Neutral Barium in Solid Neon: Optical Spectroscopy and First Excited State Lifetime

This study presents optical spectroscopy and the first lifetime measurement (0.39 ± 0.02 s) of the barium 5d6s ³D₁ state in a neon matrix at 6.8 K, characterizing matrix-induced shifts and relaxation channels to inform future electron electric dipole moment searches using barium monofluoride.

The Gist

Imagine you want to study how a specific type of dancer (a Barium atom) moves when they are in a crowded, freezing dance floor made of neon gas. Normally, if you try to watch a dancer in a chaotic crowd, they bump into everyone, their moves get messy, and it's hard to tell what they are actually doing.

This paper is about a team of scientists who built a special, super-cold, and very quiet "dance floor" (a neon crystal) to watch these Barium atoms move without getting bumped around too much. They wanted to see how the atoms behave, how long they stay in certain poses, and how they glow.

Here is the story of their experiment, broken down into simple parts:

1. The Setup: Building a Frozen Stage

The scientists needed a stage that was cold enough to freeze neon gas into a solid crystal, but not so cold that the Barium atoms would freeze solid and stop moving.

• The Stage: They used a block of solid neon, cooled to about -266°C (6.8 Kelvin). This is like a perfectly still, frozen lake.
• The Dancers: They shot tiny bits of Barium metal into this frozen lake using a high-powered laser (like a tiny, super-fast hammer hitting a target). The neon gas acted like a "cooling blanket," slowing the Barium atoms down until they got stuck inside the neon ice, perfectly spaced out.
• The Goal: They wanted to see how these trapped Barium atoms reacted when they were hit with light.

2. The Light Show: Two Ways to Wake Them Up

Once the Barium atoms were trapped in the neon ice, the scientists used two different "flashlights" to wake them up and see what they did.

Method A: The "Super Flash" (Pulsed Laser)
They used a very powerful, short burst of ultraviolet light (like a camera flash going off in a dark room).

• What happened: This blast knocked the Barium atoms high up into a very excited state. It's like throwing a ball so high it hits the ceiling.
• The Result: As the atoms fell back down to their normal state, they didn't just fall in one step. They bounced off the "ceiling" and the "walls" (other energy levels) on their way down, glowing with different colors of light at each step. This created a "cascade" of light, like a waterfall of glowing water.
• Discovery: They saw 8 different colors of light. Most of them were very close to the colors the atoms would glow if they were floating in empty space, which means the neon ice didn't mess them up too much.

Method B: The "Tunable Spotlight" (Continuous Laser)
Next, they used a softer, adjustable laser that could change colors (wavelengths) slowly.

• The Trick: In normal air, Barium atoms have strict rules about which colors of light they can absorb. But inside the neon ice, the "rules" get a little fuzzy because the ice pushes on the atoms slightly.
• What happened: The scientists found that by shining a specific color of light, they could push the atoms into a "metastable" state. Think of this as a "waiting room" where the atoms sit for a long time before moving again.
• The Double Laser: They even used two lasers at the same time. One laser put the atom in the "waiting room," and the second laser kicked it up to a higher level. This allowed them to force the atoms to glow in very specific, controlled ways.

3. The Big Discovery: How Long Do They Wait?

The most important part of the paper is a measurement of time.

• One specific state the Barium atoms get stuck in is called the $5d6s \ ^3D_1$ state.
• In a vacuum (empty space), this state lasts for a certain amount of time. In solid helium (a different, even colder ice), it lasts for about 2.7 seconds.
• The Measurement: The scientists measured how long this state lasted in their neon ice. They found it lasted about 0.39 seconds.
• Why it matters: While 0.39 seconds sounds short, in the world of atoms, that is an eternity! It means the neon ice is a very gentle host. It doesn't disturb the atom enough to make it lose its energy quickly.
• Prediction: They calculated that if they could get the neon ice even colder (down to -271°C or 2 Kelvin), the atom would stay in that state for about 0.42 seconds.

4. Why Should We Care? (The "So What?")

You might ask, "Why do we care about Barium atoms in neon ice?"

The answer lies in the future of detecting the impossible.
Scientists are trying to find the electron electric dipole moment (eEDM). This is a tiny, tiny property of the electron that, if found, could explain why the universe is made of matter instead of just disappearing. It's like looking for a tiny crack in a perfect mirror to understand why the mirror exists.

To find this, they plan to use a molecule called Barium Monofluoride (BaF) trapped in neon ice.

• The Problem: When they make BaF, some regular Barium atoms (the ones they just studied) get mixed in as "impurities."
• The Solution: Before they can study the complex BaF molecule, they need to know exactly how the "impurity" Barium atoms behave. If they don't understand the Barium atoms, they might mistake the Barium's glow for the BaF's glow, ruining the experiment.

Summary

This paper is like a user manual for Barium atoms when they are trapped in neon ice.

1. They built a super-cold neon crystal.
2. They trapped Barium atoms inside.
3. They used lasers to make the atoms glow and mapped out exactly how they move and how long they stay in specific poses.
4. They proved that neon is a "gentle" host that doesn't disturb the atoms too much.

This knowledge is the essential first step to building a much more complex experiment to search for new physics that could change our understanding of the universe.

Technical Summary

1. Problem Statement and Motivation

The study addresses the need for high-precision spectroscopic data on neutral barium (Ba) atoms embedded in cryogenic noble gas matrices. This research serves as a critical testbed for future experiments aiming to measure the electron electric dipole moment (eEDM) using barium monofluoride (BaF) molecules.

• The Challenge: In the production of BaF molecules via laser ablation in a neon buffer gas, unavoidable neutral barium impurities are often co-deposited into the matrix. These neutral atoms can act as sources of background noise and systematic errors in precision measurements.
• The Gap: While Ba has been studied in heavier noble gas matrices (Argon, Krypton, Xenon) and solid Helium, its behavior in solid Neon was not well characterized. Neon is a promising host due to its low perturbation of embedded species and high optical transparency, but specific data on Ba energy level shifts, linewidths, and metastable state lifetimes in Neon were missing.

2. Methodology

The researchers employed Matrix Isolation Spectroscopy (MIS) combined with Laser-Induced Fluorescence (LIF) to study Ba atoms in a solid Neon crystal at temperatures between 6.8 K and 9 K.

• Sample Production:

  • A custom cryogenic ablation cell was used to generate neutral Ba atoms via laser ablation of a metallic target.
  • The atoms were cooled by a Neon buffer gas (5–20 SCCM flow) and deposited onto a CaF₂ substrate cooled to 6.8 K by a pulse tube cryocooler.
  • Crystal growth occurred over 20–60 minutes, creating a high-density doped Neon matrix.

• Excitation Schemes:

  1. High-Energy Pulsed Excitation: A 10-ns, 355 nm (3rd harmonic of Nd:YAG) laser was used to directly excite high-lying energy levels, triggering fluorescence cascades down to lower states.
  2. Continuous Wave (CW) Excitation: Tunable Ti:Sa lasers (700–900 nm, 10–200 mW) were used for:
     • Single-laser excitation: To probe low-lying transitions and matrix-induced symmetry breaking.
     • Double-laser excitation: Two overlapping CW lasers were used to selectively pump specific pathways (e.g., populating intermediate metastable states before exciting to higher levels).

• Detection:

  • Fluorescence was collected orthogonally to the excitation beam using a custom lens assembly.
  • Detection was performed using a spectrometer (visible range), an InGaAs spectrometer (near-infrared), and a photodiode for lifetime measurements.
  • An Electro-Optic Modulator (EOM) was used to switch the excitation laser on/off (200 ns pulses) to measure decay times.

3. Key Contributions

• First Growth of High-Quality Ba-Doped Neon Crystals: Successfully demonstrated the production of stable, high-density Ba-doped Neon crystals at 6.8–9 K.
• Comprehensive Spectroscopic Mapping: Identified and characterized multiple radiative pathways and relaxation channels for Ba in Neon, including transitions that are forbidden in the gas phase but become allowed due to matrix-induced symmetry breaking.
• First Lifetime Measurement in Neon: Measured the lifetime of the metastable $5d6s \ ^3D_1$ state of neutral barium in a Neon matrix for the first time.
• Validation of Double-Laser Pumping: Demonstrated a selective pumping scheme to populate specific high-lying states ($5d6p \ ^3F_2$) via intermediate metastable states, confirming the feasibility of complex excitation pathways in solid Neon.

4. Key Results

A. Spectral Shifts and Linewidths

• Small Perturbation: The Neon matrix caused relatively small spectral shifts compared to free gas-phase transitions (ranging from $-93$ to $+124 \ cm^{-1}$). This is significantly smaller than shifts observed in heavier noble gases (Ar, Kr, Xe), confirming Neon's low-perturbation nature.
• Linewidths: The Full Width at Half Maximum (FWHM) varied by transition. For example, the $6s6p \ ^1P_1 \to 6s^2 \ ^1S_0$ transition (Peak C) had a width of $156 \ cm^{-1}$, which is broader than in solid Helium ($30 \ cm^{-1}$) but consistent with the general trend of Neon being a "softer" host than heavier gases.
• Forbidden Transitions: Transitions forbidden in the gas phase (e.g., $5d6s \ ^3D_1 \to 6s^2 \ ^1S_0$) were observed, attributed to the breaking of atomic symmetries by the local crystal field.

B. Excitation Pathways

• Single-Laser: Indirect excitation was observed where laser light tuned to the $6s6p \ ^3P_j$ manifold (via matrix-induced relaxation) populated the long-lived $5d6s \ ^3D_1$ and $^1D_2$ states.
• Double-Laser: A specific scheme was validated:
  1. Laser 1 (749 nm) populates the $5d6s \ ^3D_2$ state.
  2. Laser 2 (780 nm) excites from $5d6s \ ^3D_2$ to $5d6p \ ^3F_2$.
  3. This resulted in a cascade fluorescence: $5d6p \ ^3F_2 \to 5d6s \ ^1D_2$ (937 nm) followed by decay to the ground state (877 nm).

C. Metastable State Lifetime

• Measured Value: The lifetime of the $5d6s \ ^3D_1$ state in solid Neon at 6.8 K was measured to be $0.39 \pm 0.02$ seconds.
• Temperature Dependence: Using a rate equation model considering radiative ($\tau_r$) and non-radiative ($\tau_{nr}$) decay channels, the authors extrapolated that the lifetime would increase to approximately $0.42 \pm 0.03$ seconds at 2 K.
• Comparison: This is significantly shorter than the lifetime in solid Helium ($2.72$ s at 1.5 K), indicating that Neon, while a low-perturbation host, still introduces non-radiative decay channels (likely phonon-assisted) that are less active in Helium.

5. Significance and Future Outlook

• Baseline for BaF eEDM Searches: The detailed spectral data provides a crucial reference for distinguishing between neutral Ba impurities and BaF molecules in future eEDM experiments. Understanding the "background" Ba signal is essential for achieving the required sensitivity.
• Host Material Evaluation: The study confirms Neon as a viable, low-perturbation host for matrix isolation, offering a stable crystalline structure suitable for long data acquisition times.
• Parahydrogen Comparison: The results serve as a benchmark for future studies in parahydrogen matrices. While Rubidium showed no fluorescence in parahydrogen, the clear emission of Ba in Neon suggests that Ba and BaF might be viable candidates for parahydrogen hosts, pending further investigation.
• Fundamental Physics: The observation of long-lived metastable states and the ability to manipulate them with double-laser schemes opens new avenues for precision spectroscopy and quantum control in solid-state environments.

In conclusion, this work establishes a robust spectroscopic foundation for using neutral barium in solid neon, directly supporting the development of next-generation experiments to probe fundamental physics violations like the electron electric dipole moment.