Symmetric Fano Profiles of Two-photon Electronic Transitions Observed in an Ultracold Rydberg Gas

Abstract

Fano profiles are caused by the interference between the discrete states and continuum. It is a very common phenomenon
 in multi-electron Rydberg states, such as the Rydberg states of group II elements in the periodic table. It often leads to
 autoionization due to the fact that a discrete state couples with an autoionizing state. Unlike the typical asymmetric line
 shape, here we report a symmetric unusual Fano line shape that has been predicted. In addition, this profile is caused
 by molecular states or polymer states instead of multi-electron atomic states. Moreover, it is shown that the fano profile
 caused by the multi-electron polymer states does not necessarily lead to autoionization. We propose that the Fano profile
 studied is caused by the interference between discrete levels and the multipole-multipole coupled continuum, or energy
 band.