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 the lifetime of these states qualitatively and explains several seemingly contradictory observations obtained using zero-kinetic-energy (ZEKE) photoelectron spectroscopy. The presence of a small homogeneous de electric field and a few ions in the sample volume causes m(l) mixing in general as well as l mixing, both of which contribute to the lengthening of the lifetimes. Consequently, the lifetime lengthening appears to be insensitive to the sample pressure. The effect of the de electric field on the lifetime is complex. Although the electric field results in l mixing, with increasing field strength it inhibits m(l) mixing, and, at still higher field strength, induces ionization. The variation 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 that different states have different lifetimes. At higher ion concentration, l and m(l) mixing are mon uniform, which lengthens the lifetimes and makes them more similar across the Stark manifold. At still higher concentrations, collisional ionization dominates, which shortens the lifetimes.