Resonant collisions of K Rydberg atoms

We have studied the collisional energy transfer process K (ns) +K (n-2) d → K (np) +K (n-1) p which is tuned into resonance with an applied static electric field. The initial Rydberg states are excited stepwise with two dye laser pulses. The resonant energy transfer is detected by selectively field ionizing the final K(np) state as the static field is swept over many laser shots. Due to fine structure splitting of the d and p levels 45 resonances are energetically allowed at ns=25. The actual number of resonances that are observed depends on the polarization of the laser light, the dipole-dipole selection rules, and the n state excited. Above ns=29 all resonances are energically forbidden. We have measured the position of the resonances in electric field from ns=25 to ns=29 and found them to be in good agreement with calculated positions