Molecular Beam Collisions with a Magnetically Trapped Target

Cold, neutral hydroxyl radicals are Stark decelerated and efficiently loaded into a permanent magnetic trap. The OH molecules are trapped in the rovibrational ground state at a density of $\sim {10}^6{\mathrm{cm}}^{-3}$ and temperature of 70 mK. Collision studies between the trapped OH sample and supersonic beams of atomic He and molecular ${\mathrm{D}}_2$ determine absolute collision cross sections. The He-OH and ${\mathrm{D}}_2\mathrm{\text{−}}\mathrm{OH}$ center-of-mass collision energies are tuned from $60{\mathrm{cm}}^{-1}$ to $230{\mathrm{cm}}^{-1}$ and $145{\mathrm{cm}}^{-1}$ to $510{\mathrm{cm}}^{-1}$, respectively, yielding evidence of quantum threshold scattering and resonant energy transfer between colliding particles.

Figure 1

Illustration of the magnetic trap and pulsed valve assemblies. The foreground contains the temperature-controlled solenoid valve and 1 mm diameter skimmer. The final stages of the Stark decelerator and permanent ring magnets of the trap are shown in the background.

Figure 2

Illustration of the trap loading sequence. (a) High voltage is applied to the surfaces of the two permanent ring magnets $1\mu \mathrm{s}$ after the final deceleration stage (shown at left) is grounded. The OH packet is stopped directly between the two magnets by the electric field gradient in $400\mu \mathrm{s}$. The stopping potential due to the applied electric field is depicted. (b) The magnet surfaces are electrically grounded, leaving the packet trapped within the displayed permanent magnetic quadrupole potential.

Figure 3

(a) Time-of-flight data (circles with error bars) and three-dimensional Monte Carlo simulations (solid line) corresponding to OH trap loading. Stopping $E$ fields are switched off at 0.4 ms, which leaves 50% of the stopped OH molecules trapped in the permanent magnetic quadrupole. (b) Measurement of the lifetime of OH trapped within the magnetic trap at a background pressure of $7.5\times {10}^{-9}\mathrm{Torr}$. A single-exponential fit (solid line) of the data yields $432\pm 47\mathrm{ms}$.

Figure 4

(a) Time dependence of OH trap loss due to collisions with a supersonic He beam. Trap density drops sharply over 1 ms upon He beam collisions, then remains constant over the time scale shown. The valve is triggered 20 ms after the magnetic trap is loaded. (b) Total collision cross sections for He-OH (open circles) and ${\mathrm{D}}_2\mathrm{\text{−}}\mathrm{OH}$ (squares) as a function of $E_{\mathrm{c}.\mathrm{m}.}$. The decrease in the He-OH cross section at low energy is attributed to reduced inelastic loss as $E_{\mathrm{c}.\mathrm{m}.}$ drops below the $84{\mathrm{cm}}^{-1}$ splitting between the $J=3/2$ and $J=5/2$ states of the OH molecule.