The decay dynamics of the high Rydberg states of N2 converging on the first few rotational levels (N+ = 0,1,2,3) of the ground vibronic X 2Σg+ (v+ = 0) state of the N2+ cation have been investigated by delayed pulsed field ionization (PFI) following two-photon enhanced (2+1′) three-photon excitation via the a″ 1Σg+ (v′ = 0) state of N2. The experiments were carried out in the presence of a weak homogeneous dc electric field and at typical ion densities of 200-2000 ions/mm3. 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 μs. 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 μs in the range n30 μs). The branching between predissociation and autoionization is explained by the effect of the dc 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 ml mixing becomes important as soon as l mixing sets in. © 1995 American Institute of Physics.
