Optical excitation and decay dynamics of ytterbium atoms embedded in a solid neon matrix

Opportunities for fundamental physics research with radioactive molecules

Molecules containing short-lived, radioactive nuclei are uniquely positioned to enable a wide range of scientific discoveries in the areas of fundamental symmetries, astrophysics, nuclear structure, and chemistry. Recent advances in the ability to create, cool, and control complex molecules down to the quantum level, along with recent and upcoming advances in radioactive species production at several facilities around the world, create a compelling opportunity to coordinate and combine these efforts to bring precision measurement and control to molecules containing extreme nuclei. In this manuscript, we review the scientific case for studying radioactive molecules, discuss recent atomic, molecular, nuclear, astrophysical, and chemical advances which provide the foundation for their study, describe the facilities where these species are and will be produced, and provide an outlook for the future of this nascent field.

Spectroscopy of Alkali Atoms in Solid Matrices of Rare Gases: Experimental Results and Theoretical Analysis

We present an experimental and theoretical investigation of the spectroscopy of dilute alkali atoms in a solid matrix of inert gases at cryogenic temperatures, specifically Rubidium atoms in a solid Argon or Neon matrix, and related aspects of the interaction energies between the alkali atoms and the atoms of the solid matrix. The system considered is relevant for matrix isolation spectroscopy, and it is at the basis of a recently proposed detector of cosmological axions, exploiting magnetic-type transitions between Zeeman sublevels of alkali atoms in a magnetic field, tuned to the axion mass, assumed in the meV range. Axions are one of the supposed constituents of the dark matter (DM) of the Universe. This kind of spectroscopy could be also relevant for the experimental search of new physics beyond the Standard Model, in particular the search of violations of time-reversal or parity-charge-conjugation (CP) symmetry. In order to efficiently resolve the axion-induced transition in alkali-doped solid matrices, it is necessary to reduce as much as possible the spectral linewidth of the electronic transitions involved. The theoretical investigation presented in this paper aims to estimate the order of magnitude of the inhomogeneous contribution to the linewidth due to the alkali–matrix interactions (Coulomb/exchange and dispersion), and to compare the theoretical results with our experimental measurements of spectra of dilute Rubidium atoms in Argon and Neon solid matrix. The comparison of the expected or measured spectral linewidths will be important for selecting the most appropriate combination of alkali atoms and matrix inert elements to be used in the proposed axion detection scheme. It is finally suggested that dilute Lithium atoms diffused in a cold parahydrogen solid matrix could be, overall, a good system upon which the proposed detector could be based.

Experimental setup for the growth of solid crystals of inert gases for particle detection

Low energy threshold detectors are necessary in many frontier fields of the experimental physics. In this work, we present a novel detection approach based on pure or doped matrices of inert gases solidified at cryogenic temperatures. The small energy release of the incident particle can be transferred directly (in pure crystals) or through a laser-driven ionization (in doped materials) to the electrons of the medium that are then converted into free electrons. The charge collection process of the electrons that consists in their drift within the crystal and their extraction through the solid-vacuum interface gives rise to an electric signal that we exploit for preliminary tests of charge collection and crystal quality. Such tests are carried out in different matrices of neon and methane using an UV-assisted apparatus for electron injection in crystals.

Longitudinal Spin Relaxation of Optically Pumped Rubidium Atoms in Solid Parahydrogen

We have grown crystals of solid parahydrogen using a single closed-cycle cryostat. We have doped the crystals with rubidium atoms at densities on the order of 10^{17}  cm^{-3} and used optical pumping to polarize the spin state of the implanted atoms. The optical spectrum of the rubidium atoms shows larger broadening than previous work in which the rubidium was implanted in solid argon or neon. However, the optical pumping behavior is significantly improved, with both a larger optical pumping signal and a longer longitudinal relaxation time. The spin relaxation time shows a strong dependence on orthohydrogen impurity levels in the crystal, as well as the applied magnetic field. Current performance is comparable to state-of-the-art solid state systems at comparable spin densities, with potential for improvement at higher parahydrogen purities.

Measurement of the hyperfine quenching rate of the clock transition in 171YB

We report the first experimental determination of the hyperfine quenching rate of the 6s(2) (1)S(0)(F = 1/2) - 6s6p (3)P(0)(F = 1/2) transition in (171)Yb with nuclear spin I = 1/2. This rate determines the natural linewidth and the Rabi frequency of the clock transition of a Yb optical frequency standard. Our technique involves spectrally resolved fluorescence decay measurements of the lowest lying (3)P(0,1) levels of neutral Yb atoms embedded in a solid Ne matrix. The solid Ne provides a simple way to trap a large number of atoms as well as an efficient mechanism for populating (3)P(0). The decay rates in solid Ne are modified by medium effects including the index-of-refraction dependence. We find the (3)P(0) hyperfine quenching rate to be (4.42 ± 0.35) × 10(-2) s(-1) for free (171)Yb, which agrees with recent ab initio calculations.

Electronic spectroscopy of ytterbium in a neon matrix

The low-lying electronic states of Yb isolated in a solid Ne matrix are characterized through absorption and emission spectroscopy. The absorption spectra of matrix isolated Yb while pumped into its triplet states have been recorded for the first time and the 6s6p (3)P(J) → 5d6s (3)D(1, 2) transition frequencies obtained. Under matrix conditions, the structure of these states is found to be qualitatively the same as in the free atom, but the intersystem crossing rate is observed to be several orders of magnitude greater. A proposed explanation for this is curve crossings between the bound potential energy surface correlated to the 6s6p (1)P(1) state and the potential energy surfaces correlated to the 5d6s (3)D(1, 2) states in isolation. The potentials of the Yb·Ne dimer in its lowest electronic states are computed ab initio and used in a pairwise cluster model to explicitly demonstrate these curve crossings.

Excitation and emission spectra of rubidium in rare-gas thin-films

To understand the optical properties of atoms in solid state matrices, the absorption, excitation, and emission spectra of rubidium doped thin-films of argon, krypton, and xenon were investigated in detail. A two-dimensional spectral analysis extends earlier reports on the excitation and emission properties of rubidium in rare-gas hosts. We found that the doped crystals of krypton and xenon exhibit a simple absorption-emission relation, whereas rubidium in argon showed more complicated spectral structures. Our sample preparation employed in the present work yielded different results for the Ar crystal, but our peak positions were consistent with the prediction based on the linear extrapolation of Xe and Kr data. We also observed a bleaching behavior in rubidium excitation spectra, which suggests a population transfer from one to another spectral feature due to hole-burning. The observed optical response implies that rubidium in rare-gas thin-films is detectable with extremely high sensitivity, possibly down to a single atom level, in low concentration samples.