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Slow, cold beams of polar molecules for precision measurements

N. E. Bulleid

Published Aug 1, 2013• 2 citations
Publisher

Abstract

This thesis reports on the use of buffer gas cooling to produce slow and intense molecular beams, with the aim of creating a source of molecules that can be used to measure the electron electric dipole moment. A beam of Yb is extracted from a buffer gas cell by entrainment in a flow of helium. The beam is characterised and simulations are performed of the helium flow through the cell to better understand the effect this has on the beam characteristics. The velocity of the beam is found to vary between 65 m/s[superscript -1] and 204 m/s[superscript -1] depending on the helium flow through the cell, in agreement with the predictions of the simulations. The temperature of the beam is 2.4 K and the minimum divergence is 12°. The percentage of Yb that is extracted from the cell is ~10%. Sources which use a pulsed valve to produce a supersonic jet are also characterised at cryogenic temperatures. The addition of a thermalisation cell is found to produce beams that are slower, colder and more intense. The peak beam intensity is 4.5×10[superscript 10] molecules per seradian per pulse, the velocity (206 ± 5) m/s and the translational temperature is (12.3 ± 0.5) K. An electric decelerator is used to decelerate a pulse of YbF molecules from 300 m/s[superscript -1] to 276 m/s[superscript -1] by confining them to a travelling trap. Simulations of the molecules in the decelerator show that its acceptance is greater than those demonstrated previously. A detailed understanding of this deceleration is developed. The Franck-Condon factors for the A[superscript 2]Π[subscript 1/2] (v = 0) − X[superscript 2]Σ[superscript +] (v - 0, 1, 2) transition of YbF are measured. The results show that, using three lasers, the molecule could be laser-cooled.

DOI
10.25560/18078

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This thesis reports on the use of buffer gas cooling to produce slow and intense molecular beams, with the aim of creating a source of molecules that can be used to measure the electron electric dipole moment. A beam of Yb is extracted from a buffer gas cell by entrainment in a flow of helium. The beam is characterised and simulations are performed of the helium flow through the cell to better understand the effect this has on the beam characteristics. The velocity of the beam is found to vary between 65 m/s[superscript -1] and 204 m/s[superscript -1] depending on the helium flow through the cell, in agreement with the predictions of the simulations. The temperature of the beam is 2.4 K and the minimum divergence is 12°. The percentage of Yb that is extracted from the cell is ~10%. Sources which use a pulsed valve to produce a supersonic jet are also characterised at cryogenic temperatures. The addition of a thermalisation cell is found to produce beams that are slower, colder and more intense. The peak beam intensity is 4.5×10[superscript 10] molecules per seradian per pulse, the velocity (206 ± 5) m/s and the translational temperature is (12.3 ± 0.5) K. An electric decelerator is used to decelerate a pulse of YbF molecules from 300 m/s[superscript -1] to 276 m/s[superscript -1] by confining them to a travelling trap. Simulations of the molecules in the decelerator show that its acceptance is greater than those demonstrated previously. A detailed understanding of this deceleration is developed. The Franck-Condon factors for the A[superscript 2]Π[subscript 1/2] (v = 0) − X[superscript 2]Σ[superscript +] (v - 0, 1, 2) transition of YbF are measured. The results show that, using three lasers, the molecule could be laser-cooled.
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.
We study the properties of a cryogenic buffer gas source that uses a low-temperature two-stage buffer gas cell to produce very slow beams of ytterbium monofluoride molecules. The molecules are produced by laser ablation inside the cell and extracted into a beam by a flow of cold helium. We measure the flux and velocity distribution of the beam as a function of ablation energy, helium flow rate, cell temperature, and the size of the gap between the first and second stages of the cell. We also compare the velocity distributions from one-stage and two-stage cells. The one-stage cell emits a beam with a speed of about 82 m s − 1 and a translational temperature of 0.63 K. The slowest beams are obtained using the two-stage cell at the lowest achievable cell temperature of 1.8 K. This beam has a peak velocity of 56 m s − 1 and a flux of 9 × 10 9 ground-state molecules per steradian per pulse, with a substantial fraction at speeds below 40 m s − 1 . These slow molecules can be decelerated further by radiation pressure slowing and then captured in a magneto-optical trap. Published by the American Physical Society 2024
We have developed a pulsed supersonic beam of slow, cold YbF molecular radicals with an intensity of 1.4 × 10 9 YbF molecules per steradian per pulse in the X 2 Σ + ( v = 0, N = 0) ground state. The translational and rotational temperatures of the beam are equal. The lowest temperature produced was 1.4 K and the slowest centre-of-mass velocity was 290 K. We show that YbF can be made either by ablating Yb metal into a fluorine-bearing carrier gas or by ablating solid precursors into a pure inert carrier gas. This source is suitable for injecting a molecule decelerator and for high-resolution laser-rf-double-resonance studies such as the measurement of the electron electric dipole moment.
This thesis reports on the combination of buffer-gas cooling with electric velocity filtering to produce a high-density guided beam of slow and internally cold polar molecules. In the continuous source, warm molecules are first cooled by collisions with a cryogenic helium buffer gas. Cold molecules are then extracted by means of an electric quadrupole guide. For formaldehyde the population of individual rotational states in the guide is monitored by depletion spectroscopy, resulting in single-state populations up to 82 %. This thesis also reports on a technique to produce pulses of slow guided molecules. The pulses exhibit a narrow velocity distribution around a tunable velocity.
Permanent electric dipole moments (EDMs) are signatures of time-reversal and parity violation and act as sensitive probes for physics beyond the Standard Model (BSM). The work in this thesis has been done as part of the NL-eEDM collaboration, which plans to measure the electron EDM using a beam of BaF molecules. The sensitivity of such a table-top precision experiment benefits from utilizing an intense, slow beam of molecules. We plan to achieve that through a combination of Stark deceleration and transverse laser cooling of a cryogenic beam of BaF. The work presented in this thesis has primarily been done to produce intense, slow molecular beams for an electron EDM measurement. We constructed a supersonic beam source of SrF and BaF molecules. The source produces high intensity beams with low translational and rotational temperatures. Time-resolved detection of laser-induced fluorescence from pulsed excitation of electronic states in BaF molecules was performed to determine the lifetimes of the A^{2}Pi_{1/2} and A^{2}Pi_{3/2} states. The lifetimes of these states were determined to be 57.1(3) ns and 47.9(7) ns, respectively. The transition between the A^{2}Pi_{1/2} and X^{2}Sigma^{+} states is the main laser cooling transition in BaF. We decelerated a beam of SrF molecules from a cryogenic source to a standstill inside a traveling-wave Stark decelerator, with the number of decelerated molecules on the order of 1000. BaF molecules will be decelerated with the same technique to perform an electron EDM measurement.
A comprehensive characterisation of cold molecular beams from a cryogenic buffer-gas cell, providing insight into the physics of buffer-gas cooling, is presented. Cold molecular beams are extracted from a cryogenic cell by electrostatic guiding, which is also used to measure their velocity distribution. The rotational-state distribution of the molecules is probed by radio-frequency resonant depletion spectroscopy. With the help of complete trajectory simulations, yielding the guiding efficiency for all of the thermally populated states, it is possible to determine both the rotational and the translational temperature of the molecules at the output of the buffer-gas cell. This thermometry method is demonstrated for various regimes of buffer-gas cooling and beam formation as well as for molecular species of different sizes (CH3 F and CF3 CCH). Comparison of the rotational and translational temperatures provides evidence of faster rotational thermalisation for the CH3 F/He system in the limit of low He density. In addition, the relaxation rates for different rotational states appear to be different.