The resonant transfer of energy from the inversion sublevels in NH3 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 ∼ 1 K. The experiments were performed in pulsed supersonic beams of NH3 seeded in He at a ratio of 1:19. The He atoms were prepared in the metastable 1s2s 3 S1 level in a pulsed electric discharge in the trailing part of the beams. The velocity slip between the heavy NH3 and the lighter metastable He was exploited to perform collision studies at center-of-mass collision speeds of ∼ 70 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.

Tunnel ionization rates of triplet Rydberg states in helium with principal quantum numbers close to 37 have been measured in electric fields at the classical ionization threshold of ∼ 197 V/cm. The measurements were performed in the time domain by combining high-resolution continuous-wave laser photoexcitation and pulsed electric field ionization. The observed tunnel ionization rates range from 10 5 s −1 to 10 7 s −1 and have, together with the measured atomic energy-level structure in the corresponding electric fields, been compared to the results of calculations of the eigenvalues of the Hamiltonian matrix describing the atoms in the presence of the fields to which complex absorbing potentials have been introduced. The comparison of the measured tunnel ionization rates with the results of these, and additional calculations for hydrogen-like Rydberg states performed using semiempirical methods, have allowed the accuracy of these methods of calculation to be tested. For the particular eigenstates studied the measured ionization rates are ∼ 5 times larger than those obtained from semi-empirical expressions.