Few-electron molecules are attractive systems for precision spectroscopy because their properties can be calculated with high accuracy by quantum-chemical Methods (1,2,3). The measurements serve to test theoretical predictions, ideally at the level where their accuracy is limited by the uncertainties of the fundamental constants or by unrecognized physical effects. I will report on precision measurements of energy intervals in cold samples of H₂. In particular, we determine the ionization energy with a precision (∆ν/ν) of 10⁻¹⁰ from high-resolution Rydberg spectra (4,5) and derive the dissociation energy with an accuracy of 350 kHz (5), approaching the level where the size of the proton and the uncertainty in the proton-to-electron mass ratio would limit the accuracy of otherwise exact calculations. Comparison will be made to recent theoretical results (2) in the context of a more-than-100-year-long series of experimental and theoretical determinations of the dissociation energy of H₂. I will also discuss the determination of an upper bound for a hypothetical global shift of the energy level structure of ortho-H₂ with respect to that of para-H₂ (6). If time permits, I will also present the results of similar experiments in He₂ (7). 1.V. I. Korobov, L. Hilico and J.-P. Karr, Phys. Rev. Lett. 118, 23, 001 (2017). 2.M. Puchalski, J. Komasa, P. Czachorowski, and K. Pachucki, Phys. Rev. Lett. 122, 103003 (2019). 3.E. Mátyus, J. Chem. Phys. 149, 194112 (2018). 4.M. Beyer, N. Hölsch, J. A. Agner, J. Deiglmayr, H. Schmutz and F. Merkt, Phys. Rev. A 97, 012501 (2018). 5.N. Hölsch, M. Beyer, E. J. Salumbides, K. S. E. Eikema, W. Ubachs, Ch. Jungen, and F. Merkt, Phys. Rev. Lett. 122, 103002 (2019). 6.M. Beyer, N. Hölsch, J. Hussels, C.-F. Cheng, E. J. Salumbides, K. S. E. Eikema, W. Ubachs , Ch. Jungen, and F. Merkt, Phys. Rev. Lett. 123, 163002 (2019). 7.L. Semeria, P. Jansen, G.-M. Camenisch, F. Mellini, H. Schmutz, and F. Merkt, to be submitted

We present gapless, high-resolution absorption and dispersion spectra obtained with quantum cascade laser frequency combs covering 55 cm⁻¹. Using a phase-sensitive dual comb design, the comb lines are gradually swept over 10 GHz, corresponding to the free spectral range of the laser devices, by applying a current modulation. We show that with interleaving the spectral point spacing is reduced by more than four orders of magnitude from 9.8 GHz down to 300 kHz over the full spectral span of the frequency comb. The noise equivalent absorbance (NEA) is 7x10⁻⁵ Hz^−1/2 and 1x10⁻³ Hz^−1/2 for strong and weak comb lines, respectively. The potential of this technique for high-precision gas sensing is illustrated by measuring the low pressure (107 hPa) absorption and dispersion spectra of methane spanning the range of 1170 cm⁻¹- 1225 cm⁻¹within only 120 ms.[1] In short, quantum cascade laser dual-comb spectroscopy with laser current modulation and interleaving enables acquisition of absorption spectra on a broad spectral range with sub-second temporal resolution, sub-MHz spectral resolution, and outstanding NEA. [1] M. Gianella et al., Optics Express 28, 6197-6208 (2020), doi: 10.1364/OE.379790.

It was recently shown that coupling of ultralight scalar dark matter to the standard model leads to oscillations of fundamental constants and, therefore, may be observed in clock-comparison experiments. Highly charged ions such as Ir¹⁷⁺ allow for the development of novel atomic clocks with high sensitivity to the variation of the fine-structure constant and, therefore, dark matter searches. The clock transitions are weak and very difficult to identity without accurate theoretical predictions. In the case of Ir¹⁷⁺, even stronger electric-dipole (E1) transitions eluded observations despite years of effort raising the possibility that theory predictions are grossly wrong. In this work, we have developed a broadly-applicable approach that drastically increases the ability to accurately predict properties of complex atoms and applied it to Ir¹⁷⁺ providing accurate predictions of transition wavelengths and E1 transition rates. Our results explain the lack of observation of the E1 transitions and provide a pathway towards detection of clock transitions. Computational advances demonstrated in this work are widely applicable to most elements in the periodic table and will allow to solve numerous problems in atomic physics, astrophysics, and plasma physics. We are currently developing an online portal with access to a database of high-precision atomic properties and a package of atomic codes that can be used to compute these properties.

We have carried out infrared-infrared double-resonance spectroscopy of the 2ν₃ A₁ - ν₃ band of methane. The ν₃ band transitions are pumped using a 90.5-THz difference-frequency-generation (DFG) source frequency-controlled with an optical frequency comb (OFC), and ten tetrahedral components of the Q(1) to Q(4) transitions from the pumped levels are observed with a linewidth of 0.8 MHz using another 88.4-THz DFG source. The transition frequencies are determined with an uncertainty of a few tens kilohertz using the OFC.

Yb-containing molecules exhibit strongly enhanced sensitivity to physics beyond the Standard Model, including the symmetry-violating electron electric dipole moment (eEDM). Laser cooling and trapping are necessary in order to fully leverage this intrinsic sensitivity. To this end, we present medium resolution laser-induced fluorescence (LIF) excitation spectra of a rotationally cold sample of YbOH in the 17300-17950 cm⁻¹ range recorded using two-dimensional (excitation and dispersed fluorescence) spectroscopy. High resolution dispersed LIF (DLIF) spectra and radiative lifetimes of numerous bands detected in the medium resolution spectra are described. The vibronic energy levels of the X̃  ²Σ⁺ state are predicted using a discrete variable representation approach and compared with observations. The DLIF spectra are analyzed to determine vibrational branching ratios and transition dipole moments, important determinants in the efficacy of laser cooling. Implications for laser cooling and trapping of YbOH are discussed.

Precise measurements of vibrational transition frequencies in the isotopes of molecular hydrogen can provide a sensitive probe of fundamental physics. Because these transitions can be predicted with high-level ab-initio theory, comparisons between theory and experiment can be used to test quantum electrodynamics, search for new physics, and determine the nucleon-electron mass ratios more precisely. In this talk, I will present our measurement of the 0−1 R(0) transition frequency in hydrogen deuteride (HD) using infrared-ultraviolet double resonance spectroscopy in a pulsed supersonic molecular beam.Fast et al., arXiv:2002.09333D molecules in the v=0, J=0 state are excited to v=1, J=1 using a tunable infrared laser stabilized to an optical frequency comb, and the excitation efficiency is determined by state-selectively ionizing the vibrationally-excited molecules using a pulsed UV laser. We have determined the absolute frequency of the transition with an uncertainty of 13 kHz (0.12 ppb relative uncertainty), limited primarily by the first order Doppler shift. Improvements in the compensation of this shift should make it possible to reduce the uncertainty by another order of magnitude.

---

Footnotes:

Fast et al., arXiv:2002.09333H

Molecules that have been laser-cooled in the lab are still limited to diatomic and linear triatomic species. Extension of laser cooling to include symmetric as well as asymmetric tops could significantly broaden the applications of cold (and ultra-cold) molecules. Recently, a practical roadmap to achieve optical cycling and laser cooling of asymmetric-top molecules has been proposed by Kozyryev et al.B. L. Augenbraun, J. M. Doyle, T. Zelevinsky, and I. Kozyryev, arXiv:2001.11020 [physics.atom-ph] (2020).uccessful optical cycling and laser cooling require a detailed understanding of ro-vibronic structure and transition probabilities between ro-vibronic levels. Most candidate molecules for laser cooling are open-shell molecules (free radicals), and degenerate or quasi-degenerate electronic states are involved. Therefore, vibronic interactions (e.g., Renner-Teller, Jahn-Teller, or pseudo-Jahn-Teller interactions) and related effects (e.g., spin-orbit and Coriolis couplings) need to be taken into account. In this talk, we will discuss (i) the rotational and fine structure of asymmetric tops in quasi-degenerate electronic and vibronic states that are subject to vibronic, spin-orbit, and Coriolis couplings; (ii) symmetry properties of the rotational energy levels and their implications on the search for the permanent electric dipole moment of the electron (eEDM) and parity-violating interactions in molecules; (iii) transition intensities between vibronic levels and the rotational branching, which determine the practicality of optical cycling and laser cooling. B. L. Augenbraun, J. M. Doyle, T. Zelevinsky, and I. Kozyryev, arXiv:2001.11020 [physics.atom-ph] (2020).S