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# Determining the nature of quantum resonances by probing elastic and reactive scattering in cold collisions

## Abstract

Scattering resonances play a central role in collision processes in physics and chemistry. They help build an intuitive understanding of the collision dynamics due to the spatial localization of the scattering wavefunctions. For resonances that are localized in the reaction region, located at short separation behind the centrifugal barrier, sharp peaks in the reaction rates are the characteristic signature, observed recently with state-of-the-art experiments in low-energy collisions. If, however, the localization occurs outside of the reaction region, mostly the elastic scattering is modified. This may occur due to above-barrier resonances, the quantum analogue of classical orbiting. By probing both elastic and inelastic scattering of metastable helium with deuterium molecules in merged-beam experiments, we differentiate between the nature of quantum resonances—tunnelling resonances versus above-barrier resonances—and corroborate our findings by calculating the corresponding scattering wavefunctions.

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## Data availability

The data that support the findings of this study are available from the corresponding authors on reasonable request. Source Data for Fig. 3 is provided with the paper.

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## Acknowledgements

We acknowledge financial support from the European Research Council and the Israel Science Foundation. Additional financial support from the German–Israeli Foundation (grant no. 1254) is gratefully acknowledged. C.P.K. is grateful for a Rosi and Max Varon Visiting Professorship. Correspondence and requests for materials should be addressed to E.N. and C.P.K.

## Author information

Authors

### Contributions

P.P. and N.D. carried out the experiments. D.M.R. performed the calculations with help from A.v.d.A. C.P.K. and E.N. conceived and supervised the work. All authors discussed the results and wrote the manuscript.

### Corresponding authors

Correspondence to Christiane P. Koch or Edvardas Narevicius.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

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## Extended data

### Extended Data Fig. 1 Partial and total rate coefficients for elastic scattering of He* with normal D2.

The black curve denotes the partial elastic scattering rate coefficient integrated only over the backward hemisphere. The red curve denotes the total elastic scattering rate coefficient (integrated over all scattering angles). Note the different y-scales for the partial and total rate coefficients.

## Source data

### Source Data Fig. 3

Rate coefficients as a function of collision energy for elastic collisions and Penning ionization of He* + D2.

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Reprints and Permissions

Paliwal, P., Deb, N., Reich, D.M. et al. Determining the nature of quantum resonances by probing elastic and reactive scattering in cold collisions. Nat. Chem. 13, 94–98 (2021). https://doi.org/10.1038/s41557-020-00578-x

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