Fractional stark state selective electric field ionization of very high-n Rydberg 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.