Atomic Spectroscopy with Twisted Photons: Separation of M1--E2 Mixed Multipoles

TL;DR

This paper explores how twisted photons with orbital angular momentum can be used to distinguish between different multipolar contributions in atomic excitations, enabling detailed analysis of mixed E2-M1 transitions.

Contribution

It introduces a method to separate multipolar contributions in atomic photoexcitation using twisted photons and position-dependent measurements, which was not previously demonstrated.

Findings

Fundamental differences in photo-absorption cross sections due to photon topology.

Ability to extract relative transition rates by measuring excitation rates at different impact parameters.

Numerical examples for Boron-like highly-charged ions showing practical implementation.

Abstract

We analyze atomic photoexcitation into the discrete states by twisted photons, or photons carrying extra orbital angular momentum along their direction of propagation. From the angular momentum and parity considerations, we are able to relate twisted-photon photoexcitation amplitudes to their plane-wave analogues, independently of the details of the atomic wave functions. We analyzed the photo-absorption cross sections of mixed-multipolarity $E2-M1$ transitions in ionized atoms and found fundamental differences coming from the photon topology. Our theoretical analysis demonstrates that it is possible to extract the relative transition rates of different multipolar contributions by measuring the photo-excitation rate as a function of the atom's position (or the impact parameter) with respect to the optical vortex center. The proposed technique for separation of multipoles can be implemented if the target's atom position is resolved with sub-wavelength accuracy, for example, with Paul traps. Numerical examples are presented for Boron-like highly-charged ions (HCI).