Large Effects of Electric Fields on Atom-Molecule Collisions at Millikelvin Temperatures

Controlling interactions between cold molecules using external fields can elucidate the role of quantum mechanics in molecular collisions. We create a new experimental platform in which ultracold rubidium atoms and cold ammonia molecules are separately trapped by magnetic and electric fields and then combined to study collisions. We observe inelastic processes that are faster than expected from earlier field-free calculations. We use quantum scattering calculations to show that electric fields can have a major effect on collision outcomes, even in the absence of dipole-dipole interactions.

Figure 1

Diagram of the collisions without (top) and with (bottom) an electric field. At zero field, ${\mathrm{ND}}_3$ orients smoothly along the intermolecular axis, and there is a low probability of reorientation after a collision with Rb. However, when an electric field (gray arrows) is applied, the orientation of the ${\mathrm{ND}}_3$ becomes confused when the potential anisotropy is similar to the electric Stark energy, leading to an increased probability of a state-changing collision.

Figure 2

Schematic representation of the experimental apparatus (not to scale). The atoms are cooled and trapped at the intersection of laser-cooling beams. A beam of cold molecules is created by a pulsed valve (lower right). The molecular beam is slowed and trapped using electric fields created by $\sim 150$ in-vacuum electrodes (metallic rods and rings). The coils forming the atomic trap are then translated across the table to overlap the atomic and molecular traps.

Figure 3

Measured decay curves and simulation results for ${\mathrm{ND}}_3$ and extracted elastic and inelastic cross sections. (a) The peak ammonia density as a function of time for various densities of Rb, with the best-fit simulations. (b) The effect of elastic collisions on the simulated ${\mathrm{ND}}_3$ decay profile. The density decay at the trap center is reduced for larger elastic cross sections because ${\mathrm{ND}}_3$ migrates towards the trap center due to sympathetic cooling. (c) Confidence contours for the elastic and inelastic cross sections extracted from simulations of the experimental decay curves. The shaded oval represents the range of possible results from field-free quantum-mechanical scattering calculations and is obtained by repeating the calculations for a variety of scaled potential-energy surfaces.

Figure 4

(a) Elastic (dashed) and inelastic (solid) cross sections at a variety of electric fields, obtained from quantum-mechanical scattering calculations on the potential-energy surface of Ref. 19. Pure $m$-changing collisions are elastic at zero field, but inelastic at finite field. (b) Adiabatic potential curves for zero-field collisions, showing crossings between curves for different total angular momentum $J$. The isotropic average potential $V_{00}(R)$ is subtracted so that the curves stay within a narrow energy range. The zero-field curves are independent of $M$. (c) Same as (b), but in a field of $5\mathrm{kV}/\mathrm{cm}$, showing an extensive series of avoided crossings where reorientation can occur. Only curves for $M=1$ are shown.