# High-resolution spectroscopy of He$_2^+$ using Rydberg-series extrapolation and Zeeman-decelerated supersonic beams of metastable He$_2$

## TL;DR

This study advances high-resolution spectroscopy of He$_2^+$ ions by improving spectral resolution, extending measurements to higher rotational levels, and analyzing spin-rotational level transitions using Zeeman-decelerated metastable helium beams.

## Contribution

The paper introduces a method combining continuous-wave UV laser spectroscopy with Zeeman deceleration to achieve higher resolution and detailed level assignment in He$_2^+$ spectroscopy.

## Key findings

- Achieved over fivefold improvement in spectral resolution.
- Measured higher rotational intervals of He$_2^+$.
- Demonstrated control of $M_J$ redistribution processes.

## Abstract

Recently, high-resolution spectroscopy of slow beams of metastable helium molecules (He$_2^*$) generated by multistage Zeeman deceleration was used in combination with Rydberg-series extrapolation techniques to obtain the lowest rotational interval in the molecular helium ion at a precision of 18 MHz [Jansen et al. Phys. Rev. Lett. 115 (13) (2015) 133202], limited by the temporal width of the Fourier-transform-limited laser pulses used to record the spectra. We present here an extension of these measurements in which we have (1) measured higher rotational intervals of He${_2}^+$, (2) replaced the pulsed UV laser by a cw UV laser and improved the resolution of the spectra by a factor of more than five, and (3) studied $M_J$ redistribution processes in regions of low magnetic fields of the Zeeman decelerator and shown how these processes can be exploited to assign transitions originating from specific spin-rotational levels ($N^{\prime\prime},J^{\prime\prime}$) of He$_2^*$.