Measurement of the lowest millimetre-wave transition frequency of the CH radical

The CH radical offers a sensitive way to test the hypothesis that fundamental constants measured on earth may differ from those observed in other parts of the universe. The starting point for such a comparison is to have accurate laboratory frequencies. Here we measure the frequency of the lowest millimetre-wave transition of CH, near 535 GHz, with an accuracy of 0.6 kHz. This improves the uncertainty by roughly two orders of magnitude over previous determinations and opens the way for sensitive new tests of varying constants.Comment: 5 pages, 5 figure

Vibrational branching ratios and hyperfine structure of 11^{11}BH and its suitability for laser cooling

The simple structure of the BH molecule makes it an excellent candidate for direct laser cooling. We measure the branching ratios for the decay of the ${\rm A}^{1}\Pi (v'=0)$ state to vibrational levels of the ground state, ${\rm X}^{1}\Sigma^{+}$, and find that they are exceedingly favourable for laser cooling. We verify that the branching ratio for the spin-forbidden transition to the intermediate ${\rm a}^{3}\Pi$ state is inconsequentially small. We measure the frequency of the lowest rotational transition of the X state, and the hyperfine structure in the relevant levels of both the X and A states, and determine the nuclear electric quadrupole and magnetic dipole coupling constants. Our results show that, with a relatively simple laser cooling scheme, a Zeeman slower and magneto-optical trap can be used to cool, slow and trap BH molecules.Comment: 7 pages, 5 figures. Updated analysis of A state hyperfine structure and other minor revision

A high quality, efficiently coupled microwave cavity for trapping cold molecules

We characterize a Fabry-Perot microwave cavity designed for trapping atoms and molecules at the antinode of a microwave field. The cavity is fed from a waveguide through a small coupling hole. Focussing on the compact resonant modes of the cavity, we measure how the electric field profile, the cavity quality factor, and the coupling efficiency, depend on the radius of the coupling hole. We measure how the quality factor depends on the temperature of the mirrors in the range from 77 to 293K. The presence of the coupling hole slightly changes the profile of the mode, leading to increased diffraction losses around the edges of the mirrors and a small reduction in quality factor. We find the hole size that maximizes the intra-cavity electric field. We develop an analytical theory of the aperture-coupled cavity that agrees well with our measurements, with small deviations due to enhanced diffraction losses. We find excellent agreement between our measurements and finite-difference time-domain simulations of the cavity.Comment: 16 pages, 8 figure

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

We report on hyperfine-resolved laser spectroscopy of the A2Π ←X2Σ+ 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 A2Π state. The radiative lifetime of the A2Π 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 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 μX=2.88(20) D and μA=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

A search for varying fundamental constants using Hz-level frequency measurements of cold CH molecules

Many modern theories predict that the fundamental constants depend on time, position, or the local density of matter. 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 \Delta\alpha/\alpha = (0.3 +/- 1.1)*10^{-7}, the strongest limit to date on such a variation of \alpha. For the electron-to-proton mass ratio we find \Delta\mu/\mu = (-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.Comment: 8 pages, 3 figure

Cryogenic Buffer Gas beams of AlF, CaF, MgF, YbF, Al, Ca, Yb and NO -- a comparison

Cryogenic buffer gas beams are central to many cold molecule experiments. Here, we use absorption and fluorescence spectroscopy to directly compare molecular beams of AlF, CaF, MgF, and YbF molecules, produced by chemical reaction of laser ablated atoms with fluorine rich reagents. The beam brightness for AlF is measured as 2 X 1012 molecules per steradian per pulse in a single rotational state, comparable to an Al atomic beam produced in the same setup. The CaF, MgF and YbF beams show an order of magnitude lower brightness than AlF, and far below the brightness of Ca and Yb beams. The addition of either NF3 or SF6 to the cell extinguishes the Al atomic beam, but has a minimal effect on the Ca and Yb beams. NF3 reacts more efficiently than SF6, as a significantly lower flow rate is required to maximise the molecule production, which is particularly beneficial for long-term stability of the AlF beam. We use NO as a proxy for the reactant gas as it can be optically detected. We demonstrate that a cold, rotationally pure NO beam can be generated by laser desorption, thereby gaining insight into the dynamics of the reactant gas inside the buffer gas cell

Microwave spectroscopy of Lambda-doublet transitions in the ground state of CH

The Lambda-doublet transitions in CH at 3.3 and 0.7 GHz are unusually sensitive to variations in the fine-structure constant and the electron-to-proton mass ratio. We describe methods used to measure the frequencies of these transitions with Hz-level accuracy. We produce a pulsed supersonic beam of cold CH by photodissociation of CHBr3, and we measure the microwave transition frequencies as the molecules propagate through a parallel-plate transmission line resonator. We use the molecules to map out the amplitude and phase of the standing wave field inside the transmission line. We investigate velocity-dependent frequency shifts, showing that they can be strongly suppressed through careful timing of the microwave pulses. We measure the Zeeman and Stark effects of the microwave transitions, and reduce systematic shifts due to magnetic and electric fields to below 1 Hz. We also investigate other sources of systematic uncertainty in the experiment.Comment: 27 pages, 12 figure

Spectroscopic characterization of singlet-triplet doorway states of aluminum monofluoride

Aluminum monofluoride (AlF) possesses highly favorable properties for laser cooling, both via the A1Π and a3Π states. Determining efficient pathways between the singlet and the triplet manifold of electronic states will be advantageous for future experiments at ultralow temperatures. The lowest rotational levels of the A1Π, v = 6 and b3Σ+, v = 5 states of AlF are nearly iso-energetic and interact via spin–orbit coupling. These levels thus have a strongly mixed spin-character and provide a singlet–triplet doorway. We here present a hyperfine resolved spectroscopic study of the A1Π , v = 6//b3Σ+, v = 5 perturbed system in a jet-cooled, pulsed molecular beam. From a fit to the observed energies of the hyperfine levels, the fine and hyperfine structure parameters of the coupled states and their relative energies as well as the spin–orbit interaction parameter are determined. The standard deviation of the fit is about 15 MHz. We experimentally determine the radiative lifetimes of selected hyperfine levels by time-delayed ionization, Lamb dip spectroscopy, and accurate measurements of the transition lineshapes. The measured lifetimes range between 2 and 200 ns, determined by the degree of singlet–triplet mixing for each level

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 kinetic temperatures