The fact that the universe is made entirely out of matter, and contains no free anti-matter, has no physical explanation. The unknown process that created matter in the universe must violate a number of fundamental symmetries, including those that forbid the existence of certain electromagnetic moments of fundamental particles - moments which are amplified by the large internal fields in polar molecules. We discuss spectroscopic and theoretical investigations into polyatomic molecules that uniquely combine high polarizability through symmetry-lowering mechanical motions with laser-coolable electronic structures. In particular we discuss YbOH, whose isotopologues have sensitivity to symmetry violations of the electron and the Yb nucleus, along with some other molecules that offer unique opportunities for precision measurement.

Aluminum monofluoride (AlF) is an excellent candidate for laser cooling on any Q-line of the A¹Π - X¹Σ transition and trapping at high densities.Truppe et al., Phys. Rev. A 100, 052513 (2019) In preparation for cooling and manipulation experiments, it is necessary to know the detailed energy structure of the involved states, as well as their lifetimes, dipole moments and the Franck-Condon factors of their transitions.
The metastable a³Π state is the ideal starting point for extensive spectroscopic investigations. Therefo re, this presentation will focus on the a³Π← X¹Σ⁺ transition. The energy levels in the X¹Σ⁺, v"=0 state and within each Ω manifold of the a³Π, v′=0 state were determined with a relative accuracy of a few kHz, using laser-radio-frequency multiple resonance and ionization detection schemes in a jet-cooled, pulsed molecular beam. All spectroscopic parameters relevant for describing the rotational and hyperfine structure were determined by fitting the eigenvalues of the molecular Hamiltonian to the data.
With this knowledge, the measured hyperfine structure in the A¹Π state could be assigned. The dipole moments of the X¹Σ⁺, A¹Π and a³Π states were determined by recording cw excitation spectra in electric fields up to 150 kV/cm.
The A¹Π - a³Π band was observed for the first time. Measurements on the transition strength showed that it is no significant loss channel for the A¹Π - X¹Σ laser cooling transition.

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Footnotes:

Truppe et al., Phys. Rev. A 100, 052513 (2019)

Precision spectroscopy of fundamental bands of molecules in the mid-infrared (MIR) region is of great interest in applications of trace detection and testing fundamental physics, where high-power and narrow-linewidth MIR lasers are needed. By controlling the phase noise of the signal and pump light of a continuous wave optical parametric oscillator (OPO), we established a broadly tunable MIR light source which has an output power of several hundred milliwatts and a linewidth of a few tens kilohertz. The long term frequency drift of the MIR laser was reduced and calibrated utilizing an optical frequency comb. The performance of the light source was investigated and tested by measuring the saturated absorption spectroscopy of a few molecular transitions at 3.3 μm.

The combination of cryogenic ion trap machines, operated close to 4 K, with different laser sources allowed the first experimental characterization of the antisymmetric stretch (ν₃) and bending (ν₂) fundamentals of the linear He-H⁺-He core of the HHe_n⁺ complexes for n=3−6. The found band origins, at around 1290 cm⁻¹ for ν₃ and around 850 cm⁻¹ for ν₂, are fully supported by first-principles quantum-chemical computations. These results are consistent with the structure for HHe₃⁺ being of T-shaped C_2v symmetry and HHe₆⁺ being of D_4h symmetry, while HHe₄⁺ is suggested to exhibit interesting dynamical phenomena related to large-amplitude motion. Very recently, using a quantum cascade laser, the linear HHe₂⁺ has been probed in high resolution, yielding molecular parameters for this fundamental three-nucleus-four-electron system for the first time.

Barium fluoride BaF is one of the heaviest molecular candidates for effective laser cooling, and as such BaF is being employed in eEDM, anapole and a variety of other experiments in a number of laboratories worldwide. Beyond the relevant recent BaF optical spectroscopy study by Steimle, et al., the purpose here is to extend and complement the existing microwave spectroscopy data for BaF. We will present high-resolution (approx. 1 kHz) data for the N = 1 - 0 and 2 - 1 transitions in the vibrational ground state for the five most abundant stable Ba isotopes. This reflects an improvement in resolution of a factor of 20 or more, and direct ablation of barium metal in the presence of a fluorine-containing buffer gas has enabled the first microwave observations of low abundance ¹³⁵BaF (6.6%) and ¹³⁴BaF (2.4%). A comparison of the resulting molecular parameters for the two odd barium isotopologues ¹³⁷BaF and ¹³⁵BaF (both I = 3/2) will be highlighted, in the context of the goal of a robust global fit of all the microwave data, which includes transitions up to N = 22 - 21 and vibrational states up to v = 4.

r0pt Figure Double resonance (DR) spectroscopy has been frequently applied in state-selective excitation and to reach energy levels forbidden to single-photon transitions. Due to the low cross section of two-photon transitions, usually high-power pulsed lasers are needed, which prevent high-precision measurements. Here we present a newly developed comb-locked cavity-assisted double resonance (COCA-DR) spectroscopy technique by simultaneously locking two diode lasers (1.60 μm and 1.67 μm) to a high-finesse cavity and an optical frequency comb. Doppler-free optical-optical DR transitions to the highly-excited (60025) (v_CO=8) "dark" state of the CO₂ molecule using the (30013) vibrational state as the intermediate. By selecting molecules with different longitudinal speeds, we confirmed a frequency accuracy of 3 kHz of the measured transitions. Rotational energies of the (60025) state and the transition dipole of the (60025)-(30013) band were determined for the first time, and they were compared with the calculated values based on effective operators. The measurements demonstrate the possibility to determine energies of highly excited states of molecules with unprecedented precision.

The accurate determination of molecular transition frequencies can provide stringent tests and constraints on fundamental physics questions [1-3]. Here, we present recent work on a comb-locked cavity ring-down spectroscopy system which probes Doppler-broadened spectra in the linear absorption regime. These measurements have relative uncertainties in transition frequency below 10⁻¹², which are among the lowest values reported for optical molecular transition frequencies. We use a probe laser which is phase-locked to a commercial optical frequency comb and subsequently coupled to a high-finesse optical cavity based on that of [4]. We observe stationary measurement statistics for measurements of more than 2000 spectra and find that measurements which are replicated in multiple experiments over several months are normally distributed. Single-spectrum signal-to-noise-ratios can exceed 50,000:1, with resulting line center uncertainties below 5 kHz. Because this method is based on linear absorption, it is an attractive alternative for the measurement of molecular transitions that cannot be probed by saturation-based Doppler-free spectroscopy. Furthermore, we demonstrate results having smaller systematic errors than those provided by more complicated nonlinear spectroscopy methods. Finally, we report measurements for 44 ¹²C¹⁶O₂ transition frequencies near 1.6 μm, obtaining combined uncertainties from approximately 200 Hz to 1 kHz. These results, which are included in a global fit yielding upper-state spectroscopic constants with reduced uncertainties, are expected to benefit spectroscopic retrieval algorithms for space-based measurements of atmospheric carbon dioxide[5]. [1] F. M. J. Cozijn et. al. Phys. Rev. Lett. 120, 153002 (2018). [2] H. Fleurbaey et. al. Phys. Rev. Lett. 120, 183001 (2018). [3] J. Baron et. al. Science 343, 269-272 (2014). [4] H. Lin et. al. J. Quant. Spectros. and Rad. Trans. 161, 11-20 (2015) [5] S. R. Kawa et. al. Tellus B: Chemical and Physical Meteorology 62, 759-769 (2010)