Determination of the normal A2Π state in MgF with application to direct laser cooling of molecules

Hyperfine-resolved optical spectroscopy of the A2Π ← X2Σ+ transition in MgF

We report on hyperfine-resolved laser spectroscopy of the A2Π ← X2Σ+ transition of magnesium monofluoride (MgF), relevant for laser cooling. We recorded 25 rotational transitions with an absolute accuracy of better than 20 MHz, assigned 56 hyperfine lines, and determined precise rotational, fine, and hyperfine structure parameters for the A2Π state. The radiative lifetime of the A2Π state was determined to be 7.2(3) ns, in good agreement with ab initio calculations. The transition isotope shift between bosonic isotopologues of the molecule is recorded and compared to predicted values within the Born–Oppenheimer approximation. We measured the Stark effect of selected rotational lines of the A2Π ← X2Σ+ transition by applying electric fields of up to 10.6 kV cm−1 and determined the permanent electric dipole moments of 24MgF in its ground X2Σ+ and first excited A2Π states to be μ X = 2.88(20) D and μ A = 3.20(22) D, respectively. Based on these measurements, we caution for potential losses from the optical cycling transition due to electric field induced parity mixing in the excited state. In order to scatter 104 photons, the electric field must be controlled to below 1 V cm−1.

High resolution laser excitation spectra and Franck-Condon factors of A2Π−X2Σ+ electronic transition of MgF

Magnesium monofluoride (MgF) is proposed as an ideal candidate radical for direct laser cooling. Here, the rotationally resolved laser spectra of MgF for the A2Π− X2Σ+ electronic transition system were recorded by using laser induced fluorescence technique. The MgF radicals were produced by discharging SF6/Ar gas mixtures between the tips of two magnesium needles in a supersonic jet expansion. We recorded a total of 19 vibrational bands belonging to three sequences of Δ v=0, ±1 in the region of 348-370 nm. Accurate spectroscopic constants for both X2Σ+ and A2Π states are determined from rotational analysis of the experimental spectra. Spectroscopic parameters, including the Franck-Condon factors (FCFs), are determined from the experimental results and the Rydberg-Klein-Rees (RKR) calculations. Significant discrepancies between the experimentally measured and RKR-calculated FCFs are found, indicating that the FCFs are nearly independent of the spin-orbit coupling in the A2Π state. Potential energy curves (PECs) and FCFs determined here provide necessary data for the theoretical simulation of the laser-cooling scheme of MgF.

Theoretical study of laser cooling of the TlF+ molecular ion

The feasibility of the thallium monofluoride TlF+ molecular ion towards laser cooling is brought into focus through an electronic structure study. Ab initio calculations are carried out to investigate the four lowest-lying electronic states, X2Σ+, (1)2Π, (2)2Σ+ and (2)2Π, including the spin-orbit coupling effect by employing the Complete Active Space Self Consistent Field (CASSCF) method at the Multireference Configuration Interaction (MRCI) level of theory while invoking Davidson correction (+Q). Potential energy and permanent dipole moment curves are used to determine the corresponding spectroscopic constants and some other equilibrium parameters. Vibrational parameters of vibrational states and transition dipole moments between possible transitions are computed. The calculated parameters are then used to conduct a theoretical study focusing on the potential possibility of TlF+ ionic molecule to be laser cooled on the (2)2Π1/2(ν')-X2Σ+1/2(ν'') transition based on Di Rosa's criteria. With the results obtained being promising, a laser cooling optical cycling scheme is proposed to illustrate the number of pump lasers needed with the corresponding wavelengths that were found to lie within the ranges covered by a specific scientific laser.

Multireference study of ionic/covalent electronic states of MF (M = Be, Mg and Ca)

Ultracold environments composed by atoms or molecules offer an opportunity to study chemical reactions at the quantum-state level, for simulation of solid-state systems, as qubits in quantum computing, and for test fundamental symmetries. Those ultracold conditions formed by molecules can be obtained from cryogenic buffer gas, via supersonic expansion, followed by deceleration or from the laser cooling process. Diatomic alkaline earth monofluoride molecules have been shown as great candidates for the laser cooling process. In this sense, the present work focuses on the characterization of the low-lying doublet electronic states correlated to the first dissociation channel of the alkaline earth monofluorides diatomic molecules MF (M = Be, Mg and Ca). The developed state-of-the-art methodology was based on a qualitative analysis of the diatomic electronic structure, employing a hypothetical potential energy curve or by a simple molecular orbital diagram combined with bond order analysis. The potential energy curves, excitation and dissociation energies, and various sets of spectroscopic parameters were calculated by the MRCI/cc-pV5Z methodology. Transition probabilities for emission and radiative lifetimes among the characterized electronic states were also calculated for the (A)²Π ⟶ (X)²Σ⁺ electronic transition. Comparing the spectroscopy properties, we were able to indicate the CaF molecule as the best candidate molecule for laser cooling devices among the studied molecules.

A Theoretical Study on Laser Cooling Feasibility of Group IVA Hydrides XH (X = Si, Ge, Sn, and Pb): The Role of Electronic State Crossing

The feasibility of direct laser cooling of SiH, GeH, SnH, and PbH is investigated and assessed based upon first principles. The internally contracted multi-reference configuration interaction method with the Davidson correction is applied. Very good agreement is obtained between our computed spectroscopic constants and the available experimental data. We find that the locations of crossing point between the B²Σ^- and A²Δ states have the tendency of moving downwards from CH to SnH relative to the bottom of the corresponding A²Δ potential, which precludes the laser cooling of GeH, SnH, and PbH. By including the spin-orbit coupling effects and on the basis of the A 2 Δ 5 / 2 → X 2 Π 3 / 2 transition, we propose a feasible laser cooling scheme for SiH using three lasers with wavelengths varying from 400 to 500 nm, which features a very large vibrational branching ratio (0.9954) and a very short radiative lifetime (575 ns). Moreover, similar studies are extended to carbon monosulfide (CS) with a feasible laser cooling scheme proposed. The importance of electronic state crossing in molecular laser cooling is underscored, and our work suggests useful caveats to the choice of promising candidates for producing ultracold molecules.

Sub-Doppler Cooling and Compressed Trapping of YO Molecules at μK Temperatures

Complex molecular structure demands customized solutions to laser cooling by extending its general set of principles and practices. Compared with other laser-cooled molecules, yttrium monoxide (YO) exhibits a large electron-nucleus interaction, resulting in a dominant hyperfine interaction over the electron spin-rotation coupling. The YO ground state is thus comprised of two manifolds of closely spaced states, with one of them possessing a negligible Landé g factor. This unique energy level structure favors dual-frequency dc magneto-optical trapping (MOT) and gray molasses cooling (GMC). We report exceptionally robust cooling of YO at 4 μK over a wide range of laser intensity, detunings (one- and two-photon), and magnetic field. The magnetic insensitivity enables the spatial compression of the molecular cloud by alternating GMC and MOT under the continuous operation of the quadrupole magnetic field. A combination of these techniques produces a laser-cooled molecular sample with the highest phase space density in free space.