Theory of Non-Dichroic Enantio-Sensitive Chiroptical Spectroscopy

TL;DR

This work identifies a symmetry-protected, non-dichroic enantiosensitive (NoDES) component in photoelectron angular distributions (PADs) arising when randomly oriented chiral molecules are ionized by elliptically polarized or orthogonal two-color fields. By decomposing the PAD into four $D_{2h}$ irreps, the NoDES signal is shown to reside in the $A_{u}$ representation, enabling enantiosensitive detection that is robust against phase variations between orthogonal field components. The authors propose and validate a practical extraction protocol using two velocity-map imaging projections, demonstrated through perturbation theory for resonantly enhanced two-photon ionization of methyloxirane and TDSE simulations of a toy-model chiral molecule across multiphoton and strong-field regimes; NoDES signals reach about 1% of the energy-resolved ionization yield. The results establish NoDES spectroscopy as a symmetry-protected, ultrafast chirality probe with broad applicability and practical robustness, with experimental confirmation reported in a companion paper.

Abstract

We show that the photoelectron angular distributions produced by elliptical and cross-polarized two-color laser fields interacting with randomly oriented chiral molecules decompose into four irreducible representations of the $D_{2h}$ point group. One of these ($A_u$) corresponds to a non-dichroic enantiosensitive (NoDES) contribution. This NoDES contribution has opposite sign for opposite enantiomers but remains invariant under reversal of the field ellipticity, enabling chirality detection that is robust against variations of the relative phase between orthogonal field components. We propose a protocol to isolate this component using only two velocity-map imaging projections and validate it through numerical simulations. Our calculations, performed in the two-photon resonantly-enhanced ionization, multi-photon, and strong-field ionization regimes with cross-polarized two-color fields show that the NoDES signal reaches about 1\% of the energy-resolved ionization yield, comparable to photoelectron circular dichroism and much larger than standard magnetic-dipole chiroptical effects. NoDES spectroscopy thus provides a symmetry-protected and phase-robust route to probe molecular chirality on the ultrafast time scale. The experimental confirmation of our theory is presented in the companion paper [L. Fede et al., arXiv:2512.19062 (2025)].

Figures (5)

• Figure 1: Perturbation-theory calculations for the lowest NoDES contribution $b_{3,-2}$ normalized to the total yield $b_{0,0}$ [Eq. (\ref{['eq:b3m2_b00']})] as a function of the photoelectron energy in two-photon resonantly enhanced ionization of methyloxirane with an orthogonal two-color field [Eq. (\ref{['eq:otc']})] for two different photon orderings. In (a) $\omega=3.56$ eV while in (b) $\omega=7.12$ eV. The ground and intermediate states are described by the HOMO and LUMO orbitals.
• Figure 2: TDSE calculations of the photoionization of a toy-model chiral molecule by an orthogonally polarized two-color laser field at various relative phases $\varphi$ [Eq. (\ref{['eq:otc']})]. The intensity of the 800 nm fundamental field component is $I=5\times 10^{12}$ W/cm$^2$ so that the interaction lies in the multiphoton regime. (a-b) Projections of the forward-backward antisymmetric part of the PAD along the $\hat{y}$ direction, in one enantiomer (a) and its mirror image (b). (c-d) NoDES signal obtained by subtracting the $\hat{x}+\hat{y}$ and $\hat{x}-\hat{y}$ projections of the forward-backward antisymmetric part of the PAD (see Sec. \ref{['sec:Extracting-the-NoDES']}), in one enantiomer (c) and its mirror image (d).
• Figure 3: Spherical harmonic decomposition of the photoelectron angular distributions obtained by TDSE simulations of the photoionization of a toy-model chiral molecule (first enantiomer in (a), second enantiomer in (b)) by an orthogonal two-color field at $5\times10^{12}$$\mathrm{W/cm}^{2}$. The enantiodifferential, forward-backward antisymmetric signal is shown in (c).
• Figure 4: Same as Fig. 2 but for $I=5\times 10^{13}$ W/cm$^2$ so that the interaction lies in the strong-field regime.
• Figure 5: Same as Fig. 3 but for $I=5\times 10^{13}$ W/cm$^2$.