Starting from an ultracold sample of ground-state 23 Na 87 Rb molecules, we investigate the lowest ro-vibrational level of the b 3 Π state with high resolution laser spectroscopy. This electronic spin-forbidden X 1 Σ + ↔ b 3 Π transition features a nearly diagonal Franck–Condon factor and has been proposed useful for probing and manipulating the ultracold molecular gas. We measure the transition strength directly by probing the ac Stark shift induced by near resonance light and determine the total excited-state spontaneous emission rate by observing the loss of molecules. From the extracted branching ratio and the theoretical modeling, we find that the leakage to the continuum of the a 3 Σ + state plays the dominant role in the total transition linewidth. Based on these results, we show that it is feasible to create optical trapping potentials for maximizing the rotational coherence with laser light tuned close to this transition.
Starting from an ultracold sample of ground-state 23 Na 87 Rb molecules, we investigate the lowest ro-vibrational level of the b 3 Π state with high resolution laser spectroscopy. This electronic spin-forbidden X 1 Σ + ↔ b 3 Π transition features a nearly diagonal Franck–Condon factor and has been proposed useful for probing and manipulating the ultracold molecular gas. We measure the transition strength directly by probing the ac Stark shift induced by near resonance light and determine the total excited-state spontaneous emission rate by observing the loss of molecules. From the extracted branching ratio and the theoretical modeling, we find that the leakage to the continuum of the a 3 Σ + state plays the dominant role in the total transition linewidth. Based on these results, we show that it is feasible to create optical trapping potentials for maximizing the rotational coherence with laser light tuned close to this transition.
We present spectroscopic studies of ultracold LiRb molecules using ionization detection. The molecules are created by cooling and trapping Li and Rb atoms in overlapping magneto-optical traps (MOTs) and using light resonant with a free-bound transition to create weakly bound excited state molecules in a process known as photoassociation (PA). We explore weakly bound vibrational levels of LiRb with PA spectroscopy using ionization detection and, where possible, compare our results with earlier measurements performed in our lab using trap-loss spectroscopy. In addition, we also probe vibrational levels of the ground triplet electronic state and excited electronic states using resonantly enhanced multiphoton ionization (REMPI) spectroscopy. We identify several vibrational levels of the α3Σ+, (3)3ΠΩ and (4)3Σ + states and compare our observations with theoretical calculations. As LiRb is one of the least studied heteronuclear diatomic molecules, these studies are aimed towards exploring the molecular structure. The spectroscopic work is also in line with the long-term project goals of transferring ultracold LiRb molecules into the lowest rotational and vibrational levels of the ground singlet electronic state. Molecules in this rovibronic ground state possess a large electric dipole moment, which is essential for application of ultracold molecules in various quantum computation schemes. The rovibronic ground state will also be an ideal starting point for investigating molecular entangled states.
In this dissertation, three distinct but related experimental works are reported. The first is a study of the highly excited 3^1 Δ_g and 4^1 Δ_g electronic states of 85Rb2. Rovibrational levels of the two electronic states were probed using the high-resolution optical-optical double resonance (OODR) technique by exciting 85Rb2 molecules from thermally populated levels of the X(_ ^1)Σ_g^+ ground state through intermediate levels of the B^1 Π_u electronic state. The (_ ^1)Δ_g resonances induced by the probe laser were observed by detecting laser induced fluorescence (LIF) from nearby, collisionally populated triplet levels to the a^3 Σ_u^+ triplet ground state. The (_ ^1)Δ_g character of the two electronic states was confirmed by showing that the probe transitions to these states abide by (_ ^1)Δ←(_ ^1)Π dipole selection rules and by observing that their lowest rotational level is J = 2. A set of molecular constants and a Rydberg-Klein-Rees (RKR) potential energy curve (PEC) were calculated from the observed term values for each electronic state and compared with ab initio predictions. This is the first observation of (_ ^1)Δ_g states in Rb2.The next results are on the highly excited 6(_ ^3)Π_0g electronic state of the 133Cs2 molecule in an energetic region where very little is known. Rovibrational levels of the excited triplet state were experimentally observed by the perturbation-facilitated-OODR (PFOODR) using a level of the A(_ ^1)Σ_u^+ ~b(_ ^3)Π_u mixed manifold as an intermediate. A progression of P and R branch probe laser resonances were detected by measuring (LIF) to the a(_ ^3)Σ_u^+ state as a function of the probe laser frequency. For select rovibrational levels of the 6(_ ^3)Π_0g state, dispersed fluorescence to the a(_ ^3)Σ_u^+ state and the A(_ ^1)Σ_u^+ ~b(_ ^3)Π_u manifold were also recorded and compared with Frank-Condon factor (FCF) calculations to determine spin multiplicity and vibrational numbering. Term values of the 284 observed rovibrational levels and their quantum numbers were fit to a Dunham expansion to obtain molecular constants, which were in turn used to generate a corresponding RKR-PEC. Comparison to ab initio results led to the assignment of 6(_ ^3)Π_0g . Accurate and high-resolution potential energy curves for the excited states of alkali dimers are crucial for quantum control and cooling experiments relying on techniques such as Stimulated Raman adiabatic passage (STIRAP) and resonantly enhanced multiphoton ionization (REMPI).Lastly, the results of producing state selective molecular angular momentum orientation using dressed states created by a cw optical field are reported. The experiment was carried out with 7Li2 molecules and a combination of left- and right-hand circularly polarized lasers. Our approach exploits the dependence of the Rabi frequency on the quantum number M, which makes it possible to achieve complete M-state selectivity and thus molecular angular momentum orientation relative to laboratory frame space-fixed axes. Using molecules in a (_ ^1)Σ state, orientation of the rotational angular momentum, R ⃑, can be achieved by spectrally resolving the Autler-Townes split M components of the total angular momentum, J ⃑. Results of orienting J ⃑ in (_ ^1)Π state molecules are also discussed. This is the first time complete angular momentum orientation has been achieved in nonpolar molecules and should aid the efforts of stereochemistry, molecular imaging, photodissociation and photoassociation, to name a few.
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.
The thesis reports transferring the ultracold RbCs molecules to the absolute ground state and a trapping light that eliminates the differential AC Stark shift. We build a laser system to perform Stimulated Raman Adiabatic Passage (STIRAP) for the molecules trapped in tweezers, which transfer the molecules from the weakly- bounded state to the absolute ground state. We use the offset cavity stabilisation technique to stabilise the laser frequencies to a passive cavity to narrow the laser linewidths while keeping the laser frequency tunable. We then align the beams to the molecules. We demonstrate a “magic” laser frequency to eliminate the differential AC Stark shift of the rotational states in the ground vibrational state of RbCs molecule and to increase the rotational coherence time of multiple states. We study the AC Stark effect caused by the trapping light. We calculate the polarisabilities of the rotational states near a narrow electronic from X ^1Σ^+ state to b^3Π_0 state and found at certain laser frequency the polarisabilities become the same. We measure the transition frequencies to different vibrational states in the excited electronic state experimentally. We perform microwave spectroscopy to measure the light shift and show that the shift vanishes at the magic frequency. We confirmed the coherence in 1 ms timescale by measuring the Ramsey fringe contrast.
Spectroscopic studies of aluminum monofluoride (AlF) have revealed its highly favorable properties for direct laser cooling. All Q lines of the strong A1Π ← X1Σ+ transition around 227 nm are rotationally closed and thereby suitable for the main cooling cycle. The same holds for the narrow, spin-forbidden a3Π ← X1Σ+ transition around 367 nm, which has a recoil limit in the µK range. We here report on the spectroscopic characterization of the lowest rotational levels in the a3Π state of AlF for v = 0–8 using a jet-cooled, pulsed molecular beam. An accidental AC Stark shift is observed on the a3Π0, v = 4 ← X1Σ+, v = 4 band. By using time-delayed ionization for state-selective detection of the molecules in the metastable a3Π state at different points along the molecular beam, the radiative lifetime of the a3Π1, v = 0, J = 1 level is experimentally determined as τ = 1.89 ± 0.15 ms. A laser/radio frequency multiple resonance ionization scheme is employed to determine the hyperfine splittings in the a3Π1, v = 5 level. The experimentally derived hyperfine parameters are compared to the outcome of quantum chemistry calculations. A spectral line with a width of 1.27 kHz is recorded between hyperfine levels in the a3Π, v = 0 state. These measurements benchmark the electronic potential of the a3Π state and yield accurate values for the photon scattering rate and for the elements of the Franck–Condon matrix of the a3Π–X1Σ+ system.
By using a combination of molecular beam (MB) excitation spectra and two distinct ultracold molecule excitation spectra (UM+ and UM-), we have assigned high vibrational levels of the A and 3 (1)Σ(+) states from absorption spectra of the mutually strongly perturbed A (1)Σ(+) - 3 (1)Σ(+) - 1 (1)Π - 2 (3)Σ(+) - b (3)Π states of ultracold (39)K(85)Rb molecules in the energy region between 15,116 and 16,225 cm(-1) above the minimum of the ground X (1)Σ(+) state. The ultracold molecules (UM+ and UM-) are formed by radiative decay following photoassociation (PA) to a specific level of the 3(0(+)) state (UM+) or to a specific level of the 3(0(-)) state (UM-). We observe that the A and 3 (1)Σ(+) states are observable in the UM+ spectra, but absent from the UM- spectra. This is explained by considering Hund's case (c) selection rules and transition dipole moments between the upper excited A (1)Σ(+) (2(0(+))) state and the three Ω components (0(+), 0(-), and 1) at the ground-state dissociation limit. We propose further investigations of the extended potential wells of the A and 3 (1)Σ(+) states by combining short-range MB excitation spectra in a narrow Franck-Condon (FC) window near R(e) of the X (1)Σ(+) state, and long-range UM (and PA) excitation spectra, which have much larger FC windows.