# Optical Quenching of Metastable Helium Atoms using Excitation to the $4P$ State

## TL;DR

This paper introduces an optical method to selectively deplete metastable helium atoms in a beam by exciting them to the 4$^1$P state, enabling precise control and measurement of their state populations.

## Contribution

The study presents a novel optical depletion technique for metastable helium atoms using excitation to the 4$^1$P state, with demonstrated near-unity efficiency across various beam velocities.

## Key findings

- Achieved near 100% depletion efficiency at all studied velocities.
- Enabled direct measurement of singlet-to-triplet ratio in helium beams.
- Discovered variation in the singlet-to-triplet ratio at the trailing edges of gas pulses.

## Abstract

Discharge and electron-impact excitation lead to the production of metastable helium atoms in two metastable states, 2$^1$S$_0$ and 2$^3$S$_1$. However, many applications require pure beams of one of these species or at least a detailed knowledge of the relative state populations. In this paper, we present the characterization of an original experimental scheme for the optical depletion of He(2$^1$S$_0$) in a supersonic beam which is based on the optical excitation of the 4$^1$P$_1 \leftarrow 2^1$S$_0$ transition at 397 nm using a diode laser. From our experimental results and from a comparison with numerical calculations, we infer a near unit depletion efficiency at all beam velocities under study (1070 m/s $\leq v \leq$ 1750 m/s). Since the technique provides a direct means to determine the singlet-to-triplet ratio in a pulsed supersonic helium beam, our results show that the intrabeam singlet-to-triplet ratio is different at the trailing edges of the gas pulse.