Abstract

The two-step excitation-ionization, has been studied in electric fields (1–8 keV/cm) using linearly polarized (it or a) VUV and UV laser light. Tuning the VUV to selected components in the (n=2) Stark manifold and scanning the UV, the H* excitation energy ranged from ~500 cm^-1 below to ~100 cm^-1 above the zero-field ionization potential, IP_O. Ionization spectra taken for (π,π), (π,σ), (σ,π), (σ,σ) polarization, showed the sharp line structure from states between the field-ionization saddle point energy and the IP_O, in agreement with theory (Fig. 1). In the (π,π) case oscillating resonances extending beyond the IP_O were observed. As an example, Fig. 1 shows the ionization spectrum for (π,π) excitation at field strength F = 5714 V/cm. The lines are Identified by upper state parabolic quantum numbers |n₁,n₂,|m|> with n = n₁ + n₂ + |m| + 1. Lifetimes of the field ionizing states have been measured and the dependence of the oscillations on the electric field has been studied.

© 1984 Optical Society of America