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Wavelength-Resolved Timing of Near-Infrared Scintillation in Gaseous Xenon

This study characterizes the wavelength-resolved timing of near-infrared scintillation in gaseous xenon by decomposing the signal into fast atomic and slow continuum components, ultimately identifying an ionic-dimer transition as the most plausible source of the dominant microsecond-scale emission through experimental measurements and theoretical modeling.

Original authors: Robert Hammann, Steffen Form, Humaira Nazneen, Gabriel Schöpfer, Milan Ončák, Teresa Marrodán Undagoitia

Published 2026-10-08

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CC BY 4.0

Original authors: Robert Hammann, Steffen Form, Humaira Nazneen, Gabriel Schöpfer, Milan Ončák, Teresa Marrodán Undagoitia

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Xenon is a heavy, invisible gas that scientists have long relied on to catch the faintest whispers of the universe. In massive detectors buried deep underground, this gas acts as a target for rare particles, such as those from dark matter or the mysterious decay of atomic nuclei. When a particle strikes a xenon atom, the gas flashes with light. For decades, researchers have focused on the ultraviolet part of this flash, the high-energy light that is easiest to catch with standard sensors. However, xenon also emits a second, dimmer glow in the near-infrared range, a color of light that sits just beyond what the human eye can see. This infrared signal has been known for some time, but its behavior has remained a mystery. Understanding exactly how and why this light appears, and how long it lasts, is crucial. If scientists can learn to read this infrared signal, they could improve the precision of their detectors, potentially unlocking new ways to see the invisible universe.

A team of researchers recently set out to solve this puzzle by watching how the infrared light behaves in gaseous xenon. They built a specialized chamber, a stainless steel cube filled with the gas, and bombarded it with alpha particles, a type of radiation that mimics the interactions they hope to study. To see the light, they used sensitive cameras capable of detecting individual photons across a wide range of infrared colors, from 950 to 1620 nanometers. By carefully timing when each photon arrived after a particle hit the gas, they could map out the life cycle of the flash. They discovered that the light does not appear all at once. Instead, it arrives in two distinct waves. The first is a very quick burst that lasts only a few billionths of a second. The second is a much slower, lingering glow that persists for several millionths of a second.

The researchers found that these two waves come from different sources. The fast, brief flash matches the exact colors of light emitted by individual, excited xenon atoms. It is the signature of single atoms returning to a calm state. The slow, long-lasting glow, however, tells a different story. Its color is not a sharp line but a broad, smooth band of light centered around 1300 nanometers. This suggests the light comes from pairs of atoms that have temporarily stuck together, forming a short-lived molecule known as an excimer. By measuring how the timing of this slow glow changed as they adjusted the pressure of the gas, the team was able to deduce the steps involved in its creation. They determined that the molecule forms through a three-way collision involving three particles, and then decays on its own. This decay process is surprisingly slow, with the light persisting for about 3 microseconds before fading away.

To understand exactly which type of molecule was responsible, the team turned to powerful computer simulations. They calculated the energy levels and behaviors of various possible pairings of xenon atoms, including neutral pairs and pairs that had lost an electron to become ions. They tested these models against their experimental data to see which one fit best. The simulations ruled out several popular theories. For instance, they found that a previously suggested type of neutral molecule would likely decay far too quickly to match the slow glow they observed. The calculations pointed instead to two strong candidates: a neutral pair of atoms in a specific excited state, and a pair of atoms where one has become an ion.

The evidence leans heavily toward the ion-based pair. This specific molecular configuration matched the color, the width, and the shape of the observed light band almost perfectly. Furthermore, the theoretical lifetime of this ion pair was consistent with the measured duration of the glow. This finding also helps explain why certain impurities affect the light. The researchers noted that oxygen gas strongly suppresses this infrared glow, while sulfur hexafluoride does not. Their model suggests that oxygen can steal an electron from the ion pair, breaking it apart and stopping the light, a process that sulfur hexafluoride cannot do because of its different energy requirements. While the team cannot yet say with absolute certainty which molecule is the sole source, the ionic pair provides the most consistent picture of the data.

This work transforms a vague signal into a detailed map of atomic behavior. By separating the fast atomic flashes from the slow molecular glow, and by identifying the likely molecular partner responsible for the infrared light, the researchers have provided a robust foundation for future experiments. Their findings suggest that this near-infrared signal is not just background noise, but a rich source of information about the microscopic interactions within the gas. As detectors grow larger and more sensitive, the ability to read this slow, infrared pulse could become a vital tool for distinguishing between different types of particles, bringing scientists one step closer to understanding the fundamental nature of the universe.

Technical Summary: Wavelength-Resolved Timing of Near-Infrared Scintillation in Gaseous Xenon

Problem Statement
While gaseous xenon (GXe) is a primary target for rare-event searches (e.g., neutrinoless double-beta decay and dark matter) due to its high vacuum ultraviolet (VUV) light yield, it also exhibits substantial near-infrared (NIR) scintillation. Although the NIR signal offers potential for improved particle discrimination and energy resolution, its microscopic origin and kinetic properties remain poorly understood. Previous studies identified a broad continuum centered around 1.3 µm and a slow time constant (µs-scale), but the specific molecular transitions responsible and the formation/decay kinetics were not established. Furthermore, the relationship between the NIR signal and atomic emission lines required clarification.

Methodology
The authors developed a new experimental setup consisting of a stainless steel cell filled with GXe (up to 1.7 bar) irradiated by a collimated $^{241}$ Am $\alpha$ -particle source. Scintillation light was detected by VUV-sensitive photomultiplier tubes (PMTs) and a NIR-sensitive PMT (Hamamatsu H10330C-75) equipped with a quick-release mount for optical filters.

Experimental Approach: The team measured the time response of the NIR signal in 12 discrete wavelength bands (950–1620 nm) using long-pass filters. Data were acquired at a pressure of $\sim$ 1000 mbar and across a pressure range of 200–1700 mbar to study kinetic dependencies.
Data Analysis: Arrival times of NIR photons were histogrammed relative to VUV trigger signals. The time distributions were modeled as a sum of fast (ns-scale) and slow (µs-scale) components, described by consecutive first-order reaction kinetics.
Theoretical Approach: The authors performed ab initio calculations using Multi-Reference Configuration Interaction (MRCI) with spin-orbit coupling to model excited states of neutral xenon dimers ( $Xe_2^*$ ) and ionic xenon dimers ( $Xe_2^+$ ). They calculated potential energy curves, transition energies, oscillator strengths, and lifetimes to identify candidates matching the experimental continuum.

Key Results

Spectral Decomposition: The NIR time response was successfully decomposed into two components:
- Fast Component: Coincides with strong atomic emission lines (predominantly $6p \to 6s$ transitions) below 1000 nm.
- Slow Component: Follows the broad continuum centered around 1.3 µm. This component dominates the signal in bands above 1000 nm.
Kinetic Modeling: The pressure dependence of the slow component in the (1300–1350) nm band was described by a two-step kinetic model involving a precursor state and an emitting state.
- The formation rate ( $\Gamma_1$ ) includes a three-body term ( $k_3 n^2$ ) and a density-independent term ( $k'$ ). The fitted $k_3 \approx 2.8 \times 10^{-31} \text{ cm}^6 \text{ s}^{-1}$ is consistent with known three-body association rates for $Xe_2^*$ and $Xe_2^+$ .
- The decay rate ( $\Gamma_2$ ) includes a two-body collisional term ( $k_2 n$ ) and a density-independent radiative term ( $k$ ). The fitted $k_2 \approx 3.5 \times 10^{-14} \text{ cm}^3 \text{ s}^{-1}$ suggests inefficient collisional de-excitation.
- Interpreting the density-independent decay term as radiative decay yields an intrinsic lifetime of $\sim 3 \mu\text{s}$ ( $1/k \approx 3140 \text{ ns}$ ).
Theoretical Identification:
- Calculations ruled out previously proposed transitions, such as the $(3)0^+_u \to (1)0^+_g$ transition, due to their short lifetimes (dominated by fast decay to the ground state) and spectral mismatches.
- Two candidates were identified that avoid fast ground-state decay channels:
  - Neutral Excimer: $(2)0^-_u \to (1)0^-_g$ . While parity-forbidden to the ground state, the calculated spectrum is redshifted by $\sim 2500 \text{ cm}^{-1}$ relative to the measurement, and the calculated lifetime (200 ns) is shorter than the measured value.
  - Ionic Dimer: $II(1/2)_u \to I(1/2)_g$ in $Xe_2^+$ . This transition reproduces the measured spectral position (7900 vs. 7700 cm $^{-1}$ ), width (800 vs. 1000 cm $^{-1}$ ), and shape. The calculated radiative lifetime (600 ns) is within a factor of five of the measured $\sim 3 \mu\text{s}$ .
- The ionic-dimer model offers a qualitative explanation for impurity dependence: charge transfer with $O_2$ (ionization energy $\sim 12.1$ eV) is energetically plausible for quenching, whereas $SF_6$ (ionization energy $\sim 15.3$ eV) is not, consistent with experimental observations.

Significance and Claims
The paper establishes a robust characterization of the NIR scintillation signal in GXe, distinguishing between atomic line emission and a molecular continuum. By combining wavelength-resolved timing with pressure-dependent kinetics and theoretical calculations, the authors propose that the dominant NIR continuum arises from an ionic-dimer transition ( $II(1/2)_u \to I(1/2)_g$ ) or potentially a neutral-excimer transition, rather than the previously hypothesized high-lying neutral states.

The authors state that identifying the specific transition is a prerequisite for utilizing the NIR signal to enhance the performance of xenon-based detectors, particularly regarding energy resolution and particle discrimination. They acknowledge that while the ionic-dimer transition provides the most consistent description of the data, a firm assignment requires improved calculations and further experimental constraints, such as measurements with controlled impurity admixtures and extended acquisition windows. The work does not claim to have definitively solved the microphysics but provides the most consistent model to date for the observed continuum.

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