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

Abstract

Matrix isolation spectroscopy enables probing atomic properties in controlled cryogenic environments. We present a spectroscopic study on neutral barium atoms embedded in a neon cryogenic crystal at 6.8 K, extending previous investigations performed in other noble gas hosts. The visible and near-infrared emission spectra were recorded under two different laser excitation schemes. First, 10-ns laser pulses at 355 nm were used to directly excite high-lying energy levels of barium, enabling the observation of fluorescence cascades. Second, a tunable continuous-wave laser operating between 700 nm and 900 nm allowed us to determine the matrix-induced shifts of barium energy levels relative to their vacuum values, as well as the inhomogeneous linewidths of the observed transitions and to perform lifetime measurements. Our results confirm multiple radiative pathways and matrix-induced relaxation channels affecting the 5d6s and 6s6p barium manifolds. Furthermore, we present the first lifetime measurement of the barium 5d6s 3D1 state in a neon crystal, yielding 0.39 \pm 0.02 s, with a predicted increase of about 10% at 2 K. The study of fluorescence and spectroscopic properties of barium isolated in neon represents an important step toward future searches for the electron electric dipole moment using barium monofluoride in neon matrices, where neutral barium atoms may act as unavoidable impurities and potential sources of background and systematic limitations.

Figures (11)

• Figure 1: Top view sketch of the experimental setup including production of a gas phase barium beam, crystal growth, excitation, and fluorescence detection: A) Barium production and doped neon crystal growth setup: two lasers are simultaneously used. An Nd:YAG laser (light blue arrows in the figure) is used for the ablation of the metallic barium target. An Avesta laser (red arrows in the figure) at 12636 cm$^{-1}$ (791.13 nm) is used to probe the presence of barium atoms, ensuring efficient production both inside and outside the cell. B) Excitation and fluorescence detection scheme: the gas composed of Ba and neon is sprayed onto the CaF$_2$ substrate (orange rectangle) at 6.8 K, which is permanently tilted to minimize the collection of excitation light in the fluorescence detection system. Purple arrows indicate the excitation laser. A custom built lens assembly allows us to fix the first collection lens at about 30 mm from the crystal growth spot, enabling efficient collection of the emitted signal which is then focused on either a spectrometer or a photodiode, depending on the measurement.
• Figure 2: Energy level diagram (not to scale) of neutral barium showing the electronic states relevant to this work. The ground state 6s$^2\; ^1\text{S}_0$ is shown alongside the low-lying excited states 5d6s $^1\text{D}_2$ and 5d6s $^3\text{D}_{1,2,3}$. Higher-lying manifolds include $6s6p \; ^1\text{P}_1$, 6s6p $^3\text{P}_{0,1,2}$, 5d6p $^3\text{F}_{2,3,4}$, 6s6d $^3\text{D}_{1,2,3}$ and 6s9s $^3$S$_1$. These states are involved in the excitation and fluorescence processes discussed throughout the paper. Red and green lines indicates fluorescence cascade transitions identified and shown in Table \ref{['Table_3rd_harm']}. Blue lines represent third harmonic excitation at 355 nm.
• Figure 3: Fluorescence spectrum of atomic barium in a neon cryogenic matrix at 6.8 K when excited with a third harmonic (355 nm) of a Nd:YAG laser with a repetition rate of 10 Hz. In gray: experimental data (baseline-subtracted); black solid: multi-Gaussian total fit; black dashed: individual fitted transitions. The region above 750 nm is magnified ×5. List of cascade fluorescence detected: A - 6s9s $^3\text{S}_1$ to 6s6p $^1\text{P}_1$; B - 6s6d $^3\text{D}_1$ to 6s6p $^3\text{P}_0$; C - 6s6p $^1\text{P}_1$ to 6s$^2$$^1\text{S}_0$; D - 6s6d $^3\text{D}_3$ to 6s6p $^3\text{P}_2$; E - 5d6p $^3\text{F}_2$ to 5d6s $^3\text{D}_1$; F - 5d6p $^3\text{F}_2$ to 5d6s $^3\text{D}_2$; G - 6s6p $^3\text{P}_1$ to 6s$^2$$^1\text{S}_0$; H - 5d6s $^1\text{D}_2$ to 6s$^2$$^1\text{S}_0$.
• Figure 4: Energy level diagram (not to scale) of the single laser pumping scheme of neutral barium atoms in a neon matrix. Both excitation lasers, at 13 514 cm$^{-1}$ (749.8 nm, green) and 13 369 cm$^{-1}$ (740.5 nm, red), populate the 6s6p $^3$P$_j$ manifold. The system is relaxed to the states 5d6s $^3$D$_1$ and 5d6s $^1$D$_2$, which then decay to the ground state. These population transitions are detected respectively at 9 033.4 cm$^{-1}$ (1107.1 nm, green) and 11 494.2 cm$^{-1}$ (870.0 nm, red).
• Figure 5: 2D excitation–emission spectrum of barium in a neon crystal at 6.8 K when shined with a single laser at 35 mW on a 1 mm wide spot. A strong emission band is observed, with a maximum intensity at 11 415 cm$^{-1}$ (876.0 nm), corresponding to population of the 5d6s $^1$D$_2$ state ($+20$ cm$^{-1}$ shift), when the laser is tuned at 13 504.3 cm$^{-1}$ (740.5 nm). A long blue tail is visible until the reach of our laser at 700 nm of excitation. The residual signal of the excitation laser appears at longer wavelengths due to the reduced efficiency of the optical filtering.