Multiphoton ionization spectroscopy of organometallics: The Cr(CO) ₆, Cr(CO)₃C₆H₆, Cr(C ₆H₆)₂ series

The multiphoton ionization (MPI) spectra of the title compounds have been studied in the 3533-6000 Å region in an effort to elucidate the fragmentation and ionization pathways. At wavelengths longer than 4000 Å, the compounds dissociate more rapidly than they are ionized, thereby forming Cr(I) which then can be ionized in a (2 + 1) resonant process. The resonance wavelengths are identical in the three compounds, and only Cr⁺ is observed in the MPI-produced mass spectra. At wavelengths shorter than 4000 Å, both Cr⁺ and CrCO⁺ appear in the mass spectra of Cr(CO)₆ and Cr(CO)₃C₆H₆. Surprisingly, however, no ionization of the unbound ligands was observed in any of the complexes studied. Electron time-of-flight measurements in Cr(CO) ₆ show that at wavelengths shorter than 4000 Å ionization of the parent-molecule resonant state and fragmentation to neutrals with eventual ionization of Cr(I) are important ionization paths. The observation of severe ac Stark broadening of the Cr(I) MPI resonances implies that a rate-equation approach to ion production will be inadequate, except at very low flux. Additionally, the exclusive production of Cr⁺ indicates extremely rapid dissociation of the intermediate molecular fragments. The breakdown of the rate-equation approach leads to effective power exponents which are smaller than those required by the known Cr⁺ mass-spectrometric appearance potentials; this indeed is found experimentally. The branching ratios for production of various excited states of Cr(I) are dependent upon the precursor molecule and the incident laser flux.