Multiphoton ionization of vanadocene and ferrocene at 248 and 193 nm. Wavelength-dependent competition between dissociation and ionization

Multiphoton ionization upon nanosecond laser excitation at 248 and 193 nm of vanadocene and ferrocene in comparison with electron impact ionization is reported. A similar behaviour was found for both compounds. Upon laser excitation at 248 nm extensive fragmentation is observed. The ionic decay pattern is only slightly dependent on the laser intensity and is dominated by the metal ion, which is formed after absorption of four photons. At 193 nm the fragmentation is strongly dependent on the laser intensity. Up to moderate laser intensity the molecular ion is dominant and no metal ion is detected. Increasing the laser intensity a higher degree of fragmentation is observed and the metal ion, formed after absorption of three photons, becomes more prominent. These results are consistent with a fast dissociation to metal atoms after biphotonic excitation at 248 nm on an excited repulsive surface, followed by ionization of these metal atoms. The slower dissociation at 193 nm points to a tendency to a statistical redistribution of the excitation energy prior to dissociation. The absorbed photon energy is corroborated by ionization and appearance energies measured by calibrated electron impact ionization. The absorption pathway and the ionic fragmentation mechanism are investigated by analysis of the metastable ion decay.