# Quantum-State-Specific Reaction Rate Measurements for the Photo-induced Reaction Ca$^+$ + O$_2$ $\rightarrow$ CaO$^+$ + O

## TL;DR

This study measures how the reaction rate of Ca$^+$ with O$_2$ depends on the specific quantum state of Ca$^+$, using laser cooling and trapping to control and analyze state-dependent reactivity.

## Contribution

It introduces a method to control and measure reaction rates for specific quantum states of ions, providing detailed insights into state-dependent reaction mechanisms.

## Key findings

- Reaction rates differ significantly between excited states.
- Laser detuning controls the population of excited states.
- Independent rate constants were measured for each state.

## Abstract

Atoms and molecules often react at different rates depending on their internal quantum states. Thus, controlling which internal states are populated can be used to manipulate the reactivity and can lead to a more detailed understanding of reaction mechanisms. We demonstrate this control of reactions by studying the excited state reaction reaction Ca$^+$ + O$_2$ $\rightarrow$ CaO$^+$ + O. This reaction is exothermic only if Ca$^+$ is in one of its excited electronic states. Using laser-cooling and electrodynamic trapping, we cool and trap Ca$^+$ at millikevin temperatures for several minutes. We can then change the fraction of time they spend in each of the two excited states by adjusting the detunings of the cooling lasers. This allows us to disentangle the reactions that begin with Ca$^+$ in the $^2$P$_{1/2}$-state from the ones where Ca$^+$ is in the $^2$D$_{3/2}$-state. Using time-of-flight mass spectrometry, we determine independent reaction rate constants for Ca$^+$ in both electronically excited quantum states.