Multiphoton excitation of tricarbonyl—cyclooctatetraene—iron. Wavelength-dependent competition between ionization and dissociation

Mass spectra of (cot)Fe(CO)3 (cot = C8H8, cyclooctatetraene) were recorded on multiphoton ionization at 351, 308, 248 and 193 nm, and compared with electron impact ionization. The spectra at 351 and 308 nm are, independent of the laser intensity, dominated by Fe+ ions which are formed after absorption of four photons. At 248 nm molecular ions (formed after absorption of two photons) and fragment ions are detected at low laser intensities with the fragmentation pattern being dominated by Fe+ ions, formed after absorption of three photons. At 193 nm and low laser intensities (cot)Fe+ and (C6H6)Fe+ ions are dominant, whereas at higher laser intensities Fe+ ions are prevailing. Fragmentation channels are elucidated by direct metastable ion analysis. The ionization/fragmentation mechanism is explained by a primary dissociation of the neutral parent molecule to metal atoms at 351 and 308 nm, followed by ionization of these metal atoms. At 248 nm, molecular ionization can compete, to some extent, with this dissociation. At 193 nm, most probably, molecular ionization precedes a fast elimination of the three CO ligands.