The effect of DC electric fields on nP cold Rydberg collisions*

We have investigated the role of the dc Stark effect on population transfer to the nS state 100 ns after the excitation of nP Rydberg atoms in a Rb magneto-optical trap (MOT) for 32 $\le $ n $\le $ 36. The nS population was measured as a function of the principal quantum number, as a function of the dc electric field, and as a function of the nP atomic density. The time evolution of the nS population was also measured up to 10 $\mu $s after the nP+nP state excitation. The electric fields are scanned trough the Forster resonance between 0 and 12 V/cm. The experimental results are compared to a theoretical model to calculate the probability of a transfer of population between the two states, taking into account dipole-dipole, dipole-quadrupole, and quadrupole-quadrupole interactions as well as the dc Stark effect. We should stress that the nP case is simpler than our previous work involving nD+nD states [1], since there are fewer potential curves involved in the process.\\[4pt] [1] J S Cabral \textit{et al}., New J. Phys. \textbf{12,} 093023 (2010).