Geometric magnetism and anomalous enantio-sensitive observables in photoionization of chiral molecules

Abstract

Chiral molecules are instrumental for molecular recognition in living organisms. Distinguishing between two opposite enantiomers, the mirror twins of the same chiral molecule, is both vital and challenging. Geometric magnetism enables a broad class of phenomena in solids including the anomalous electron velocity, the Hall effect, and related topological observables. Here we show that ultrafast electron currents excited in chiral molecules can generate geometric fields which enable a class of anomalous enantio-sensitive observables in photoionization. Next, we introduce the first member of this class: enantio-sensitive orientation of chiral molecules via photoionization. This effect opens unexplored routes to both enantio-separation and imaging of chiral dynamics on ultrafast time scales. Our work suggests that geometric magnetism in photoionization provides the bridge between the two geometrical properties, chirality and topology.

Capturing electron-driven chiral dynamics in UV-excited molecules

“…The electron dynamics uncovered in this work underlies yet another consequence on the molecular response of photoexcited chiral systems: the coherent superposition of excited states induced by the pump allows to selectively filter—within a few femtoseconds—specific molecular orientations through enantiosensitive photoionization 10 . An electron moving within the coherent state superposition creates an electronic current 35 , which reduces to when expanding Φ on the (real) 3d and 4p bound eigenstates.…”

“…Its relation with the number of molecules pointing forward and backward with respect to the light-propagation axis, and , respectively, is derived from the simple orientation model introduced in ref. 10 : the distribution of oriented molecules is described by the wavefunction , in which and are the usual spherical harmonics and . Note that higher ( l , m ) orders are not necessary to describe basically a forward/backward asymmetry.…”

“…To quantify the degree of orientation of the photoionized molecules, we select a unitary vector fixed to the internal C–C bond of the methyl lactate cation, as illustrated in the inset of Fig. 4e , and calculate its averaged value over the probe-filtered molecular orientations in the laboratory frame as 10 in which is the vector passively rotated in the laboratory frame and is the ionization rate associated to helicity h = ± 1 and photoelectrons of energy ε . Notably, the averaged orientation of the cations depends on ε because the ε dependence of the underlying photoionization yield is not the same for all orientations .…”

“…Notably, the averaged orientation of the cations depends on ε because the ε dependence of the underlying photoionization yield is not the same for all orientations . The x and y components of are found to be zero and only the z component survives the averaging 10 (see Supplementary Fig. 9 ), leading to in which θ ion is the angle between the internal C–C bond and the probe propagation axis (see inset of Fig.…”

“…Attosecond technology might enable influence over such interactions, given that it can probe and even direct electron motion within molecules on the intrinsic electronic timescale 6 and thereby control reactivity 7 – 9 . Electron currents in photoexcited chiral molecules have indeed been predicted to enable enantiosensitive molecular orientation 10 , but electron-driven chiral dynamics in neutral molecules have not yet been demonstrated owing to the lack of ultrashort, non-ionizing and perturbative light pulses. Here we use time-resolved photoelectron circular dichroism (TR-PECD) 11 – 15 with an unprecedented temporal resolution of 2.9 fs to map the coherent electronic motion initiated by ultraviolet (UV) excitation of neutral chiral molecules.…”