  The resonant transfer of energy from the inversion sublevels in NH$_3$ to He
atoms in triplet Rydberg states with principal quantum number $n=38$ has been
controlled using electric fields below 15 V/cm in intrabeam collisions at
translational temperatures of $\sim1$ K. The experiments were performed in
pulsed supersonic beams of NH$_3$ seeded in He at a ratio of 1:19. The He atoms
were prepared in the metastable 1s2s $^3$S$_1$ level in a pulsed electric
discharge in the trailing part of the beams. The velocity slip between the
heavy NH$_3$ and the lighter metastable He was exploited to perform collision
studies at center-of-mass collision speeds of $\sim70$ m/s. Resonant energy
transfer in the atom-molecule collisions was identified by
Rydberg-state-selective electric-field ionization. The experimental data have
been compared to a theoretical model of the resonant dipole-dipole interactions
between the collision partners based on the impact parameter method.
