The dynamics of high autoionizing Rydberg states of Ar
Authors: Bixon M. , Jortner Joshua 
Published: July 2002

In this paper we present a theoretical study of the autoionization dynamics of high 2P1/2np′[3/2]1 Rydbergs (with the principal quantum numbers n=100–280) of Ar in weak homogeneous electric fields (F=0.01–1.0 V/cm), which were experimentally interrogated by time-resolved zero-electron kinetic energy (ZEKE) spectroscopy [M. Mühlpfordt and U. Even, J. Chem. Phys. 103, 4427 (1995)], and which exhibit a marked dilution (i.e., ∼2 orders of magnitude lengthening) of the lifetimes relative to those inferred on the basis of the n3 scaling law for the spectral linewidths of the np′ (n=12–24) Rydbergs. The multichannel effective Hamiltonian (Heff) with several doorway state(s) (for excitation and decay) and pure escape states (for decay) was advanced and utilized to treat the dynamics of the mixed Stark manifold of the ZEKE Rydbergs. Heff of dimension 2n−1 is then constructed for a n Rydberg manifold using independent experimental information on the (l dependent) quantum defects δ(l) and the (l, K, J dependent) decay widths, which are of the form Γ0(lKJ)/(n−δ(l))3, with Γ0(lKJ) being the decay widths constants. Here, l, K, and J are the azimuthal, the electronic and the total electronic angular momentum quantum numbers, respectively. Two coupling ranges are distinguished according to the strength of the reduced electric field F̄(n,p′)=(F/V cm−1)n5/ 3.4×109[δ(p′)(mod1)].
Range (A); The onset of the effective coupling of the doorway and escape states, i.e., 0.7≤F̄(n,p′)≤2. Range (B); The strong mixing domain F̄(n,p′)≥3. The lifetimes in range (B) can be well represented by a nearly democratic mixing of all the doorway and escape states (lKJ), with the average value 〈τ(n)〉≂〈τSM(n)〉= 2n4ℏ/[𝒥(lJK)Γ0(lJK)]. In range (B) 〈τ(n)〉 increases with increasing n and is only weakly F dependent. Range (A) is characterized by a hierarchy of two time scales for the decay, with a short decay component, which manifests the residue of the doorway state, and a distribution of very long lifetimes with an average value 〈τLONG(n)〉≂η(n)〈τSM(n)〉, where η(n)≂2–5. In range (A), 〈τLONG(n)〉 decreases with increasing n and decreases with increasing F, manifesting the enhancement of mixing. We identified range (B) for n=150–280, where a semiquantitative agreement between the experimental ZEKE lifetimes and spectra and our theory was obtained. A tentative identification of range (A) for lower n (=100–150) values was accomplished. We have also performed a theoretical study of the Ar autoionization dynamics via the 2P1/2nd′[3/2]1 doorway state, which was experimentally studied by Merkt [J. Chem. Phys. 100, 2623 (1994)].
The onset of range (A) was identified in the region n=70–80, with the estimated lifetimes near the onset being in agreement with experiment. Our analysis explains the higher n onset for the np′ doorway state mixing (n≂100 and F≂0.1 V/cm) than for the np′ doorway state mixing (n′=70–80 for F≂0.1 V/cm). Experimental values of 〈τLONG(n)〉 (around n≂90) in range (A), excited via the 2P1/2nd′[3/2]1 doorway state, are considerably longer than those predicted by our theory for l mixing. The discrepancy may be due to (lml) mixing, which presumably originates from Rydberg–ion collisions.