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A new scheme for the detection of ions produced by delayed pulsed field ionization of long-lived high-n Rydberg states (ZEKE Rydberg states) is presented. The equivalence of this new ion detection scheme with high resolution zero kinetic energy photoelectron (ZEKE) spectroscopy is proven by the simultaneous measurement of electrons and ions originating from the same ZEKE Rydberg states, taking into account the effects of pulsed and static low electric fields on the ZEKE Rydberg states. Using this ion detection, a rotationally resolved mass analyzed threshold ionization (MATI) spectrum of benzene with the same resolution (the rotational structure of the benzene cation) as the simultaneously recorded ZEKE photoelectron spectrum has been obtained.

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A detection scheme for zero-kinetic-energy photoelectron spectroscopy (ZEKE spectroscopy) adapted to the time structure of multibunch vacuum ultraviolet synchrotron radiation from BESSY is described. A resolution of 50 cm−1 (6 meV) is achieved at high stored ring currents providing high photon flux and resolution for one-photon ZEKE spectroscopy and electron-ion coincidence measurements.

Correlated pair fragmentation processes, for C 1s core level excitation of free CO2 molecules and CO2 clusters, have been investigated by “Zero Kinetic Energy” (ZEKE) electron spectroscopy and a (triple) ZEKE electron‐ion‐ion coincidence technique. For free CO2, the main fragmentation channels observed are the correlated ion pairs C+/O+, O+/O+, O+/CO+. These correlated pairs of cations originate from fragmentation of a CO2 dication which is formed by Auger‐type relaxation of the primary 1 s−1 core hole state. Surprisingly, for CO2 clusters this fragmentation channel is quenched, and correlated pair formation between intact cluster ions, i.e. CO2+/CO2+, CO2+/(CO2)2+ and (CO2)2+/(CO2)2+, become the dominant channels. This observation is explained by a fast charge transfer within the cluster, leading to redistribution of the two positive charges. For excitation to the CO22πu(π*) resonance, a possible mechanism for this intra‐cluster charge transfer is the participator valence Auger process following Auger core hole relaxation.