THE DYNAMICS OF HIGH AUTOIONIZING RYDBERG STATES OF AR

In this paper we present a theoretical study of the autoionization dynamics of high (2)P(1/2)np'[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. Muhlpfordt and U. Even, J. Chem. Phys. 103, 4427 (1995)], and which exhibit a marked dilution (i.e., similar to 2 orders of magnitude lengthening) of the lifetimes relative to those inferred on the basis of the n(3) scaling law for the spectral linewidths of the np' (n = 12-24) Rydbergs. The multichannel effective Hamiltonian (H-eff) 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. H-eff of dimension 2n-1 is then constructed for a n Rydberg manifold using independent experimental information on the (l dependent) quantum defects delta(l) and the (l, K, J dependent) decay widths, which are of the form Gamma(0)(lKJ)/(n - delta(1))(3) with Gamma(0)(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 (F) over bar(n,p') = (F/V cm(-1))n(5)/ 3.4 X 10(9)[delta(p')(mod1)]. Range (A); The onset of the effective coupling of the doorway and escape states, i.e., 0.7 less than or equal to (F) over bar(n,p') less than or equal to 2. Range (B); The strong mixing domain (F) over bar(n,p') greater than or equal to 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 [GRAPHICS] In range (B) [tau(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 [tau(LONG)(n)] similar or equal to eta(n)[tau(SM)(n)], where eta(n) similar or equal to 2-5. In range (A), [tau(LONG)(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 (2)P(1/2)nd'[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 similar or equal to 100 and F similar or equal to 0.1 V/cm) than for the np' doorway state mixing (n' = 70-80 for F similar or equal to 0.1 V/cm). Experimental values of [tau(LONG)(n)] (around n similar or equal to 90) in range (A), excited via the (2)P(1/2)nd'[3/2](1) doorway state, are considerably longer than those predicted by our theory for l mixing. The discrepancy may be due to (lm(l)) mixing, which presumably originates from Rydberg-ion collisions. (C) 1995 American Institute of Physics.