Decelerating molecules with microwave fields

We report on the experimental realization of a microwave decelerator for neutral polar molecules, suitable for decelerating and focusing molecules in high-field-seeking states. The multistage decelerator consists of a cylindrical microwave cavity oscillating in the TE₁₁ₙ mode, with $n=12$ electric field maxima along the symmetry axis. By switching the microwave field on and off at the appropriate times, a beam of state-selected ammonia molecules with an incident mean velocity of 25 m/s is guided while being spatially focused in the transverse direction and bunched in the forward direction. Deceleration from 20.0 to 16.9 m/s and acceleration from 20.0 to 22.7 m/s are demonstrated.