Spectroscopic and thermodynamic investigations of transition-metal cluster ions in the gas phase photodissociation of MFe+

Heteronuclear metal dimer ions, MFe/sup +/ (M + Sc, Ti, V, Cr, Fe, Co, Ni, Cu, Nb, Ta), were generated and isolated in the gas phase with use of Fourier transform mass spectrometry. Observations of these ionic species in a ligand-free environment provides a method of probing the fundamental bonding nature between two bare transition-metal atoms. The photodissociation spectra of MFe/sup +/, obtained by monitoring the fragmentation of MFe/sup +/ as a function of wavelength, provide both spectroscopic and thermodynamic information. Both M/sup +/ and Fe/sup +/ are observed as photoproducts, with the metal having the lowest ionization potential predominating. The photodissociation spectra reveal broad absorption in the ultraviolet and visible spectral regions with a range of cross sections from 0.06 A for VFe/sup +/ to 0.62 A/sup 2/ for CrFe/sup +/. Bond energies obtained by observing photoappearance onsets are in the range of 48 kcal/mol for ScFe/sup +/ to 75 kcal/mol for VFe/sup +/ and show good agreement with values obtained by ion-molecule bracketing techniques, suggesting that the observed thresholds are thermodynamic and not spectroscopic. The metal cluster ions are found in general to be bound more strongly than their neutral metal dimer counterparts. The ionization potentials for MFemore » can be calculated from D/sup 0/ (M-Fe) and D/sup 0/(M/sup +/-Fe) and are found to be in the range of 5.4 eV for VFe to 7.4 eV for TaFe.« less