ROTATIONAL AUTOIONIZATION DYNAMICS IN HIGH RYDBERG STATES OF NITROGEN

The decay dynamics of the high Rydberg states of N-2 converging on the first few rotational levels (N+ =0,1,2,3) of the ground vibronic X (2) Sigma(g)(+) (upsilon(+) =0) state of the N-2(+) cation have been investigated by delayed pulsed field ionization (PFI) following two-photon enhanced (2+1') three-photon excitation via the alpha'' (1) Sigma(g)(+) (upsilon' =0) state of N-2. The experiments were carried out in the presence of a weak homogeneous dc electric field and at typical ion densities of 200-2000 ions/mm(3). All Rydberg states in the range of principal quantum number n = 140-200 exhibit extreme stability against autoionization and predissociation and some have,lifetimes which exceed 30 mu s. The decay of the highest Rydberg states beyond n = 200 is induced by external perturbations (field ionization and collisional ionization) and no Rydberg states beyond n = 350 can be observed by delayed PFI. The Rydberg states which converge on the N+ =0 and 1 rotational levels of the ion, and which therefore are not subject to rotational autoionization, decay into neutral products (by a process presumed to be predissociation) in less than 7 mu s in the range n<100. The importance of predissociation is greatly reduced beyond n = 100 and becomes negligible on our experimental timescale (30 mu s) above n = 140. The decay of the Rydberg states converging on the N+ =2 and 3 rotational levels of the ion is more complex. Below n = 100, only 30%-40% of the Rydberg population decays by fast rotational autoionization whereas 60%-70% decays by predissociation. The importance of predissociation decreases rapidly above n = 100 and becomes negligible beyond n = 140. The decay by rotational autoionization can be observed at all n values but becomes noticeably slower beyond n = 100. In the range n = 140-200 it exhibits a marked biexponential decaying behavior with 30% of the population decaying within a few microseconds and 70% displaying long term stability (tau>30 mu s). The branching between predissociation and autoionization is explained by the effect of the de electric field which mixes strongly the optically accessible p Rydberg series with the high l manifold beyond n = 100. The long lifetimes observed experimentally indicate that mi mixing becomes important as soon as l mixing sets in. (C) 1995 American Institute of Physics.