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Journal of Physics B Atomic Molecular and Optical Physics

A jet beam source of cold YbF radicals

Michael R. Tarbutt, J. J. Hudson, B. E. Sauer, E. A. Hinds, Vyacheslav A. Ryzhov, V. L. Ryabov, В. Ф. Ежов

Published Nov 27, 2002• 58 citations
Publisher

Abstract

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.

DOI
10.1088/0953-4075/35/24/306

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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 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.
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.
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 source of cold LiH molecules for Stark deceleration and trapping experiments. Lithium metal is ablated from a solid target into a supersonically expanding carrier gas. The translational, rotational, and vibrational temperatures are 0.9+/-0.1, 5.9+/-0.5, and 468+/-17 K, respectively. Although they have not reached thermal equilibrium with the carrier gas, we estimate that 90% of the LiH molecules are in the ground state, X (1)Sigma(+)(v=0,J=0). With a single 7 ns ablation pulse, the number of molecules in the ground state is 4.5+/-1.8 x 10(7) molecules/sr. A second, delayed, ablation pulse produces another LiH beam in a different part of the same gas pulse, thereby almost doubling the signal. A long pulse, lasting 150 micros, can make the beam up to 15 times more intense.
A supersonic beam source for SrF and BaF molecules is constructed by combining the expansion of carrier gas (a mixture of 2% SF6 and 98% argon) from an Even–Lavie valve with laser ablation of a barium/strontium metal target at a repetition rate of 10 Hz. Molecular beams with a narrow translational velocity spread are produced at relative values of Δv/v = 0.053(11) and 0.054(9) for SrF and BaF, respectively. The relative velocity spread of the beams produced in our source is lower in comparison with the results from other metal fluoride beams produced in supersonic laser ablation sources. The rotational temperature of BaF is measured to be 3.5 K. The source produces 6 × 108 and 107 molecules per steradian per pulse in the X2Σ+ (ν = 0, N = 1) state of BaF and SrF molecules, respectively, a state amenable to Stark deceleration and laser cooling.
We demonstrate a cryogenic buffer gas-cooled molecular beam source capable of producing bright, continuous beams of cold and slow free radicals via laser ablation over durations of up to 60 seconds. The source design uses a closed liquid helium reservoir as a large thermal mass to minimize heating and ensure reproducible beam properties during operation. Under typical conditions, the source produces beams of our test species SrF, containing $5\ifmmode\times\else\texttimes\fi{}{10}^{12}$ molecules per steradian per second in the $X{\phantom{\rule{0.16em}{0ex}}}^{2}\mathrm{\ensuremath{\Sigma}}(v=0,N=1)$ state with a rotational temperature of 1.0(2) K and a forward velocity of 140m/s140\phantom{\rule{0.28em}{0ex}}\mathrm{m}/\mathrm{s}140m/s. The beam properties are robust and unchanged for multiple cell exit geometries, but depend critically on the helium buffer gas flow rate, which must be $\ensuremath{\ge}10$ standard cubic centimeters per minute to produce bright, continuous beams of molecules for an ablation repetition rate of 55 Hz.