We report a generally applicable computational and experimental approach to determine vibronic branching ratios in linear polyatomic molecules to the 10−5 level, including for nominally symmetry-forbidden transitions. These methods are demonstrated in CaOH and YbOH, showing approximately two orders of magnitude improved sensitivity compared with the previous state of the art. Knowledge of branching ratios at this level is needed for the successful deep laser cooling of a broad range of molecular species.

Medium resolution (∆ν ∼ 3 GHz) laser-induced fluorescence (LIF) excitation spectra of a rotationally cold sample of YbOH in the 17300-17950 cm -1 range have been recorded using twodimensional (excitation and dispersed fluorescence) spectroscopy. High resolution (∆λ ∼ 0.65 nm) dispersed laser induced fluorescence (DLIF) spectra and radiative decay curves of numerous bands detected in the medium resolution LIF excitation spectra were recorded. The vibronic energy levels of the 2 X + Σ  state were predicted using a discrete variable representation approach and compared with observations. The radiative decay curves were analyzed to produce fluorescence lifetimes.DLIF spectra resulting from high resolution (∆ν < 10 MHz) LIF excitation of individual lowrotational lines in the 2 2 1/ 2 (0,0,0) (0,0,0)bands were also recorded. The DLIF spectra were analyzed to determine branching ratios which were combined with radiative lifetimes to obtain transition dipole moments. The implications for laser cooling and trapping of YbOH are discussed.3

An extension of the exact two-component theory with atomic mean-field integrals (the X2CAMF scheme) to the treatment of the Breit term together with efficient implementation using an atomic Dirac−Coulomb−Breit Hartree−Fock program is reported. The accuracy of the X2CAMF scheme for treating the contributions from the Breit term to the molecular properties is demonstrated using benchmark calculations of equilibrium bond lengths, harmonic frequencies, and dipole moments for molecules containing elements across the periodic table. Calculations of the properties for molecules containing period four elements aiming at high accuracy as well as for Th-and U-containing molecules are also presented and compared with experimental results to demonstrate the usefulness of the X2CAMF scheme in combination with accurate treatments of electron correlation by the coupled-cluster (CC) methods. The combination of CC methods and the X2CAMF scheme shows potential to extend the accuracy of CC calculations to heavy elements, e.g., to computational heavy-element thermochemistry.

Development of analytic gradients for relativistic coupled-cluster singles and doubles augmented with a non-iterative triples [CCSD(T)] method using an all-electron exact two-component Hamiltonian with atomic mean-field spin-orbit integrals (X2CAMF) is reported. This enables efficient CC geometry optimizations with spin-orbit coupling included in orbitals. The applicability of the implementation is demonstrated using benchmark X2CAMF-CCSD(T) calculations of equilibrium structures and harmonic vibrational frequencies for methyl halides, CH3X, X=Br, I, At, as well as calculations of rotational constants and infrared spectrum for RaSH+, a radioactive molecular ion of interest to spectroscopic study.

Benchmark spinor-based relativistic coupled-cluster calculations for the ionization energies of the uranium atom, the uranium monoxide molecule (UO), and the uranium dioxide molecule (UO 2 ) and for the bond dissociation energies of UO and UO 2 are reported. The accuracy of these calculations in the treatments of relativistic, electroncorrelation, and basis-set effects is analyzed. The intrinsic convergence of the computed results and the favorable comparison with the experimental values demonstrate the unique applicability of the spinor representation of quantum-chemical methods to open-shell uranium-containing atomic and molecular species with uranium oxidation states ranging from U(0) to U(V).

Multiphoton excitation promises opportunities for opening new photochemical reaction pathways and controlling photoproduct distributions. We demonstrate photonic control of the 6π photocyclization of ortho-terphenyl to make 4a,4b-dihydrotriphenylene (DHT). Using pump–repump–probe spectroscopy we show that 1 + 1′ excitation to a high-lying reactant electronic state generates a metastable species characterized by a red absorption feature that accompanies a repump-induced depletion in the one-photon trans-dihydro product (trans-DHT); signatures of the new photoproduct are clearer for a structural analogue of the reactant that is sterically inhibited against one-photon cyclization. Quantum-chemical computations support assignment of this species to cis-DHT, which is accessible photochemically along a disrotatory coordinate from high-lying electronic states reached by 1 + 1′ excitation. We use time-resolved spectroscopy to track photochemical dynamics producing cis-DHT. In total, we demonstrate that selective multiphoton excitation opens a new photoreaction channel in these photocyclizing reactants by taking advantage of state-dependent correlations between reactant and product electronic states.

A study of the intensity-borrowing mechanisms important to optical cycling transitions in laser-coolable polyatomic molecules arising from non-adiabatic coupling, contributions beyond the Franck–Condon approximation, and Fermi resonances is reported. It has been shown to be necessary to include non-adiabatic coupling to obtain computational accuracy that is sufficient to be useful for laser cooling of molecules. The predicted vibronic branching ratios using perturbation theory based on the non-adiabatic mechanisms have been demonstrated to agree well with those obtained from variational discrete variable representation calculations for representative molecules including CaOH, SrOH, and YbOH. The electron-correlation and basis-set effects on the calculated transition properties, including the vibronic coupling constants, the spin–orbit coupling matrix elements, and the transition dipole moments, and on the calculated branching ratios have been thoroughly studied. The vibronic branching ratios predicted using the present methodologies demonstrate that RaOH is a promising radioactive molecule candidate for laser cooling.