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# Reaction blockading in a reaction between an excited atom and a charged molecule at low collision energy

## Abstract

Recent advances have enabled studies of atom–ion chemistry at unprecedentedly low temperatures, allowing precision observation of chemical reactions and novel chemical dynamics. So far, these studies have primarily involved reactions between atoms and atomic ions or non-polar molecular ions, often in their electronic ground state. Here, we extend this work by studying an excited atom–polar-molecular-ion chemical reaction (Ca* + BaCl+) at low temperature in 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 differ dramatically 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. This reaction blockading effect, which greatly suppresses the reactivity of short-lived excited states, provides a means for directly probing the reaction range and also naturally suppresses unwanted chemical reactions in hybrid trapping experiments.

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## Data availability

The experimental data sets (displayed in Figs. 1 and 2) are available from the Harvard Dataverse online repository at https://dataverse.harvard.edu/dataverse/cabacl.

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## Acknowledgements

This work was supported by grants from the National Science Foundation (PHY-1205311, PHY-1806653 and DGE-1650604) and the Army Research Office (W911NF-15-1-0121, W911NF-14-1-0378 and W911NF-13-1-0213).

## Author information

P.P. and M.M. acquired and analysed all experimental data presented in the work. P.P evaluated the phase space model for interpreting the experimental branching ratios. I.S., R.C. and P.P. provided the framework for the presented long-range capture model, while J.A.M. performed the electronic structure calculations utilized to understand short-range reaction dynamics. P.P., M.M., I.S., J.A.M. and R.C. contributed to the figures presented in the work. C.S. provided valuable experimental insight, and A.G.S. and E.R.H. provided guidance for the entire project and played key roles in merging the theoretical calculations with the experimental findings. P.P. and E.R.H. prepared the manuscript and all authors provided useful comments.

### Competing interests

The authors declare no competing interests.

Correspondence to Prateek Puri.

## Supplementary information

### Supplementary Information

Supplementary methods, Supplementary Figs. 1 and 2

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• #### DOI

https://doi.org/10.1038/s41557-019-0264-3