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arXiv:1412.5790 · Physical Review A

Collisional trap losses of cold magnetically trapped Br atoms

Jessica Lam, Chris Rennick, T. P. Softley

Published Dec 11, 2014• 5 citations
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Abstract

Near-threshold photodissociation of ${\mathrm{Br}}_{2}$ from a supersonic beam produces slow bromine atoms that are trapped in the magnetic-field minimum formed between two opposing permanent magnets. Here we quantify the dominant trap-loss rate due to collisions with two sources of residual gas: the background limited by the vacuum chamber base pressure and the carrier gas during the supersonic gas pulse. The loss rate due to collisions with residual Ar in the background follows pseudo-first-order kinetics and the bimolecular rate coefficient for collisional loss from the trap is determined by measurement of this rate as a function of the background Ar pressure. This rate coefficient is smaller than the total elastic collision rate coefficient, as it only samples those collisions that lead to trap loss, and is determined to be $\ensuremath{\langle}\ensuremath{\nu}\ensuremath{\sigma}\ensuremath{\rangle}=(1.12\ifmmode\pm\else\textpm\fi{}0.09)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}9}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{3}\phantom{\rule{0.16em}{0ex}}{\mathrm{s}}^{\ensuremath{-}1}$. The calculated differential cross section can be used with this value to estimate a trap depth of $293\ifmmode\pm\else\textpm\fi{}24\phantom{\rule{0.28em}{0ex}}\mathrm{mK}$. Carrier-gas collisions occur only during the tail of the supersonic beam pulse. Using the differential cross section verified by the background-gas collision measurements provides an estimate of the peak molecular-beam density of $(3.0\ifmmode\pm\else\textpm\fi{}0.3)\ifmmode\times\else\texttimes\fi{}{10}^{13}\phantom{\rule{0.28em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, in good agreement with the prediction of a simple supersonic expansion model. Finally, we estimate the trap-loss rate due to Majorana transitions to be negligible, owing to the relatively large trapped-atom phase-space volume.

DOI
10.1103/physreva.90.063419

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Magnetic trapping of bromine atoms at temperatures in the millikelvin regime is demonstrated for the first time. The atoms are produced by photodissociation of Br2 molecules in a molecular beam. The lab-frame velocity of Br atoms is controlled by the wavelength and polarization of the photodissociation laser. Careful selection of the wavelength results in one of the pair of atoms having sufficient velocity to exactly cancel that of the parent molecule, and it remains stationary in the lab frame. A trap is formed at the null point between two opposing neodymium permanent magnets. Dissociation of molecules at the field minimum results in the slowest fraction of photofragments remaining trapped. After the ballistic escape of the fastest atoms, the trapped slow atoms are lost only by elastic collisions with the chamber background gas. The measured loss rate is consistent with estimates of the total cross section for only those collisions transferring sufficient kinetic energy to overcome the trapping potential.
This thesis demonstrates the feasibility of producing the first cold source of halogen atoms, using Br atoms as the focus. Br atoms are produced by photodissociation of Br2, and detected by (2+1) REMPI and time-of-flight measurements. Ground- and excited-state Br fragments are formed with a recoil velocity directed along the molecular beam axis. The excess energy from the dissociation laser provides the backscattered Br fragments with sufficient recoil velocity to match and cancel out the average velocity of the molecular beam. The Br fragments which undergo sufficient velocity cancellation remain in the laser detection volume for up to 5 ms. A magnetic trap, composed of two permanent bar magnets with their North poles facing each other, is placed around the detection volume and with an axis perpendicular to the molecular beam and laser axes. The centres of the trap and of the laser interaction volume are overlapped. The magnetic field is linear near the centre of the trap and forms a 3-dimensional well with a depth of 0.22 T, equivalent to a trap depth of U0/kB = 255 mK, with the upper and lower standard error [215,325] mK, for ground-state Br. With this configuration, Br atoms have been detected up to delays of 99 ms, to the next laser pulse, suggesting the possibility of accumulating density over successive molecular beam cycles. The decay of Br atoms from the trap is measured, and a Monte Carlo Markov Chain method is implemented to extract the intensity of the Br signal, which decreases to be on the order of the background noise at delays close to 99 ms. Monte-Carlo simulations demonstrate that the trap loss mechanisms are primarily due to collisions with the molecular beam and background gas, inhibiting the ability to accumulate trap density. These simulations also show that Majorana transitions to higher quantum states are minimal and can be ignored. Experimental measurements confirm that near the peak of the molecular beam, when the strongest signal of Br is observed at long delays, around 60% of initially trapped Br atoms are lost due to molecular beam collisions, and (34 ±3)% due to collisions with the background gas. To minimise molecular beam collisions, a chopper construct is designed to reduce the beam pulse width and is placed between the molecular beam valve and detection volume. The chopper runs from 3,000 rpm to 80,000 rpm, during which the duration of the molecular beam at the detection volume is shortened from 130 μs (measured at the full width at half maximum) to between 80 μs and 13 μs, respectively. However, the chopper significantly reduces the initial Br2 density in the trapping region, and the influence of the chopper construct in reducing molecular beam collisions requires further experimental work. Further work is also necessary to improve the operational components, such as reducing the base pressure in the detection chamber to lower background gas collisions, or using a heavier carrier gas to increase density of initially trapped Br atoms. These improvements can lead us closer toward building density of cold Br atoms in the trap over successive molecular beam cycles, with which a source of cold, dense halogen atoms can be realised.
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We present measurements and calculations of the trap loss rate for laser-cooled 87Rb^{87}\text{R}\text{b}87Rb atoms confined in either a magneto-optic trap (MOT) or a magnetic quadrupole trap when exposed to a room-temperature background gas of Ar. We study the loss rate as a function of trap depth and find that copious glancing elastic collisions, which occur in the so-called quantum diffractive regime and impart very little energy to the trapped atoms, result in significant differences in the loss rate for the MOT compared to a pure magnetic trap due solely to the difference in potential depth. This finding highlights the importance of knowing the trap depth when attempting to infer the total collision cross section from measurements of trap loss rates. Moreover, this variation in trap loss rate with trap depth can be used to extract information about the differential cross section.
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