Two-body Forster resonance involving Rb nD states in a quasi-electrostatic trap

In this work, we excite nD(5/2) Rydberg states in a dense and cold Rb atomic sample held in a 10.6 mu m quasi-electrostatic trap using narrow-bandwidth laser pulses. Our goal is to study the Forster resonance process nD(5/2) + nD(5/2) -> (n - 2)F + (n + 2) P-3/2 at zero electric field as a function of the total atomic density using pulsed-field ionization in the range n = 37-47. Such a process is almost degenerate for n = 43. Younge and coworkers studied this process [K. C. Younge, A. Reinhard, T. Pohl, P. R. Berman, and G. Raithel, Phys. Rev. A 79, 043420 (2009)] and attributed the observed saturation to many-body effects. Our results show that as the ground-state atomic density increases, the nD(5/2) state population and the population transfer starts to saturate, which is consistent with the onset of Rydberg atom blockade and previously published results. However, since our experiment allows the independent measurement of the nD(5/2) and (n + 2)P-3/2 state populations, we were able to obtain the (n + 2) P-3/2 state population density dependence. Our results clearly show that the (n + 2)P-3/2 state population depends quadratically on the total Rydberg atomic population, and consequently, the Forster resonance is a two-body process for a ground-state atomic density below 3 x 10(11) cm(-3).