Perturbation-assisted observation of the lowest vibrational level of the b ³ Π ₀ state of ultracold LiK molecules

The narrow transition from the lowest rovibrational level of the X¹Σ⁺ electronic ground state to the lowest vibrational level of the b³Π₀ potential provides opportunities for achieving magic-wavelength trapping of ultracold bialkali molecules for enhancing their rotational coherence times. Guided by existing spectroscopic data of several perturbed and deeply-bound rovibrational states of the A¹Σ⁺ potential [Grochola <i>et al.</i>, <i>Chem. Phys. Lett.</i>, 2012, <b>535</b>, 17-20], we conducted a targeted spectroscopic search and report the first observation of the lowest vibrational level of the b³Π₀ state in ⁶Li⁴⁰K. The transition frequency from |X¹Σ⁺, <i>v</i> = 0, <i>J</i> = 0〉 to |b³Π₀, <i>v</i>' = 0, <i>J</i>' = 1〉 is determined to be 314 230.5(5) GHz. Assisted by microwave spectroscopy, we resolved the rotational structure of |b³Π₀, <i>v</i>' = 0〉 and extracted a rotational constant of <i>h</i> × 8.576(44) GHz for the b³Π₀ state. From this, we deduced an energy separation between |b³Π₀, <i>v</i>' = 0, <i>J</i>' = 0〉 and |X¹Σ⁺, <i>v</i> = 0, <i>J</i> = 0〉 of <i>hc</i> × 10 481.03(2) cm^-1. Our work provides timely and precise information on the deeply-bound region of the b³Π₀ triplet excited potential of LiK, and benefits future applications of ultracold LiK isotopologues in quantum simulation and quantum computation that demand long coherence times.