Ionization and Photofragmentation of Isolated Metalloporphyrin Cations Investigated by VUV Action Spectroscopy

We investigated the photoionization and fragmentation of isolated metal protoporphyrin IX cations (MPPIX with M = Fe, Co, Zn) by means of VUV action spectroscopy in the energy range of 8.5-35 eV. Experiments were carried out in the gas phase by interfacing an electrospray ionization tandem mass spectrometer with a synchrotron beamline. The mass spectra and partial ion yields show that photoexcitation of the precursor ions predominantly leads to ? CH 2 COOH radical side-chain losses of the macrocycle with additional methyl radical ( ? CH 3 ) side-chain losses. Ionization, in contrast, leads to the formation of the intact ionized precursor and various doubly charged fragments which are mostly due to side-chain cleavages. Although statistical fragmentation dominates, we found evidence for non-statistical processes such as new fragments involving e.g. single and double H 2 O losses, indicating that different relaxation mechanisms are at play upon photoionization compared to photoexcitation. The measured ionization energies were 9.6 ± 0.2 eV, 9.4 ± 0.2 eV and 9.6 ± 0.2 eV for FePPIX + , CoPPIX + and ZnPPIX + , respectively