We have observed 10-MHz-wide energy-transfer resonances between colliding Na Rydberg atoms. The narrow linewidth allows us to apply a modest 5–40-G static magnetic field in addition to a static tuning electric field and resolve new resonances arising from magnetic-field-induced splitting of the Stark energy levels. When B is parallel to E, the nine resolved resonances observed with only an E field are further split into 144 possible resonances. However, only the 21 resonances corresponding to collisions in which the spins are unchanged are observable. When B is perpendicular to E, the spectrum of collisional resonances is similar to that of the vertical field case, except that the final spin states cannot be determined. With both parallel and perpendicular fields present, linear and quadratic terms of the B field interaction create more complicated spectra than with either case alone. The experimental spectra agree quantitatively with the spectra calculated using a simple Zeeman model of the energy levels.

• Received 14 April 1994

DOI:https://doi.org/10.1103/PhysRevA.50.4077