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Recently, we have demonstrated a method to record the longitudinal phase-space distribution of a pulsed cryogenic buffer gas cooled beam of barium fluoride molecules. In this paper, we use this method to determine the influence of various source parameters. Besides the expected dependence on temperature and pressure, the forward velocity of the molecules is strongly correlated with the time they exit the cell, revealing the dynamics of the gas inside the cell. Three observations are particularly noteworthy: (1) The velocity of the barium fluoride molecules increases rapidly as a function of time, reaches a maximum 50-200 $\mu$s after the ablation pulse and then decreases exponentially. We attribute this to the buffer gas being heated up by the plume of hot atoms released from the target by the ablation pulse and subsequently being cooled down via conduction to the cell walls. (2) The time constant associated with the exponentially decreasing temperature increases when the source is used for a longer period of time, which we attribute to the formation of a layer of isolating dust on the walls of the cell. By thoroughly cleaning the cell, the time constant is reset to its initial value. (3) The velocity of the molecules at the trailing end of the molecular pulse depends on the length of the cell. For short cells, the velocity is significantly higher than expected from the sudden freeze model. We attribute this to the target remaining warm over the duration of the molecular pulse giving rise to a temperature gradient within the cell. Our observations will help to optimize the source parameters for producing the most intense molecular beam at the target velocity., Comment: 17 pages, 7 figures

We demonstrate a spin-precession method to observe and analyze multi-level coherence between all hyperfine levels in the $X ^2\Sigma^+,N=0$ ground state of barium monofluoride ($^{138}$Ba$^{19}$F). The signal is sensitive to the state-preparation Rabi frequency and external electric and magnetic fields applied in searches for a permanent electric dipole moment (EDM). In the obtained interference spectrum, the electric field and Rabi frequency become observable simultaneously with the EDM. This method reduces systematic biases and the number of auxiliary measurements for such precision measurements., Comment: 5 pages, 4 figures

We report on the electrostatic trapping of neutral SrF molecules. The molecules are captured from a cryogenic buffer-gas beam source into the moving traps of a 4.5 m long traveling-wave Stark decelerator. The SrF molecules in $X^2\Sigma^+(v=0, N=1)$ state are brought to rest as the velocity of the moving traps is gradually reduced from 190 m/s to zero. The molecules are held for up to 50 ms in multiple electric traps of the decelerator. The trapped packets have a volume (FWHM) of 1 mm$^{3}$ and a velocity spread of 5(1) m/s which corresponds to a temperature of $60(20)$ mK. Our result demonstrates a factor 3 increase in the molecular mass that has been Stark-decelerated and trapped. Heavy molecules (mass$>$100 amu) offer a highly increased sensitivity to probe physics beyond the Standard Model. This work significantly extends the species of neutral molecules of which slow beams can be created for collision studies, precision measurement and trapping experiments.

Time resolved detection of laser induced fluorescence from pulsed excitation of electronic states in barium monofluoride (BaF) molecules has been performed in order to determine the lifetimes of the $A^2\Pi_{1/2}$ and $A^2\Pi_{3/2}$ states. The method permits control over experimental parameters such that systematic biases in the interpretation of the data can be controlled to below $10^{-3}$ relative accuracy. The statistically limited values for the lifetimes of the $A^2\Pi_{1/2}(\nu=0)$ and $A^2\Pi_{3/2}(\nu=0)$ states are 57.1(3) ns and 47.9(7)~ns, respectively. The ratio of these values is in good agreement with scaling for the different excitation energies. The investigated molecular states are of relevance for an experimental search for a permanent electric dipole moment (EDM) of the electron in BaF.

We report on the deceleration of a beam of SrF molecules from 290 to 120~m/s. Following supersonic expansion, the molecules in the $X^2\Sigma$ ($v=0$, $N=1$) low-field seeking states are trapped by the moving potential wells of a traveling-wave Stark decelerator. With a deceleration strength of 9.6 km/s$^2$ we have demonstrated the removal of 85 % of the initial kinetic energy in a 4 meter long modular decelerator. The absolute amount of kinetic energy removed is a factor 1.5 higher compared to previous Stark deceleration experiments. The demonstrated decelerator provides a novel tool for the creation of highly collimated and slow beams of heavy diatomic molecules, which serve as a good starting point for high-precision tests of fundamental physics.

We search for a dependence of the lifetime of $^{20}\text{Na}$ nuclei on the nuclear spin direction. Such a directional dependence would be evidence for Lorentz-invariance violation in weak interactions. A difference in lifetime between nuclei that are polarized in the east and west direction is searched for. This difference is maximally sensitive to the rotation of the Earth, while the sidereal dependence is free from most systematic errors. The experiment sets a limit of $2\times 10^{-4}$ at 90 % C.L. on the amplitude of the sidereal variation of the relative lifetime differences, an improvement by a factor 15 compared to an earlier result., Comment: 23 pages, 8 figures, 4 tables

A novel mid-infrared/near-infrared double resonant absorption setup for studying infrared-inactive vibrational states is presented. A strong vibrational transition in the mid-infrared region is excited using an idler beam from a singly resonant continuous-wave optical parametric oscillator, to populate an intermediate vibrational state. High output power of the optical parametric oscillator and the strength of the mid-infrared transition result in efficient population transfer to the intermediate state, which allows measuring secondary transitions from this state with a high signal-to-noise ratio. A secondary, near-infrared transition from the intermediate state is probed using cavity ring down spectroscopy, which provides high sensitivity in this wavelength region. Due to the narrow linewidths of the excitation sources, the rovibrational lines of the secondary transition are measured with sub-Doppler resolution. The setup is used to access a previously unreported symmetric vibrational state of acetylene, $\nu_1+\nu_2+\nu_3+\nu_4^1+\nu_5^{-1}$ in the normal mode notation. Single-photon transitions to this state from the vibrational ground state are forbidden. Ten lines of the newly measured state are observed and fitted with the linear least-squares method to extract the band parameters. The vibrational term value was measured to be at 9775.0018(45) $\text{cm}^{-1}$, the rotational parameter $B$ was 1.162222 $\text{cm}^{-1}$, and the quartic centrifugal distortion parameter $D$ was 3.998(62)$\times 10^{-6} \text{cm}^{-1}$, where the numbers in the parenthesis are one-standard errors in the least significant digits.

A radioactive beam of 20Na is stopped in a gas cell filled with Ne gas. The stopped particles are polarized by optical pumping. The degree of polarization that can be achieved is studied. A maximum polarization of 50% was found. The dynamic processes in the cell are described with a phenomenological model., Comment: 16 pages, 6 figures

We report on the production, deceleration and detection of a SrF molecular beam. The molecules are captured from a supersonic expansion and are decelerated in the X$^2\Sigma^+ (v=0, N=1)$ state. We demonstrate the removal of up to 40% of the kinetic energy with a 2 meter long modular traveling-wave decelerator. Our results demonstrate a crucial step towards the preparation of ultracold gases of heavy diatomic molecules for precision spectroscopy.

A new test of Lorentz invariance in the weak interactions has been made by searching for variations in the decay rate of spin-polarized 20Na nuclei. This test is unique to Gamow-Teller transitions, as was shown in the framework of a recently developed theory that assumes a Lorentz symmetry breaking background field of tensor nature. The nuclear spins were polarized in the up and down direction, putting a limit on the amplitude of sidereal variations of the form |(\Gamma_{up} - \Gamma_{down})| / (\Gamma_{up} + \Gamma_{down}) < 3 * 10^{-3}. This measurement shows a possible route toward a more detailed testing of Lorentz symmetry in weak interactions., Comment: 11 pages, 6 figures

The diatomic molecule radium monofluoride (RaF) has recently been proposed as a versatile probe for physics beyond the current standard model. Herein, a route towards production of a RaF molecular beam via radium ions is proposed. It takes advantage of the special electronic structure expected for group 2 halides and group 2 hydrides: The electronic ground state of neutral RaF and its monocation differ in occupation of a non-bonding orbital of $\sigma$ symmetry. This implies similar equilibrium distances and harmonic vibrational wavenumbers in the two charge states and thus favourable Franck--Condon factors for neutralisation without dissociation in neutralising collisions. According to the calculated ionisation energy of RaF, charge exchange collisions of RaF$^+$ with sodium atoms are almost iso-enthalpic, resulting in large cross-sections for the production of neutral radium monofluoride., Comment: 3 pages, 1 figure, 1 table, 1 ancillary file; references added in ancillary file

Lorentz invariance in the weak interaction has been tested rather poorly compared to the electromagnetic interaction. In this work we show which tests on the weak interaction should be considered. We focus on one particular test that explores the spin degree of freedom in {\beta} decay. To connect various phenomenological tests of Lorentz invariance in the weak interaction, we exploit a new theoretical model that may provide a framework that relates the different tests., Comment: 12 pages, 4 figures

The parameter $W_\mathrm{a}$, which characterizes nuclear spin-dependent parity violation effects within the effective molecular spin-rotational Hamiltonian, was computed for the electronic ground state of radium fluoride (RaF) and found to be one of the largest absolute values predicted so far. These calculations were performed with the complex generalised Hartree-Fock method within a two-component (quasi-relativistic) zeroth-order regular approximation framework. Peculiarities of the molecular electronic structure of RaF lead to highly diagonal Franck-Condon matrices between vibrational states of the electronic ground and first excited states, which renders the molecule in principle suitable for direct laser cooling. As a trapped gas of cold molecules offers a superior coherence time, RaF can be considered a promising candidate for high-precision spectroscopic experiments aimed at the search of molecular parity-violation effects., Comment: 4.5 pages, 1 figure, 2 tables. Supplementary material can be requested from the authors. Minor changes to version 1

We demonstrate a combination of an isotopically purified atom beam and a magneto-optical trap which enables the single atom detection of all stable isotopes of calcium (40, 42, 43, 44, 46 and 48). These isotopes range in abundance from 96.9 % (40Ca) to 0.004 (46Ca). The trap is loaded from an atomic beam which is decelerated in a Zeeman slower and subsequently deflected over an angle of 30 degrees by optical molasses. The isotope selectivity of the Zeeman slower and the deflection stage is investigated experimentally and compared with Monte Carlo simulations., Comment: 8 pages, 9 figures, accepted for publication (PRA)