Cold molecules are an invaluable tool for modern physics. Cold atoms have already revolutionized precision metrology, allowed for spectacular findings in quantum physics and opened the door for the creation and observation of novel states of matter. Cold molecules bring with them additional degrees of freedom which give rise to unique properties that enable new kinds of experiments. For these experiments it is of vital importance to start with a sufficiently large number of cold molecules. There are multiple paths for the slowing and cooling of atoms and most of the slowing techniques utilizing lasers for direct cooling have been transferred to the molecular case with its specific requirements. The Zeeman slowing technique is advantageous for loading a trap with cold particles but had not yet been transferred from atoms to the molecular case. In this thesis I will show the first realization of the Zeeman slowing technique for diatomic molecules. This thesis will discuss several preparatory steps: Our group has had some ideas to collimate the molecular beam with a conical nozzle outlet at the buffer gas cell which I investigated on potassium atoms generated by the ablation of 39K35Cl inside the buffer gas cell. I also realized a fluorescence spectroscopy of the |XΣ1/2, N = 1, v = 0⟩ → |AΠ1/2, N = 0, v = 0⟩ transition in magnetic fields up to 300 G and confirmed simulations regarding our two laser cycling scheme in the Paschen-Back regime by measuring the scattering rate of light on one molecule. Finally, I created pre-cooled 40Ca19F with a forward velocity of around 150 m s−1 from a 40Ca19F2 target with laser ablation inside a cryogenic buffer gas cell. With the Zeeman slowing technique I slowed the molecules down to 25 m s−1, a velocity where trapping can be possible. I have also shown the characteristic compression into specific velocity classes for not yet trappable velocities. Together these two findings demonstrate that molecular Zeeman slowing is possible and a real promising tool for future experiments.
Cold molecules are an invaluable tool for modern physics. Cold atoms have already revolutionized precision metrology, allowed for spectacular findings in quantum physics and opened the door for the creation and observation of novel states of matter. Cold molecules bring with them additional degrees of freedom which give rise to unique properties that enable new kinds of experiments. For these experiments it is of vital importance to start with a sufficiently large number of cold molecules. There are multiple paths for the slowing and cooling of atoms and most of the slowing techniques utilizing lasers for direct cooling have been transferred to the molecular case with its specific requirements. The Zeeman slowing technique is advantageous for loading a trap with cold particles but had not yet been transferred from atoms to the molecular case. In this thesis I will show the first realization of the Zeeman slowing technique for diatomic molecules. This thesis will discuss several preparatory steps: Our group has had some ideas to collimate the molecular beam with a conical nozzle outlet at the buffer gas cell which I investigated on potassium atoms generated by the ablation of 39K35Cl inside the buffer gas cell. I also realized a fluorescence spectroscopy of the |XΣ1/2, N = 1, v = 0⟩ → |AΠ1/2, N = 0, v = 0⟩ transition in magnetic fields up to 300 G and confirmed simulations regarding our two laser cycling scheme in the Paschen-Back regime by measuring the scattering rate of light on one molecule. Finally, I created pre-cooled 40Ca19F with a forward velocity of around 150 m s−1 from a 40Ca19F2 target with laser ablation inside a cryogenic buffer gas cell. With the Zeeman slowing technique I slowed the molecules down to 25 m s−1, a velocity where trapping can be possible. I have also shown the characteristic compression into specific velocity classes for not yet trappable velocities. Together these two findings demonstrate that molecular Zeeman slowing is possible and a real promising tool for future experiments.
Laser cooling and trapping are central to modern atomic physics. It has been roughly three decades since laser cooling techniques produced ultracold atoms, leading to rapid advances in a vast array of fields and a number of Nobel prizes. Prior to the work presented in this thesis, laser cooling had not yet been extended to molecules because of their complex internal structure. However, this complexity makes molecules potentially useful for a wide range of applications. The first direct laser cooling of a molecule and further results we present here provide a new route to ultracold temperatures for molecules. In particular, these methods bridge the gap between ultracold temperatures and the approximately 1 kelvin temperatures attainable with directly cooled molecules (e.g. with cryogenic buffer gas cooling or decelerated supersonic beams). Using the carefully chosen molecule strontium monofluoride (SrF), decays to unwanted vibrational states are suppressed. Driving a transition with rotational quantum number R=1 to an excited state with R'=0 eliminates decays to unwanted rotational states. The dark ground-state Zeeman sublevels present in this specific scheme are remixed via a static magnetic field. Using three lasers for this scheme, a given molecule should undergo an average of approximately 100,000 photon absorption/emission cycles before being lost via unwanted decays. This number of cycles should be sufficient to load a magneto-optical trap (MOT) of molecules. In this thesis, we demonstrate transverse cooling of an SrF beam, in both Doppler and a Sisyphus-type cooling regimes. We also realize longitudinal slowing of an SrF beam. Finally, we detail current progress towards trapping SrF in a MOT. Ultimately, this technique should enable the production of large samples of molecules at ultracold temperatures for molecules chemically distinct from competing methods.
Slowing, cooling, and trapping molecules One of the major achievements in physics during the last two decades of the twentieth century was the ability to cool and trap atoms [1]. Laser cooling can produce previously inconceivable atomic temperatures of less than 1 μK. Atoms with sufficiently low kinetic energy can be trapped in magnetic and optical traps and thus studied for a prolonged time, thereby improving the resolution of spectroscopy by many orders of magnitude as well as reducing Doppler broadening. Furthermore, besides the external degrees of freedom, the internal degrees of freedom of the atoms can also be manipulated, leading to full control of the atoms. One of the holy grails of physics, the achievement of Bose–Einstein condensation in a dilute gas, was achieved mainly because of laser cooling and trapping of atoms. One of the natural extensions of laser cooling and trapping of atoms is the application of the techniques to molecules. Molecules offer a much richer internal structure than atoms, and thus provide extended possibilities to experimentalists. However, atomic laser cooling is based on the repetitive scattering of light and requires a closed, or at least nearly closed, two-level system. Because of the complicated internal structure of molecules, a closed two-level system cannot usually be found because they have many different electronic, vibrational, and rotational states and in general can decay to many different states after absorption of light (see Chap. 16). This requires a complete rethinking of the schemes that are used to cool atoms. Still, because of the extended possibilities cold molecules can offer with respect to cold atoms, there have been a large number of techniques developed to slow, cool, and trap them. These techniques can be divided into two classes, namely direct and indirect cooling. In the first case, molecules at cryogenic or room temperature are directly slowed and cooled. Although many different slowing techniques are used, this chapter focuses on Stark slowing of molecules since it was the first technique developed. Apart from the Stark shift of the molecules, the Zeeman and light shift of molecules can also be used to slow them. The idea behind Stark slowing is to subject the moving molecules to a continuously increasing electric field so that their kinetic energy is converted into potential energy.
Abstract In this paper we investigate the feasibility of Zeeman slowing calcium monofluoride molecules originating from a cryogenic buffer gas cell. We measure the A 2 Π 1 / 2 ( v = 0 , J = 1 2 ) − X 2 Σ 1 / 2 ( v = 0 , N = 1 ) hyperfine spectrum of CaF in the Paschen–Back regime and find excellent agreement with theory. We then investigate the scattering rate of the molecules in a molecular Zeeman slower by illuminating them with light from a 10 mW broad repumper and a 10 mW multi-frequency slowing laser. By comparing our results to theory we can calculate the photon scattering rate at higher powers, leading to a force profile for Zeeman slowing. We show results from a simple 1D simulation demonstrating that this force is narrow enough in velocity space to lead to significant velocity compression, and slowing of the molecules to trappable velocities.
Slowing down atoms using laser cooling, together with techniques like magnetic trapping and evaporative cooling, has led to numerous exciting results over the last decade [1], with the realization of Bose-Einstein condensation as the most spectacular achievement [2]. Laser-cooling schemes are applicable to atoms since closed multilevel systems can be realized and many consecutive absorptionemission cycles can be induced. This leads to a significant momentum transfer from the photons of the laser to the atom, resulting in a change of the velocity of the atom. There currently is a great interest in producing cold molecules for the study of cold molecule-molecule collisions, molecular quantum collective effects, Doppler-free spectroscopy, frequency standards, as well as many other applications already realized for atoms [3]. Slowing down of neutral molecules, a prerequisite for being able to trap them, has proven to be considerably more difficult to be achieved, however. The above-mentioned laser-cooling scheme cannot be applied to slow down molecules as no simple closed-level systems can be found; the unavoidable off-resonant radiative decay of molecules from the excited state to other vibrational levels in the electronic ground state hampers efficient momentum transfer [4]. Once molecules are present at sufficiently low kinetic energies, below a fraction of a cm21, they can be trapped in various ways. Trapping of neutral molecules in inhomogeneous magnetic [3] and electric fields [5] and trapping based on the use of evanescent fields close to the dielectricvacuum interface have been proposed [6]. It has also been suggested that extremely high laser powers might be used to induce a dipole moment in polarizable molecules, leading to trapping of these molecules in the laser beam [7]. Other methods to produce slow molecules have been put forward and have recently been successfully applied to produce the first samples of trapped molecules. By injecting molecules in a cold buffer gas, sufficiently low kinetic energies of the injected molecules can be obtained after thermalization that they can subsequently be trapped in an inhomogeneous magnetic field. This has recently been demonstrated for CaH injected in 3He buffer gas at temperatures of around 0.3 K [8]. Associative collisions between ultracold trapped atoms [9] have proven to be
We present a novel slowing scheme for beams of laser-coolable diatomic molecules reminiscent of Zeeman slowing of atomic beams. The scheme results in efficient compression of the one-dimensional velocity distribution to velocities trappable by magnetic or magneto-optical traps. We experimentally demonstrate our method in an atomic testbed and show an enhancement of flux below v = 35 m s −1 by a factor of ≈20 compared to white light slowing. 3D Monte Carlo simulations performed to model the experiment show excellent agreement. We apply the same simulations to the prototype molecule 88 Sr 19 F and expect 15% of the initial flux to be continuously compressed in a narrow velocity window at around 10 m s −1 . This is the first experimentally shown continuous and dissipative slowing technique in molecule-like level structures, promising to provide the missing link for the preparation of large ultracold molecular ensembles.
This thesis reports on the production and characterisation of the first slow and cold beam of YbF molecules using buffer gas cooling. These molecules are being used to measure the electron’s electric dipole moment, and an intense source of slow-moving molecules is desirable for this experiment. The molecules are loaded into a buffer gas cell via laser ablation where they thermalise with cold helium buffer gas. They are then detected inside the cell using laser absorption imaging and spectroscopy on the X2Σ+→A2II1/2 transition. The formation, diffusion and thermalisation dynamics of the molecules inside the cell are studied. Measurements of laser absorption versus intensity reveal that saturation of the absorption is due to a competition between optical pumping into dark states and repopulation of the addressed level by inelastic and velocity-changing collisions. A beam of YbF molecules is extracted through an aperture in the buffer gas cell and characterised using laser induced fluorescence detection. Peak fluxes of 1010 molecules per steradian per pulse, in the rotational and vibrational ground state, are obtained. The translational and rotational temperatures are in equilibrium with the cell temperature of 4 K. The forward velocity of the pulses can be varied between 130 m/s and 200 m/s by changing the buffer gas pressure. This source is an order of magnitude brighter and more than three times slower than a supersonic source of YbF molecules and provides an excellent starting point for improving the measurement of the electron’s electric dipole moment and for deceleration and trapping experiments. In order to reduce the helium load on the vacuum system and to shorten the molecular pulses, a second set-up, delivering the buffer gas into an open copper cylinder in pulses rather than in a continuous flow,is characterised and shows promising first results.