Possible evidence for the production of Ar$_2^{*-}$ metastable negative molecular ions in gaseous argon of two-phase detectors for dark matter searches

TL;DR

This study provides evidence for the existence of two types of metastable negative argon molecular ions in gaseous argon, inferred from delayed electroluminescence signals in two-phase detectors used for dark matter searches.

Contribution

The paper presents the first experimental evidence for metastable negative argon molecular ions, identified through pressure-dependent electroluminescence measurements.

Findings

Detection of two slow components in electroluminescence signals.

Identification of two metastable negative argon molecular ions.

Pressure dependence supports the ion states hypothesis.

Abstract

Our recent studies of electroluminescence (EL) properties in two-phase argon detectors for dark matter searches have revealed the presence of unusual delayed pulses in the EL signal in the form of two slow components with time constants of about 5 and 50 $\mu$s. These components were shown to be present in the charge signal itself, which clearly indicates that drifting electrons are temporarily trapped on two states of metastable negative argon ions which have never been observed before. In this work, using the pressure dependence of the ratio of slow component contributions measured in experiment, it is deduced that these states are those of two types of metastable negative molecular ions, $\mathrm{Ar}_2^{*-}(b \ ^4\Sigma_u^-)$ and $\mathrm{Ar}_2^{*-}(a \ ^4\Sigma_g^+)$ for the higher and lower energy level respectively.