THE DYNAMICS OF HIGH AUTOIONIZING RYDBERG STATES OF AR

In this paper we present a theoretical study of the autoionization dyn amics of high (2)P(1/2)np'[3/2](1) Rydbergs (with the principal quantu m numbers n = 100-280) of Ar in weak homogeneous electric fields (F = 0.01-1.0 V/cm), which were experimentally interrogated by time-resolve d 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 l ifetimes relative to those inferred on the basis of the n(3) scaling l aw for the spectral linewidths of the np' (n = 12-24) Rydbergs. The mu ltichannel effective Hamiltonian (H-eff) with several doorway state(s) (for excitation and decay) and pure escape states (for decay) was adv anced and utilized to treat the dynamics of the mixed Stark manifold o f 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) dec ay 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 momentu m 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 wit h increasing n and is only weakly F dependent. Range (A) is characteri zed by a hierarchy of two time scales for the decay, with a short deca y component, which manifests the residue of the doorway state, and a d istribution 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 equa l to 2-5. In range (A), [tau(LONG)(n)] decreases with increasing n and decreases with increasing F, manifesting the enhancement of mixing. W e identified range (B) for n = 150-280, where a semiquantitative agree ment between the experimental ZEKE lifetimes and spectra and our theor y was obtained. A tentative identification of range (A) for lower n (= 100-150) values was accomplished. We have also performed a theoretica l 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. Ph ys. 100, 2623 (1994)]. The onset of range (A) was identified in the re gion n = 70-80, with the estimated lifetimes near the onset being in a greement 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(LO NG)(n)] (around n similar or equal to 90) in range (A), excited via th e (2)P(1/2)nd'[3/2](1) doorway state, are considerably longer than tho se predicted by our theory for l mixing. The discrepancy may be due to (lm(l)) mixing, which presumably originates from Rydberg-ion collisio ns. (C) 1995 American Institute of Physics.