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New sources of parity and time-reversal violation are predicted by well motivated extensions of the Standard Model and can be effectively probed by precision spectroscopy of atoms and molecules. Chiral molecules have distinguished enantiomers which are related by parity transformation. Thus they are promising candidates to search for parity violation at molecular scales, which has yet to be observed. In this work, we show that precision spectroscopy of the hyperfine structure of chiral molecules is sensitive to new physics sources of parity and time-reversal violation. In particular, such a study can be sensitive to regions unexplored by terrestrial experiments of a new chiral spin-1 particle that couples to nucleons. We explore the potential to hunt for time-reversal violation in chiral molecules and show that it can be a complementary measurement to other probes. We assess the feasibility of such hyperfine metrology and project the sensitivity in ${\text{CHDBrI}}^{+}$. Published by the American Physical Society2024 

We report the hyperfine-resolved rotational spectrum of the gas-phase phenoxy radical in the 8−25 GHz frequency range using cavity Fourier transform microwave spectroscopy. A complete assignment of its complex but well-resolved fine and hyperfine splittings yielded a precisely determined set of rotational constants, spin-rotation parameters, and nuclear hyperfine coupling constants. These results are interpreted with support from high-level quantum chemical calculations to gain detailed insight into the distribution of the unpaired π electron in this prototypical resonance-stabilized radical. The accurate laboratory rest frequencies enable studies of the chemistry of phenoxy in both the laboratory and space. The prospects of extending the present experimental and theoretical techniques to investigate the rotational spectra of isotopic variants and structural isomers of phenoxy and other important gas-phase radical intermediates that are yet undetected at radio wavelengths are discussed. 

Large-amplitude vibrational motion influences the rovibrational structure of molecules that tunnel between multiple wells. Reaction path (RP) Hamiltonians, and curvilinear coordinates more gen- erally, are useful for modelling pure vibrational motion in these systems and provide a practical framework for calculating accurate ab initio anharmonic vibrational energies and tunnelling split- tings with perturbation theory. These computational tools also offer the means to address rotation- vibration coupling associated with large-amplitude motion in rotating molecules. In this paper, we incorporate the reduced axis system (RAS) frameembeddingwithRPHamiltoniansandsecond-order vibrational Møller-Plesset perturbation theory (VMP2). Because the RP-RAS Hamiltonian eliminates rotation-vibration momentumcoupling everywhere along a one-dimensional reaction path, it is well suited for rovibrational VMP2 methods, the convergence of which relies critically on approximate vibration-vibration and vibration-rotation separability. The accuracy of this combined RP-RAS-VMP2 scheme is demonstrated by comparisons with numerically exact variational calculations and VMP2 parameters based on traditional Eckart embeddings for reduced-dimension models of torsional tunnelling in hydrogenperoxideandinversion tunnelling in cyclopropyl radical. Thefavourablecom- putational scaling ofVMP2makes it a promising strategy for calculating accurate tunnelling-rotation parameters for medium-sized and larger molecules in full dimensionality. 

Recent advances in circumstellar metal chemistry and laser-coolable molecules have spurred interest in the spectroscopy and electronic properties of alkaline earth metal-bearing polyatomic molecules. We report the microwave rotational spectra of two members of this important chemical family, the linear magnesium- carbon chains MgC4H and MgC3N, detected with cavity Fourier transform microwave spectroscopy of a laser ablation-electric discharge expansion. The rotation, fine, and hyperfine parameters have been derived from the precise laboratory rest frequencies. These experimental results, combined with a theoretical quantum chemical analysis, confirm the recent identification of MgC4H and MgC3N in the circumstellar envelope of the evolved carbon-rich star IRC+10216. The spectroscopic data also provide insight into the structural and electronic properties that influence the metal-based optical cycling center in this unique class of laser-coolable polyatomics. 

We report the hyperfine-resolved rotational spectrum of gas-phase phenoxy radical in the 8–25 GHz frequency range using cavity Fourier transform microwave spectroscopy. A complete assignment of its complex but well-resolved fine and hyperfine splittings has yielded a precisely determined set of rotational constants, spin-rotation parameters, and nuclear hyperfine coupling constants. These results are interpreted with support from high-level quantum chemical calculations to gain detailed insight into the distribution of the unpaired π electron in this prototypical resonance-stabilized radical. The accurate laboratory rest frequencies enable studies of the chemistry of phenoxy in both the laboratory and in space. The prospects of extending the present experimental and theoretical techniques to investigate the rotational spectra of isotopic variants and structural isomers of phenoxy and other important gas-phase radical intermediates yet undetected at radio wavelengths are discussed. 

Polycyclic aromatic hydrocarbons (PAHs) are organic molecules containing adjacent aromatic rings. Infrared emission bands show that PAHs are abundant in space, but only a few specific PAHs have been detected in the interstellar medium. We detected 1-cyanopyrene, a cyano-substituted derivative of the related four-ring PAH pyrene, in radio observations of the dense cloud TMC-1, using the Green Bank Telescope. The measured column density of 1-cyanopyrene is $\sim \text{1}{\text{.52×10}}^{\text{12}}$cm⁻², from which we estimate that pyrene contains up to 0.1% of the carbon in TMC-1. This abundance indicates that interstellar PAH chemistry favors the production of pyrene. We suggest that some of the carbon supplied to young planetary systems is carried by PAHs that originate in cold molecular clouds. 

The unique optical cycling efficiency of alkaline earth metal–ligand molecules has enabled significant advances in polyatomic laser cooling and trapping. Rotational spectroscopy is an ideal tool for probing the molecular properties that underpin optical cycling, thereby elucidating the design principles for expanding the chemical diversity and scope of these platforms for quantum science. We present a comprehensive study of the structure and electronic properties in alkaline earth metal acetylides with high-resolution microwave spectra of 17 isotopologues of MgCCH, CaCCH, and SrCCH in their²Σ⁺ground electronic states. The precise semiexperimental equilibrium geometry of each species has been derived by correcting the measured rotational constants for electronic and zero-point vibrational contributions calculated with high-level quantum chemistry methods. The well-resolved hyperfine structure associated with the^1,2H,¹³C, and metal nuclear spins provides further information on the distribution and hybridization of the metal-centered, optically active unpaired electron. Together, these measurements allow us to correlate trends in chemical bonding and structure with the electronic properties that promote efficient optical cycling essential to next-generation experiments in precision measurement and quantum control of complex polyatomic molecules.