Assignment of excited-state bond lengths using branching-ratio measurements: The B 2Σ+ state of BaH molecules

Abstract

Vibrational branching ratios in the B 2 Σ + -X 2 Σ + and A 2 Π -X 2 Σ + optical-cycling transitions of BaH molecules are investigated using measurements and ab initio calculations. The experimental values are determined using fluorescence and absorption detection. The observed branching ratios have a very sensitive dependence on the difference in the equilibrium bond length between the excited and ground state, ∆re: a 1 pm (.5%) displacement can have a 25% effect on the branching ratios but only a 1% effect on the lifetime. The measurements are combined with theoretical calculations to reveal a preference for a particular set of published spectroscopic values for the B 2 Σ + state (∆r B−X e = +5.733 pm), while a larger bond-length difference (∆r B−X e = 6.3 − 6.7 pm) would match the branching-ratio data even better. By contrast, the observed branching ratio for the A 2 Π 3/2 -X 2 Σ + transition is in excellent agreement with both the ab initio result and the spectroscopically measured bond lengths. This shows that care must be taken when estimating branching ratios for molecular laser cooling candidates, as small errors in bond-length measurements can have outsize effects on the suitability for laser cooling. Additionally, our calculations agree more closely with experimental values of the B 2 Σ + state lifetime and spin-rotation constant, and revise the predicted lifetime of the H 2 ∆ state to 9.5 µs. II. SPECTROSCOPY BACKGROUNDThe optical and near-infrared spectra of BaH [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] are dominated by the three 5d-complex states that correlate to the 5d state of the Ba atom: B 2 Σ + , A 2 Π, and H 2 ∆. Since all three reach below the ground-state dissociation threshold, the only decay mechanisms are radiative. In addition, these three low-lying excited states possess spectroscopic parameters that closely resemble the X 2 Σ + ground state. The resulting diagonal Franck-Condon (FC) factors ensure absorption-emission cycles arXiv:1904.07326v2 [physics.atom-ph]

A theoretical study on laser cooling feasibility of XH (X = As, Sb and Bi): effects of intersystem crossings and spin–orbit couplings

“…Lastly, we compare AsH and SbH with earlier laser cooling candidates, such as BaH, 60,61 CH 51,52 and SiH. 13 It can be noted that the effects of electronic-state crossings on laser cooling of the group IVA and VA hydrides are more significant than those of the group IIA hydrides.…”

Large molasses-like cooling forces for molecules using polychromatic optical fields: A theoretical description

“…In this system, many of the criteria listed before are fulfilled -both FCFs are greater than 0.95 [58] and both transitions are far apart in the frequency space (λ AΠ ≈ 1060.7868 nm and λ BΣ ≈ 905.3197 nm). As was detailed in Sec.…”

“…Here, excited states |E1 and |E2 are two A 2 Π Ω states with a different total angular momentum projection of Ω = 1/2 and Ω = 3/2. In fact, in some molecules (like BaH) such scheme might work better, due to more beneficial FCF's compared to the B 2 Σ + state [58]. In this configuration, we exploit the fact that the excited states are parity doublets and given positive parity of the ground states, we can choose excited states of negative parity to create our light-coupled 2-level systems.…”

“…Motivated by the prospects of ultracold hydrogen production via molecular laser cooling followed by photo-dissociation [59], recently there has been an increased experimental [11,60] and theoretical [58,61,62] interest in direct laser cooling and trapping of alkaline-earthmetal monohydrides. High mass imbalance, low Doppler cooling limit, and small photon recoil velocity make BaH an extremely attractive candidate for producing ultracold atomic hydrogen via zero-energy photo-fragmentation [59].…”

Absolute frequency metrology of buffer-gas-cooled molecular spectra at the 10-12 accuracy level

“…Here, the main purpose is the determination of vibronic branching ratios to the 10 −5 level, which is essentially accomplished by application of laser-induced fluorescence (LIF) on cryogenic buffer gas beams [17]. For example, this produced accurate characterization of the B 2 Σ + state of BaH [18], and of the X 2 Σ 1/2 and A 2 Π 1/2 states in CaOH and SrOH [19].…”