ISSN:
1089-7690
Source:
AIP Digital Archive
Topics:
Physics
,
Chemistry and Pharmacology
Notes:
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 2Σ+g (v+=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 a″ 1Σ+g (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 n〈100. The importance of predissociation is greatly reduced beyond n=100 and becomes negligible on our experimental timescale (30 μs) 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 (τ(approximately-greater-than)30 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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1063/1.470639
