Controlled formation of weakly bound atomic negative ions by electron transfer from state-selected Rydberg atoms

Using crossed atomic beams and mass spectrometric ion detection we have studied the formation of negative Ca ions in slow collisions between laser-excited state-selected Ne** (ns, J = 2) and Ne** (nd, J = 4) Rydberg atoms and ground state Ca atoms. The rate coefficients exhibit resonant behaviour with maxima at an effective principal quantum number n* of about 11.5 and an estimated peak value of about 10(-8) cm(3) s(-1). Two different theoretical calculations, based on a curve-crossing model and on a modified adiabatic theory, respectively, yield n*-dependences of the rate coefficients in good agreement with the experimental results.