arXiv subjects

G. Meijer

Publications and source records attributed to G. Meijer.

17 recordsLinked to original sources

arXiv2026

Non-stick vacuum wall collisions with a laser-coolable molecule

Molecular species that are suitable for direct laser cooling are typically considered lost or destroyed if they collide with an ambient temperature vacuum wall. Here, we study surface collisions with aluminum monofluoride (AlF), a laser-coolable molecule that survives this process with unusually high probability. We detect the outgoing AlF molecules from a single wall collision via Doppler-sensitive laser-induced fluorescence spectroscopy, using incoming supersonic (pulsed) and thermochemical (continuous) molecular beams. The angular, velocity and rovibrational level distributions of the outgoing molecules show near-complete thermalisation to the wall in a single collision event. We determine an upper limit to the surface residence time of about 5$~\mu$s, and by monitoring the decay in density of pulses of molecules loaded into a small storage volume, we deduce the surface sticking probability for different materials. For a siloxane-coated metallic surface, the sticking probability of AlF is about 0.015, allowing us to accumulate molecules from the thermochemical source into an ambient temperature storage vessel at densities near $10^{8}~$cm$^{-3}$. This provides a route to compact, portable traps for neutral molecules.

physics.atom-ph↗

arXiv2026

A dipolar Bose-Bose mixture of Dysprosium isotopes with controllable interspecies interactions

We report on the realization of a quantum-degenerate Bose-Bose mixture of 162Dy and 164Dy. Owing to the near-identical mass and polarizability of the two isotopes, the mixture thermalizes efficiently, with evaporation trajectories closely following those of the single-isotope case. Using a broad interspecies Feshbach resonance, we explore a miscible-immiscible transition between the two Bose-Einstein condensates. The tunability of the interspecies interaction, combined with the large magnetic dipole moment of Dy, makes this platform well suited for exploring dipolar effects in ultracold mixtures, including multi-component supersolidity.

cond-mat.quant-gas↗

arXiv2026

Dual-isotope narrow-line MOT of dysprosium by phase modulation

We report on the characterization of a narrow-line magneto-optical trap (MOT), where two different isotopes of the dysprosium (Dy) atom can be simultaneously loaded and trapped. We rely on phase modulation via high-power, free-space electro-optic modulators (EOMs) to generate optical sidebands at the correct detuning for two different isotopes. This technique is applied on the slowing transition at 421 nm and the MOT transition at 626 nm. Using modular resonant electronic circuits, we match the sideband separation to the isotope shift among the two target isotopes, realizing mixtures of the bosonic isotopes 164Dy -162Dy and 162Dy -160Dy. We exploit the interplay between radiation forces and gravity in the narrow-line MOT to control the spatial position of the two isotopes with the EOM drive frequency. We demonstrate full control of the isotopic mixture with the EOM drive, offering new and cost-effective capabilities for controlling mixtures of atomic species that can be trapped in a narrow-line MOT.

physics.atom-ph↗

arXiv2025

Magneto-optical trapping of aluminum monofluoride

Magneto-optical trapping of molecules has thus far been restricted to molecules with $^2\Sigma$ 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 $^1\Sigma^+$ electronic ground state. The MOT operates on the strong A$^1\Pi\leftarrow{}$X$^1\Sigma^+$ transition near 227.5~nm, whose Q$(J)$ lines are all rotationally closed. We demonstrate a MOT of about $6\times 10^4$ 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$^3\Pi \leftarrow{}$X$^1\Sigma^+$ transition for precision spectroscopy and narrow-line cooling.

physics.atom-ph↗

arXiv2025

A large magneto-optical trap of cadmium atoms loaded from a cryogenic buffer gas beam

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 $ ^1S_0 \rightarrow{} ^1P_1 $ transition at 229 nm, we capture up to $ 1.1(2) \times 10^7$ $^{112}$Cd atoms in 10 ms, achieving a peak density of $2.5 \times 10^{11}$cm$^{-3}$ and a phase-space density of $ 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 $^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-UV (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.

physics.atom-ph↗

arXiv2024

Triplet Rydberg states of aluminum monofluoride

Aluminum monofluoride (AlF) is a suitable molecule for laser cooling and trapping. Such experiments require an extensive spectroscopic characterization of the electronic structure. Two of the theoretically predicted higher lying triplet states of AlF, the counterparts of the well-characterized D$^1\Delta$ and E$^1\Pi$ states, had experimentally not been identified yet. We here report on the characterization of the d$^3\Pi$ ($v=0-6$) and e$^3\Delta$ ($v=0-2$) states, confirming the predicted energetic ordering of these states (J. Chem. Phys. 88 (1988) 5715-5725), as well as of the f$^3\Sigma^+$ ($v=0-2$) state. The transition intensity of the d$^3\Pi, v=3$ $-$ a$^3\Pi, v=3$ band is negligibly small. This band gets its weak, unexpected rotational structure via intensity borrowing from the nearby e$^3\Delta, v=2$ $-$ a$^3\Pi, v=3$ band, made possible via spin-orbit and spin-rotation interaction between the d$^3\Pi$ and e$^3\Delta$ states. This interaction affects the equilibrium rotational constants in both states; their deperturbed values yield equilibrium internuclear distances that are consistent with the observations. We determine the ionization potential of AlF to be 78492(1) cm$^{-1}$ by ionization from the d$^3\Pi$ state.

physics.chem-ph↗

arXiv2024

Understanding inner-shell excitations in molecules through spectroscopy of the 4f hole states of YbF

Molecules containing a lanthanide atom have sets of electronic states arising from excitation of an inner-shell electron. These states have received little attention, but are thought to play an important role in laser cooling of such molecules and may be a useful resource for testing fundamental physics. We study a series of inner-shell excited states in YbF using resonance-enhanced multi-photon ionisation spectroscopy. We investigate the excited states of lowest energy, 8474, 9013 and 9090 cm$^{-1}$ above the ground state, all corresponding to the configuration 4f$^{13}$6s$^{2}$ ${}^{2}F_{7/2}$ of the Yb$^+$ ion. They are metastable, since they have no electric dipole allowed transitions to the ground state. We also characterize a state at 31050 cm$^{-1}$ that is easily excited from both the ground and metastable states, which makes it especially useful for this spectroscopic study. Finally, we study a state at 48729 cm$^{-1}$, which is above the ionization limit and features strong auto-ionizing resonances that prove useful for efficient detection of the molecules and for identifying the rotational quantum number of each line in the spectrum. We resolve the rotational structures of all these states and find that they can all be described by a very simple model based on Hund's case (c). Our study provides information necessary for laser slowing and magneto-optical trapping of YbF, which is an important species for testing fundamental physics. We also consider whether the low-lying inner-shell states may themselves be useful as probes of the electron's electric dipole moment or of varying fundamental constants, since they are long-lived states in a laser-coolable molecule featuring closely-spaced levels of opposite parity.

physics.atom-ph↗

arXiv2024

Reconciling mean-squared radius differences in the silver chain through improved measurement and {\it ab initio} calculations

Nuclear charge radius differences in the silver isotopic chain have been reported through different combinations of experiment and theory, exhibiting a tension of two combined standard errors. This study investigates this issue by combining high-accuracy calculations for six low-lying states of atomic silver with an improved measurement of the $5s ^2S_{1/2} - 5p ^2P_{3/2}$ transition optical isotope shift. Our calculations predict measured electronic transition energies in Ag I at the 0.3\% level, the highest accuracy achieved in this system so far. We calculate electronic isotope shift factors by employing analytical response relativistic coupled-cluster theory, and find that a consistent charge radius difference between $^{107,109}$Ag is returned when combining our calculations with the available optical isotope shift measurements. We therefore recommend an improved value for the mean-squared charge radius difference between $^{107}$Ag and $^{109}$Ag as $0.207(3)[4]$ fm$^2$, within one combined error from the value derived from muonic Ag experiments, and an updated set of charge radii differences across the isotopic chain.

physics.atom-ph↗

arXiv2022

Isotope Shifts in Cadmium as a Sensitive Probe for Physics Beyond the Standard Model

Isotope shifts (ISs) in 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 D1 and D2 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 these constants as calculated via different many-body methods. 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-theart sensitivity to a new heavy boson by up to two orders of magnitude.

physics.atom-ph↗

arXiv2022

The chemistry of AlF and CaF production in buffer gas sources

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.

physics.comp-ph↗

arXiv2022

Hyperfine resolved optical spectroscopy of the A$^2Π\leftarrow $X$^2Σ^+$ transition in MgF

We report on hyperfine-resolved laser spectroscopy of the A$^2Π\leftarrow $X$^2Σ^+$ transition of 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 A$^2Π$ state. The radiative lifetime of the A$^2Π$ state was determined to be 7.2(3) ns, in good agreement with \textit{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 A$^2Π\leftarrow $X$^2Σ^+$ transition by applying electric fields of up to 10.6 kV cm$^{-1}$ and determined the permanent electric dipole moments of $^{24}$MgF in its ground X$^2Σ^+$ and first excited A$^2Π$ 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 $10^4$ photons, the electric field must be controlled to below 1 V cm$^{-1}$.

physics.atom-ph↗

arXiv2019

Spectroscopic characterization of aluminum monofluoride with relevance to laser cooling and trapping

Here we report on spectroscopic measurements of the aluminum monofluoride molecule (AlF) that are relevant to laser cooling and trapping experiments. We measure the detailed energy level structure of AlF in the X$^1Σ^+$ electronic ground state, in the A$^1Π$ state, and in the metastable a$^3Π$ state. We determine the rotational, vibrational and electronic branching ratios from the A$^1Π$ state. We also study how the rotational levels split and shift in external electric and magnetic fields. We find that AlF is an excellent candidate for laser cooling on any Q-line of the A$^1Π$ - X$^1Σ^+$ transition and for trapping at high densities.

physics.atom-ph↗

arXiv2013

Imaging Cold Molecules on a Chip

We present the integrated imaging of cold molecules in a microchip environment. The on-chip de- tection is based on REMPI, which is quantum-state-selective and generally applicable. We demon- strate and characterize time-resolved spatial imaging and subsequently use it to analyze the effect of a phase-space manipulation sequence aimed at compressing the velocity distribution of a molec- ular ensemble with a view to future high-resolution spectroscopic studies. The realization of such on-chip measurements adds the final fundamental component to the molecule chip, offering a new and promising route for investigating cold molecules.

physics.atom-ph↗

arXiv2010

Secondary structure of Ac-Ala$_n$-LysH$^+$ polyalanine peptides ($n$=5,10,15) in vacuo: Helical or not?

The polyalanine-based peptide series Ac-Ala_n-LysH+ (n=5-20) is a prime example that a secondary structure motif which is well-known from the solution phase (here: helices) can be formed in vacuo. We here revisit this conclusion for n=5,10,15, using density-functional theory (van der Waals corrected generalized gradient approximation), and gas-phase infrared vibrational spectroscopy. For the longer molecules (n=10,15) α-helical models provide good qualitative agreement (theory vs. experiment) already in the harmonic approximation. For n=5, the lowest energy conformer is not a simple helix, but competes closely with α-helical motifs at 300K. Close agreement between infrared spectra from experiment and ab initio molecular dynamics (including anharmonic effects) supports our findings.

physics.bio-ph↗

arXiv2009

The Structure of Rh$_8^+$ in the Gas Phase

The geometric structure of the Rh$_8^+$ cation is investigated using a combination of far-infrared multiple photon dissociation spectroscopy and density functional theory (DFT) calculations. The energetic ordering of the different structural motifs is found to depend sensitively on the choice of pure or hybrid exchange functionals. Comparison of experimental and calculated spectra suggests the cluster to have a close-packed, bicapped octahedral structure, in contrast to recent predictions of a cubic structure for the neutral cluster. Our findings demonstrate the importance of including some exact exchange contributions in the DFT calculations, via hybrid functionals, when applied to rhodium clusters, and cast doubt on the application of pure functionals for late transition metal clusters in general.

physics.atm-clus↗

arXiv2007

Cold SO_2 molecules by Stark deceleration

We produce SO_2 molecules with a centre of mass velocity near zero using a Stark decelerator. Since the initial kinetic energy of the supersonic SO_2 molecular beam is high, and the removed kinetic energy per stage is small, 326 deceleration stages are necessary to bring SO_2 to a complete standstill, significantly more than in other experiments. We show that in such a decelerator possible loss due to coupling between the motional degrees of freedom must be considered. Experimental results are compared with 3D Monte-Carlo simulations and the quantum state selectivity of the Stark decelerator is demonstrated.

physics.atom-ph↗

arXiv2003

Slowing heavy, ground-state molecules using an alternating gradient decelerator

Cold supersonic beams of molecules can be slowed down using a switched sequence of electrostatic field gradients. The energy to be removed is proportional to the mass of the molecules. Here we report deceleration of YbF, which is 7 times heavier than any molecule previously decelerated. We use an alternating gradient structure to decelerate and focus the molecules in their ground state. We show that the decelerator exhibits the axial and transverse stability required to bring these molecules to rest. Our work significantly extends the range of molecules amenable to this powerful method of cooling and trapping.

physics.atom-ph↗