point in the graph shows the number of measured events for a direction of light propagation in spherical coordinates in the molecular system. The position of the molecule in the selected molecular system is indicated by the position of the fragments’ momenta. (b) The colored sphere represents the differential count rate in the molecular system. The count rate is represented by the distance from the C atom and the color.

Figure 3

Differential PICD in the four-body fragmentation of CHBrClF. (a) $\mathrm{PICD}[\phi ,\cos (\theta )]=100*\{\mathbf{(}{S}_{\mathrm{RCP}}[\phi ,\cos (\theta )]-S_{\mathrm{LCP}}[\phi ,\cos (\theta )]\mathbf{)}/\mathbf{(}{S}_{\mathrm{RCP}}[\phi ,\cos (\theta )]+S_{\mathrm{LCP}}[\phi ,\cos (\theta )]\mathbf{)}\}$ for the $S$ enantiomer. (b) PICD as in (a) for the $R$ enantiomer. For small values of $\cos \theta$ there is a deviation from the antisymmetry in the PICD pattern.

Figure 4

Circular dichroism in the single ionization of methyloxirane. (a) Circular dichroism in the ion yield calculated by $\mathrm{CD}(\mathrm{TOF})=50\{[R_{\mathrm{LCP}}(\mathrm{TOF})-S_{\mathrm{LCP}}(\mathrm{TOF})]/[R_{\mathrm{LCP}}(\mathrm{TOF})+S_{\mathrm{LCP}}(,\mathrm{TOF})]\}-50\{[R_{\mathrm{RCP}}(\mathrm{TOF})-S_{\mathrm{RCP}}(\mathrm{TOF})]/[R_{\mathrm{RCP}}(\mathrm{TOF})+S_{\mathrm{RCP}}(,\mathrm{TOF})]\}-0.4$. Different measuring times and the smallest errors directly influence the CD. The applied small correction cannot be determined from the available data. A detailed discussion of the errors can be found in the Supplemental Material [5]. (b) As in (a) with the additional condition that the location of the ion hit on the detector in the light propagation direction is larger (smaller) than 3.5 mm ($-3.5\mathrm{mm}$), represented by the red (blue) line. With this condition, a subset of molecular orientations is selected. (c) $\mathrm{PICD}(X_{\mathrm{ION}},\mathrm{TOF})=50\{[R_{\mathrm{LCP}}(X_{\mathrm{ION}},\mathrm{TOF})-S_{\mathrm{LCP}}(X_{\mathrm{ION}},\mathrm{TOF})]/[R_{\mathrm{LCP}}(X_{\mathrm{ION}},\mathrm{TOF})+S_{\mathrm{LCP}}(X_{\mathrm{ION}},\mathrm{TOF})]\}-50\{[R_{\mathrm{RCP}}(X_{\mathrm{ION}},\mathrm{TOF})-S_{\mathrm{RCP}}(X_{\mathrm{ION}},\mathrm{TOF})]/[R_{\mathrm{RCP}}(X_{\mathrm{ION}},\mathrm{TOF})+S_{\mathrm{RCP}}(X_{\mathrm{ION}},\mathrm{TOF})]\}$ as a function of the ionic TOF and $X_{\mathrm{ION}}/mm$ the position of impact onto the detector of the ion in the direction of light propagation. The count rate for each TOF and for each enantiomer and each helicity is normalized to one, whereby only the differential signal is represented; different integral ionization probabilities play no role in this representation. (d) Absolute ion count rate as a function of the ion TOF and position of impact on the ion detector. The transparent areas indicate the selection in (b). The vertical lines and their labels show the average TOF of the ions for the stated mass-to-charge ratios.