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Aluminium monofluoride (AlF) is a promising candidate for laser cooling and trapping at high densities. We show efficient production of AlF in a bright, pulsed cryogenic buffer gas beam, and demonstrate rapid optical cycling on the Q rotational lines of the A 1 Π ↔ X 1 Σ + transition. We measure the brightness of the molecular beam to be >10 12 molecules per steradian per pulse in a single rotational state and present a new method to determine its velocity distribution in a single shot. The photon scattering rate of the optical cycling scheme is measured using three different methods, and is compared to theoretical predictions of the optical Bloch equations and a simplified rate equation model. Despite the large number of Zeeman sublevels (up to 216 for the Q(4) transition) involved, a high scattering rate of at least 17(2) × 10 6  s −1 can be sustained using a single, fixed-frequency laser without the need to modulate the polarisation. We deflect the molecu-lar beam using the radiation pressure force and measure an acceleration of 8.7(1.5) × 10 5 m s −2 . Losses from the optical cycle due to vibrational branching to X 1 Σ + , v ″ = 1 are addressed efficiently with a single repump laser. Further, we investigate two other loss channels, parity mixing by stray electric fields and photo-ionisation. The upper bounds for these effects are sufficiently low to allow loading into a magneto‐optical trap.

Magneto-optical trapping and sub-Doppler cooling of atoms has been instrumental for research in ultracold atomic physics. This regime has now been reached for a molecular species, CaF. Magneto-optical trapping and sub-Doppler cooling have been essential to most experiments with quantum degenerate gases, optical lattices, atomic fountains and many other applications. A broad set of new applications await ultracold molecules 1 , and the extension of laser cooling to molecules has begun 2 , 3 , 4 , 5 , 6 . A magneto-optical trap (MOT) has been demonstrated for a single molecular species, SrF 7 , 8 , 9 , but the sub-Doppler temperatures required for many applications have not yet been reached. Here we demonstrate a MOT of a second species, CaF, and we show how to cool these molecules to 50 μK, well below the Doppler limit, using a three-dimensional optical molasses. These ultracold molecules could be loaded into optical tweezers to trap arbitrary arrays 10 for quantum simulation 11 , launched into a molecular fountain 12 , 13 for testing fundamental physics 14 , 15 , 16 , 17 , 18 , and used to study collisions and chemistry 19 between atoms and molecules at ultracold temperatures.

We present the properties of a magneto-optical trap (MOT) of CaF molecules. We study the process of loading the MOT from a decelerated buffer-gas-cooled beam, and how best to slow this molecular beam in order to capture the most molecules. We determine how the number of molecules, the photon scattering rate, the oscillation frequency, damping constant, temperature, cloud size and lifetime depend on the key parameters of the MOT, especially the intensity and detuning of the main cooling laser. We compare our results to analytical and numerical models, to the properties of standard atomic MOTs, and to MOTs of SrF molecules. We load up to 2 × 10 4 molecules, and measure a maximum scattering rate of 2.5 × 10 6 s−1 per molecule, a maximum oscillation frequency of 100 Hz, a maximum damping constant of 500 s−1, and a minimum MOT rms radius of 1.5 mm. A minimum temperature of 730 K is obtained by ramping down the laser intensity to low values. The lifetime, typically about 100 ms, is consistent with a leak out of the cooling cycle with a branching ratio of about 6 × 10 − 6 . The MOT has a capture velocity of about 11 m s−1.

Isotope shifts (ISs) of atomic energy levels are sensitive probes of nuclear structure and new physics beyond the standard model. We present an analysis of the ISs of the cadmium atom (Cd I) and singly charged cadmium ion (Cd II). ISs of the 229 nm, 326 nm, 361 nm and 480 nm lines of Cd I are measured with a variety of techniques; buffer–gas-cooled beam spectroscopy, capturing atoms in a magneto-optic-trap, and optical pumping. IS constants for the D 1 and D 2 lines of Cd II are calculated with high accuracy by employing analytical response relativistic coupled-cluster theory in the singles, doubles and triples approximations. Combining the calculations for Cd II with experiments, we infer IS constants for all low-lying transitions in Cd I. We benchmark existing calculations via different many-body methods against these constants. Our calculations for Cd II enable nuclear charge radii of Cd isotopes to be extracted with unprecedented accuracy. The combination of our precise calculations and measurements shows that King plots for Cd I can improve the state-of-the-art sensitivity to a new heavy boson by up to two orders of magnitude.

Using frequency-chirped radiation pressure slowing, we precisely control the velocity of a pulsed CaF molecular beam down to a few m s-1, compressing its velocity spread by a factor of 10 while retaining high intensity: at a velocity of 15 m s-1 the flux, measured 1.3 m from the source, is 7 × 105 molecules per cm2 per shot in a single rovibrational state. The beam is suitable for loading a magneto-optical trap or, when combined with transverse laser cooling, improving the precision of spectroscopic measurements that test fundamental physics. We compare the frequency-chirped slowing method with the more commonly used frequency-broadened slowing method.

Many modern theories predict that the fundamental constants depend on time, position or the local density of matter. Here we develop a spectroscopic method for pulsed beams of cold molecules, and use it to measure the frequencies of microwave transitions in CH with accuracy down to 3 Hz. By comparing these frequencies with those measured from sources of CH in the Milky Way, we test the hypothesis that fundamental constants may differ between the high- and low-density environments of the Earth and the interstellar medium. For the fine structure constant we find Δ α / α =(0.3±1.1) × 10 −7 , the strongest limit to date on such a variation of α . For the electron-to-proton mass ratio we find Δ μ / μ =(−0.7±2.2) × 10 −7 . We suggest how dedicated astrophysical measurements can improve these constraints further and can also constrain temporal variation of the constants. Some theories predict that fundamental constants may depend on time, position or the local density of matter. Truppe et al. compare new precise frequency measurements of microwave transitions in cold CH with Milky Way data, placing a new limit on variation in the fine structure constant.

We demonstrate rapid loading of a magneto‐optical trap (MOT) of cadmium atoms from a pulsed cryogenic helium buffer gas beam, overcoming strong photoionization losses. Using the 1S0→1P1 $ ^1S_0 \\rightarrow {} ^1P_1$transition at 229 nm, we capture up to 1.1(2)×107112Cd $ 1.1(2) \\times 10^{7\\;\\;112}{\\rm Cd}$atoms in 10 ms, achieving a peak density of 2.5×1011 $2.5 \\times 10^{11}$cm−3 $^{-3}$and a phase‐space density of 2×10−9 $ 2 \\times 10^{-9}$ . The large scattering force in the deep ultraviolet enables Zeeman slowing within 5 cm of the trap, yielding a capture velocity exceeding 200 m/s. We measure the MOT trap frequency and damping constant, and determine the absolute photoionization cross‐section of the 1P1 $^1P_1$state. Photoionization losses are mitigated via dynamic detuning of the trapping light's frequency, allowing efficient accumulation of multiple atomic pulses. Our results demonstrate the benefits of deep ultraviolet (DUV) transitions and cryogenic beams for loading high‐density MOTs, especially for species with significant loss channels in their main cooling cycle. The cadmium MOT provides a robust testbed that benchmarks our DUV laser cooling system and establishes the foundation for trapping and cooling polar AlF molecules, which share many optical and structural properties with Cd. We demonstrate laser cooling and trapping of cadmium atoms using deep‐ultraviolet light at 229 nm, capturing more than 1×107 $1\\times10^{7}$atoms within 10 ms from a compact 5 cm Zeeman slower and cryogenic beam source. We achieve peak densities of 2.5×1011cm−3 $2.5\\times10^{11} \\,\\text{cm}^{-3}$and phase‐space densities of 2×10−9 $2\\times10^{-9}$ . By dynamically tuning the cooling light's frequency we suppress photionization losses enbling efficient accumulation from multiple atomic pulses. These results establish cadmium as a powerful new platform for precision measurement and quantum science, with direct applications to optical lattice clocks, isotope‐shift spectroscopy, and the laser cooling of molecules.

This cover image sequence visualizes the dynamics of an ultracold cloud containing about ten million cadmium atoms held in a magneto‐optical trap. Padilla‐Castillo et al. intentionally displace the atomic cloud from its equilibrium position using a brief pulse of laser light. The subsequent images capture the cloud's damped harmonic motion as it oscillates back toward the trap center. By analyzing this oscillation, the researchers can precisely measure the trap's fundamental frequency and damping coefficient. This characterization is essential for optimizing the system for applications in quantum simulation and the development of next‐generation optical atomic clocks. More details can be found in the Research Article by Stefan Truppe and co‐workers (DOI: 10.1002/ntls.70023).

Magneto-optical trapping of molecules has thus far been restricted to molecules with \\(^2\\) electronic ground states. These species are chemically reactive and only support a simple laser cooling scheme from their first excited rotational level. Here, we demonstrate a magneto-optical trap (MOT) of aluminum monofluoride (AlF), a deeply bound and intrinsically stable diatomic molecule with a \\(^1^+\\) electronic ground state. The MOT operates on the strong A\\(^1\\)X\\(^1^+\\) transition near 227.5~nm, whose Q\\((J)\\) lines are all rotationally closed. We demonstrate a MOT of about \\(6 10^4\\) molecules for the \\(J=1\\) level of AlF, more than \\(10^4\\) molecules for \\(J=2\\) and \\(3\\), and with no fundamental limit in going to higher rotational levels. Laser cooling and trapping of AlF is conceptually similar to the introduction of alkaline-earth atoms into cold atom physics, and is key to leveraging its spin-forbidden a\\(^3 \\)X\\(^1^+\\) transition for precision spectroscopy and narrow-line cooling.

In this work, we explore the role of chemical reactions on the properties of buffer gas-cooled molecular beams. In particular, we focus on scenarios relevant to the formation of AlF and CaF via chemical reactions between the Ca and Al atoms ablated from a solid target in an atmosphere of a fluorine-containing gas, in this case, SF6 and NF3. Reactions are studied following an ab initio molecular dynamics approach, and the results are rationalized following a tree-shaped reaction model based on Bayesian inference. We find that NF3 reacts more efficiently with hot metal atoms to form monofluoride molecules than SF6. In addition, when using NF3, the reaction products have lower kinetic energy, requiring fewer collisions to thermalize with the cryogenic helium. Furthermore, we find that the reaction probability for AlF formation is much higher than for CaF across a broad range of temperatures.