Fractional Stark State Selective Electric Field Ionization of Very High-nRydberg States of Molecules

For the first time fractional Stark state selective electric field ionization of very high- n ( $n>250$) molecular Rydberg states is observed. An ``offset'' electric pulse selectively ionizes the more fragile ``red'' (downshifted in energy) Stark states. The more resilient ``bluer,'' or upshifted, ones survive and are shifted down in energy upon application of a second (``probe'') pulse of opposite direction (diabatic Stark states' inversion). Hence, even for smaller probe than offset fields ionization is observed. The offset/probe ratio allows one to control spectral peak shapes in zero-kinetic-energy photoelectron spectroscopy.