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Experimental Investigation of a Rubidium-Argon Dual Species Magneto-Optical Trap

H. C. Busch

Published Jan 1, 2004• 0 citations

Abstract

The first simultaneous cooling and confinement of two different atomic species from opposite sides of the periodic table in a dual magneto optical trap (DMOT) has been accomplished. The alkali-metal 85Rb and the noble gas 40Ar* have been simultaneously confined, characterized, and interspecies interaction parameters have been measured. The DMOT confined 1.2 × 106 85Rb atoms at a density of 1 × 1010/cm3 and 1.4 × 106 40Ar* atoms with a density of 1.2 × 1010/cm3. A collisional loss rate coefficient for Rb-Ar* has been determined to be γRb-Ar, = (4.8 ± 1.6) × 10−11cm3/s. A typical reduction of 3% of the florescence of the trapped 85Rb was observed when the 40Ar* trap was present. The loss of atoms from the dual trap is presumed to be caused by radiative escape and ionization losses, for an 40Ar* MOT it is due to Penning and associative ionization losses. An Ar* PI/AI ionization ratio of 6.7 ± 3.6 was determined, which makes the Penning ionization rate the dominant loss mechanism. The ionization rate was obtained with a SRS RGA200 mass spectrum analyzer specifically modified to work in conjunction with an ion optical field plate setup to collect the ions expelled from the MOT. The 85Rb was trapped with a 780 nm diode laser while the 40Ar* was slowed and trapped with a 811 nm Ti:sapphire laser, which was specifically modified to be locked to within 1 MHz of the atomic transition with a linewidth of ∼1 MHz. The metastable state 40Ar* was produced with a rf driven resonating cavity discharge which produced an atomic beam with an angular flux density of 4 × 1014 atoms s−1 sr−1.

DOI: 10.25777/1v0d-fw62

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The first simultaneous cooling and confinement of two different atomic species from opposite sides of the periodic table in a dual magneto optical trap (DMOT) has been accomplished. The alkali-metal 85Rb and the noble gas 40Ar* have been simultaneously confined, characterized, and interspecies interaction parameters have been measured. The DMOT confined 1.2 × 106 85Rb atoms at a density of 1 × 1010/cm3 and 1.4 × 106 40Ar* atoms with a density of 1.2 × 1010/cm3. A collisional loss rate coefficient for Rb-Ar* has been determined to be γRb-Ar, = (4.8 ± 1.6) × 10−11cm3/s. A typical reduction of 3% of the florescence of the trapped 85Rb was observed when the 40Ar* trap was present. The loss of atoms from the dual trap is presumed to be caused by radiative escape and ionization losses, for an 40Ar* MOT it is due to Penning and associative ionization losses. An Ar* PI/AI ionization ratio of 6.7 ± 3.6 was determined, which makes the Penning ionization rate the dominant loss mechanism. The ionization rate was obtained with a SRS RGA200 mass spectrum analyzer specifically modified to work in conjunction with an ion optical field plate setup to collect the ions expelled from the MOT. The 85Rb was trapped with a 780 nm diode laser while the 40Ar* was slowed and trapped with a 811 nm Ti:sapphire laser, which was specifically modified to be locked to within 1 MHz of the atomic transition with a linewidth of ∼1 MHz. The metastable state 40Ar* was produced with a rf driven resonating cavity discharge which produced an atomic beam with an angular flux density of 4 × 1014 atoms s−1 sr−1.

We have simultaneously confined rubidium and metastable argon in a dual-species magneto-optical trap (MOT). Here a binary mixture of atomic species from different groups of the periodic table have been optically confined at ultracold temperatures. We describe the apparatus and characterize the individual, single species MOTs and the dual-species MOT. Both fluorescence and ion production are monitored. With both species trapped, we observe $\ensuremath{\sim}5\ifmmode\times\else\texttimes\fi{}{10}^{6}$

{}^{85}\mathrm{Rb}

atoms and $\ensuremath{\sim}2\ifmmode\times\else\texttimes\fi{}{10}^{6}$

{}^{40}{\mathrm{Ar}}^{*}

atoms. Realization of the dual-species trap opens the way for detailed studies of Penning and associative ionization, photoassociative spectroscopy, and eventually for the production of bound, ultracold RbAr molecules.

We have investigated trap loss in a dual-species magneto-optical trap (MOT) comprised of

^{85}\mathrm{Rb}

and metastable

^{40}\mathrm{Ar}

. We measure the trap loss rate coefficients for each species due to the presence of the other as a function of trap light intensity. We clearly identify both Penning ionization of Rb by

{\mathrm{Ar}}^{*}

and associative ionization to form the molecular ion

^{40}\mathrm{Ar}

as two of the trap loss channels. We have also measured the trap loss rate coefficient for the

{\mathrm{Ar}}^{*}

MOT alone and observe production of

{\mathrm{Ar}}^{+}

and

\mathrm{Ar}_{2}{}^{+}

ions.

We present results of the study of a simultaneously confined metastable helium (He

{}^{*}

) and rubidium magneto-optical trap (MOT). By monitoring the trap decay of the

^{87}\mathrm{Rb}

MOT with and without a He

{}^{*}

MOT present, we find the light-assisted, two-body loss rate to be ${\ensuremath{\beta}}_{\mathrm{Rb}--{\mathrm{He}}^{*}}=(6\ifmmode\pm\else\textpm\fi{}2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}10} {\mathrm{cm}}^{3}/\mathrm{s}$. Moreover, we find that it is possible to create a large, robust

^{87}\mathrm{Rb}

-He

{}^{*}

MOT, opening the possibility of creating a

^{87}\mathrm{Rb}

-He

{}^{*}

Bose-Einstein condensate. This would be the first dual-species condensate incorporating an alkali metal ground-state atom and an excited-state noble gas atom.

Abstract We describe a three-dimensional (3D) magneto-optical trap (MOT) capable of simultaneously capturing 85Rb and 133Cs atoms. Unlike conventional setups, our system utilizes two separate laser systems that are combined before entering the vacuum chamber, enabling the simultaneous trapping of two different atomic species. We trapped 85Rb and 133Cs atoms using relatively low total power: 8 mW cooling and 4 mW repump for 85Rb, and 7.5 mW cooling and 1.5 mW repump for 133Cs. The number of trapped atoms was 1.6 × 108 for 85Rb and 1.4 × 108 for 133Cs. The optical depths were 3.71 for 85Rb and 3.45 for 133Cs. The temperature of trapped atoms was ∼200 μK for 85Rb and ∼200 μK for 133Cs. Our 3D MOT setup allows full horizontal optical access to the trapped atomic ensembles without spatial interference from the trapping or repump laser beams. Our vacuum system is also quite simple, avoiding much of the complexity typically encountered in similar dual-species systems. However, the red detuning of the cooling laser used for atomic trapping in our system is relatively small, leaving room for further optimization. This system offers a versatile platform for exploring complex phenomena in ultracold atom physics, such as Rydberg molecule formation and interspecies interactions.

This dissertation presents the findings of two experimental investigations in ultracold atomic and molecular physics: The study of the dual species trap loss in a rubidium - metastable argon magneto-optical trap and the photoassociative spectroscopy of ultracold metastable argon. The interspecies trap loss rate coefficients have been measured for ultracold collisions between 85Rb and 40Ar* in a dual-species magneto-optical trap (MOT) and the two rates have been found to be approximately equal over the range of intensities studied with values of β'Rb–Ar* = 3.0 ± 1.3 × 10-11 cm3/s and β'Ar*–Rb = 1.9 ± 0.9 × 10-11 cm3/s where β' Rb–Ar* is the trap loss coefficient for Rb in the presence of Ar* and β'Ar*–Rb is the reciprocal term. In addition, the trap loss rate coefficient for cold collisions in a metastable argon MOT alone have been measured with an average value of β Ar* = 5.2 ± 1.6 × 10-10 cm3/s. Using a quadrupole mass spectrometer, the production of Ar +, Ar+2, Rb+, and RbAr+ ions in the dual MOT have been observed, clearly identifying heteronuclear Penning and associative ionization as trap loss mechanisms. In the second experiment, the first ever investigation of the photoassociative (PA) spectroscopy of Ar* has been made. The exploratory study focuses on PA spectroscopy near the 4s[3/2]2 + 4p[5/2]3 asymptote of the Ar2* diatomic molecule over a 10 GHz range red detuned of the atomic resonance. With a range of probe laser intensities from ∼ 102–105ISat, 12 resonances have been observed. The spectra have been analyzed using the near-dissociation LeRoy-Bernstein method. Through this analysis, the spectra seem to best correlate with excitations to the 5g state.

We realize a combined trap for bosonic chromium (52Cr) and rubidium (87Rb) atoms. Initial experiments focus on exploring a suitable loading scheme for the combined trap and on studies of new trap loss mechanisms originating from simultaneous trapping of two species. By comparing the trap loss from the 87Rb magneto-optical trap (MOT) in the absence and presence of magnetically trapped ground state 52Cr atoms we determine the scattering cross-section of σ inel,RbCr = 5.0 ± 4.0 × 10−18 m2 for light-induced inelastic collisions between the two species. Studying the trap loss from the Rb magneto-optical trap induced by the Cr cooling-laser light, the photoionization cross-section of the excited 5P3/1 state at an ionizing wavelength of 426 nm is measured to be σ P = 1.1 ± 0.3 × 10−21 m2.