The YbF molecule is a sensitive system for measuring the electron’s electric dipole moment. The precision of this measurement can be improved by direct laser cooling of the molecules to ultracold temperature. However, low-lying electronic states arising from excitation of a 4f electron may hinder laser cooling. One set of these “4f hole” states lies below the A2Π1/2 excited state used for laser cooling, and radiative decay to these intermediate levels, even with branching ratios as small as 10−5, can be a hindrance. Other 4f hole states lie very close to the A2Π1/2 state, and a perturbation results in states of mixed character that are involved in the laser cooling cycle. This perturbation may enhance the loss of molecules to states outside of the laser cooling cycle. We model the perturbation of the A2Π1/2 state to determine the strength of the coupling between the states, the de-perturbed potential energy curves, and the radiative branching ratios to various vibrational levels of the ground state, X2Σ+. We use electronic structure calculations to characterize the 4f hole states and the strengths of transitions between these states and the A2Π1/2 and X2Σ+ states. We identify a leak out of the cooling cycle with a branching ratio of roughly 5×10−4, dominated by the contribution of the ground state configuration in a 4f hole state. Finally, we assess the impact of these results for laser cooling of YbF and molecules with similar structure.
The vibrational branching ratios of SrOH for radiative decay to the ground electronic state, $X{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}^{+}$, from the first two electronically excited states, $A{\phantom{\rule{0.16em}{0ex}}}^{2}\mathrm{\ensuremath{\Pi}}$ and $B{\phantom{\rule{0.16em}{0ex}}}^{2}{\mathrm{\ensuremath{\Sigma}}}^{+}$, are determined experimentally at the $\ensuremath{\sim}{10}^{\ensuremath{-}5}$ level. The observed small branching ratios enable the design of a full, practical laser-cooling scheme, including magnetooptical trapping and sub-Doppler laser cooling, with
>104 photon scatters per molecule. Ab initio calculations sensitive to weak vibronic transitions are performed to facilitate the experimental measurement and analysis, and show good agreement with the experiment.
The generation of a fountain of laser-cooled ytterbium fluoride, YbF, has been recently proposed\footnote{Tarbutt, M R; Sauer, B E; Hudson, J J; Hinds E A, New J. Phys 15, 053034, 2013.} as a method for long coherent observation times, thereby improving the electron electric dipole moment (eEDM) measurement. Understanding the properties of the excited electronic states of YbF is essential for the development of such a scheme for laser cooling. Here we report on the measurement of the radiative lifetimes,
τ, permanent electric dipole moments,
μel, and magnetic g-factors for the [18.6] and [18.7] excited states of YbF. The results are compared with the previously determined values for [18.1] state\footnote{Zhuang,X; Le,A.;Steimle, T C; Bulleid, N E; Smallman, I J; Hendricks, R J; Skoff, S M ; M R; Hudson, J J; Sauer, B E; Hinds, Tarbutt, M R, PCCP, 13 19103, 2011}\footnote{Condylis,P C; Hudson, J J; Tarbutt, M R; Sauer, B E; Hinds E A, J. Chem. Phys. 123, 231101, 2005}. The [18.1] state is the
Ω=1/2 spin-orbit component of the A
2Π(v=0) electronic state arising from the Yb
+(4f$^{14}
6p\pi
)F^{-}
(2p^{14}$) configuration. The experimentally determined
μel, and g-factors will be used to unravel the nature of the [18.6] and [18.7] states, which are known to be admixtures A
2Π and an additional
Ω=1/2 state of unknown electronic configuration.