# Reaction blockading in charged-neutral excited-state chemistry at low collision energy

## TL;DR

This study investigates low-energy excited-state reactions between calcium atoms and barium chloride ions, revealing that radiative lifetimes dominate reaction dynamics over potential interactions, with implications for controlling reactivity and coherence in hybrid traps.

## Contribution

It introduces a simple rule to determine when radiative lifetime effects dominate reaction dynamics, highlighting a new regime where reaction rates are suppressed by state lifetime.

## Key findings

- Reaction rates deviate from capture theory at low energies.
- Radiative lifetime dictates reaction dynamics over potential interactions.
- Short-lived states show suppressed reactivity, enabling longer coherence.

## Abstract

We study an excited atom-polar molecular ion chemical reaction (Ca$^*$ + BaCl$^+$) at low temperature by utilizing a hybrid atom-ion trapping system. The reaction rate and product branching fractions are measured and compared to model calculations as a function of both atomic quantum state and collision energy. At the lowest collision energy we find that the chemical dynamics dramatically differ from capture theory predictions and are primarily dictated by the radiative lifetime of the atomic quantum state instead of the underlying excited-state interaction potential. We provide a simple rule for calculating at what temperature this regime, where the collision complex lifetime is longer than the radiative lifetime of the quantum state, is reached. This effect, which greatly suppresses the reactivity of short-lived excited states, provides a means for directly probing reaction range. It also naturally suppresses unwanted chemical reactions in hybrid trapping experiments, allowing longer molecular ion coherence and interrogation times.