Perturbation-assisted Observation of the Lowest Vibrational Level of the
b3Π0 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 et al., Chem. Phys. Lett., 2012, 535, 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+,v=0,J=0> to
∣b3Π0,v′=0,J′=1> is determined to be 314,230.5(5)GHz. Assisted by microwave spectroscopy, we resolved the rotational structure of
∣b3Π0,v′=0> and extracted a rotational constant of
h×8.576(44) GHz for the
b3Π0 state. From this, we deducted an energy separation between
∣b3Π0,v′=0,J′=0> and
∣X1Σ+,v=0,J=0> of $hc\times$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.
We form ultracold ground-state Rb 2 molecules by photoassociating pairs of atoms in a magneto-optical trap into the 0 u + state, which decays radiatively into high vibrational levels of the X 1 Σ g + state. Sensitive and vibrationally state-selective detection is achieved by means of resonantly-enhanced two-photon ionization with a pulsed laser. Frequency scans of the detection laser reveal a long vibrational progression to a previously unobserved electronic excited state, which we identify as the 2 1 Σ u + state. Most of its vibrational spectrum is in excellent agreement with predictions based on ab initio potentials, although the lowest vibrational levels exhibit strong perturbative mixing with the triplet 2 3 Π u state. The detection method reported here, with minor variations, should be effective for the entire potential well of the X state. In this work no transitions are observed from vibrational levels above v = 118, but this turns out to be a limitation not of the detection method but rather of the photoassociative formation scheme, due to re-excitation of the highest- v levels by the same photoassociation laser that produces them.
This thesis reports the development of theoretical and experimental tools to understand and control the coupled A1Σ+ − b3Π0 system in ultracold 87Rb133Cs molecules, with applications in magic trapping and direct imaging of molecules. Building on detailed modelling of molecular structure and light–matter interactions, the work establishes conditions for long-lived rotational coherence and explores pathways towards quantum simulation and computation with RbCs. High-resolution spectroscopy of lowest vibrational levels in the coupled complex is performed, yielding vibrational splittings, rotational and hyperfine coupling constants, transition dipole moments and natural linewidths. Spectroscopy of a highly excited vibrational level of the complex is also performed, with the polarisation-dependent transition strengths and polarisabilities measured. These results demonstrate strong anisotropy of the initial state and enables direct detection of molecules. We then develop and validate a comprehensive description of rotationally magic trapping in RbCs. A general theory of dipole polarisability is applied near the transition from the rovibrational ground state in the X1Σ+ potential to the lowest vibrational states of the b3Π0+ potential, predicting rotationally magic frequencies at which differential polarisabilities between rotational states vanishes. Experimentally, we build and characterise a new magic-wavelength trapping system based on a 1145 nm laser. Measurements of magic frequencies for spinstretched superpositions, combined with long coherence times in the magic trap, confirm the model. Dephasing due to dipole–dipole interactions are observed, while long-lived rotational coherence is demonstrated with non-dipolar states. Extending the theory and experiments to non-stretched states by diagonalising the full AC Stark Hamiltonian shows nonlinear Stark shifts driven by laser-induced hyperfine couplings and identifies local magic conditions where intensity gradients vanish, enabling tailored coherence protection across a wide range of internal state.