Two spectrometers for threshold photoelectron coincidence studies of double photoionization

We present here two simple instruments for the study of double photoionization processes at threshold. They rely on the coincidence measurement of two near-zero energy electrons, which are collected by a penetrating field technique, and energy selected either by time-of-flight analysis (1st setup) or by electrostatic filtering in a hemispherical analyzer (2nd setup). Performance of the apparatus is demonstrated on Ar threshold double photoionization, and used to show the importance of two step double photoionization routes at Ar and Kr double ionization threshold.