Photoionization of Na 2 in an Electric Field: Evidence for Field-Induced Rotational Autoionization

Two-photon ionization of Na 2 has been performed in the presence of a d.c. electric field F . Using the A 1 Σ u + (υ' = 0, J ') state as an intermediate level, it is possible to excite only the (υ + = 0, N + ≃ J ' ± 1) states of the ion. At low J ' values, the field F is found to shift the ionization threshold following a - √ F law. At higher J ' values, this is not the case, and our results show that the electric field induces rotational autoionization, so that the threshold corresponds to a level N + ≪ J '.