# Precision spectroscopy of negative-ion resonances in ultralong-range Rydberg molecules

## TL;DR

This study uses ultralong-range Rydberg molecules to probe negative-ion resonances, revealing fine structures and new alignment mechanisms, thus providing detailed insights into electron-neutral scattering near the detachment limit.

## Contribution

It introduces a novel atomic-scale system for sensitive measurement of electron-neutral scattering and uncovers previously unobserved fine structures and mechanisms in negative-ion resonances.

## Key findings

- Observation of the fine structure of Rb$^-$ triplet states
- Extraction of parameters for p-wave scattering resonances
- Discovery of a spin-orbit mediated alignment mechanism

## Abstract

The level structure of negative-ions near the electron detachment limit dictates the low-energy scattering of an electron with the parent neutral atom. We demonstrate that a single ultracold atom bound inside a Rydberg orbit forming an ultralong-range Rydberg molecule provides an atomic-scale system which is highly sensitive to electron-neutral scattering and thus allows for detailed insights into the underlying near-threshold anion states. Our measurements reveal the so far unobserved fine structure of the $^3P_J$ triplet of Rb$^-$ and allow us to extract parameters of the associated $p$-wave scattering resonances which deviate from previous theoretical estimates. Moreover, we observe a novel alignment mechanism for Rydberg molecules mediated by spin-orbit coupling in the negative ion.