Ultrafast Imaging of Molecular Chirality with Photoelectron Vortices

Ultrafast imaging of molecular chirality is a key step toward the dream of imaging and interpreting electronic dynamics in complex and biologically relevant molecules. Here, we propose a new ultrafast chiral phenomenon exploiting recent advances in electron optics allowing access to the orbital angular momentum of free electrons. We show that strong-field ionization of a chiral target with a few-cycle linearly polarized 800 nm laser pulse yields photoelectron vortices, whose chirality reveals that of the target, and we discuss the mechanism underlying this phenomenon. Our Letter opens new perspectives in recollision-based chiral imaging.

Figure 1

(a) Photoionization of a chiral target with linearly polarized light produces photoelectron vortices with a helicity that encodes the handedness of the target, thus carrying opposite OAM in opposite propagation directions. The target orbital at the origin shows isosurfaces ${\chi }^{+}(\overrightarrow{r})=\pm {10}^{-3}\mathrm{a}.\mathrm{u}.$ [Eq. (2)]. (b) Standard photoelectron spectrum [Eq. (1)] resulting from strong-field ionization of a perfectly aligned chiral target $|{\chi }^{+}⟩$ [Eq. (2)] as a function of the momentum perpendicular and parallel to the laser field. (c) OAM-resolved photoelectron spectra [Eq. (5)] for OAM $l_v=+1$ (left) and $l_v=-1$ (right). (d) Difference of the OAM-resolved spectra normalized by their average [Eq. (7)] for momenta such that $(|M_{+1}^{+}{|}^2+|M_{-1}^{+}{|}^2)/2$ is at least 2% of the maximum of (b). All spectra (b)–(d) are in atomic units and were obtained for a peak intensity $I={10}^{14}\mathrm{W}/{\mathrm{cm}}^2$ ($U_p=0.22\mathrm{a}.\mathrm{u}.$), carrier frequency $\omega =0.057\mathrm{a}.\mathrm{u}.$ ($\lambda =800\mathrm{nm}$), $N=2$ cycles [Eq. (6)], and ionization potential $I_p=0.579\mathrm{a}.\mathrm{u}.$ of argon. All results were averaged over the carrier-envelope phase $\delta$ and over orientations of the target corresponding to rotations of Eq. (2) around $z$.

Figure 2

Photoelectron spectra as in Fig. 1 but for a partially aligned chiral target with a distribution of orientations given by $P(\beta )=3{\cos }^2(\beta )$ ($⟨{\cos }^2\beta ⟩=0.6$); see the Supplemental Material [31] for details.