Resonant Energy Transfer, with Creation of Hyper-Excited Atoms, and Molecular Auto-Ionization in a Cold Rydberg Gas

Abstract

A cold Rydberg gas, with its atoms prepared initially all in the excited state | n 0 > , with n 0 » 1, contains an excessive amount of energy, and presumably is to relax by the Penning-type molecular auto-ionization ( MAI ), in which a portion of excess energy of one atom is given to another near-by atom and ionizing it. Its complementary process, the resonant energy transfer ( RET ), is discussed, in which the excess energy of one atom is used on another to form a hyper-excited atomic state | n a > with n a » n 0 . This process is always present, provided certain resonance energy conditions are satisfied. In this report, the n 0 and density dependences of the RET rates are studied in detail, employing a simple model: 1) at low densities, the RET is mediated by the dipole-dipole coupling V dd and its rates are generally much smaller than that of MAI, especially for small n 0 . But 2) as the density increases, our model shows that the rates become of comparable magnitude or even larger than the MAI rates. The V dd is no longer adequate. We, then construct a semi-empirical potential to describe the RET process. 3) At high densities, we show that the atomic orbital of | n a > overlaps with that of neighboring atoms, and the electron-electron potential becomes prominent, resulting in much higher rates.