ON THE LIFETIMES OF RYDBERG STATES PROBED BY DELAYED PULSED-FIELD IONIZATION

We present a simple model to evaluate the degree of l and m(l) mixing in high Rydberg states that results from perturbations caused by weak, homogeneous de electric fields and static ions. This model predicts t he lifetime of these states qualitatively and explains several seeming ly contradictory observations obtained using zero-kinetic-energy (ZEKE ) photoelectron spectroscopy. The presence of a small homogeneous de e lectric field and a few ions in the sample volume causes m(l) mixing i n general as well as l mixing, both of which contribute to the lengthe ning of the lifetimes. Consequently, the lifetime lengthening appears to be insensitive to the sample pressure. The effect of the de electri c field on the lifetime is complex. Although the electric field result s in l mixing, with increasing field strength it inhibits m(l) mixing, and, at still higher field strength, induces ionization. The variatio n of the lifetimes with ion concentration is also complicated. At low ion concentration, the m(l) mixing varies across the Stark manifold of Rydberg states that belong to the same principal quantum number, so t hat different states have different lifetimes. At higher ion concentra tion, l and m(l) mixing are mon uniform, which lengthens the lifetimes and makes them more similar across the Stark manifold. At still highe r concentrations, collisional ionization dominates, which shortens the lifetimes.