Infrared multiphoton dissociation of unsubstituted metal carbonyls at 5 μm

A frequency-doubled carbon dioxide laser of modest output energy (1 mJ) has been used to study, for the first time, the infrared multiphoton absorption by, and dissociation of, the unsubstituted carbonyls of vanadium, chromium, iron, nickel, molybdenum, and tungsten. The multiphoton absorption cross-sections measured for Ni(CO)₄, Fe(CO)₅, Cr(CO)₆, Mo(CO)₆, and V(CO)₆ are high (σ∼2×10^-17) and ensure facile multiphoton dissociation. In focussed beams a pressure independent reaction yield proportional to the 1.5 power of the beam energy is observed for Fe(CO)₅, Cr(CO)₆, and Mo(CO)₆ implying threshold fluences of only 32, 25, and 26 mJ cm^-2, respectively. The stoichiometry of the reaction, observed by a pressure measurement technique, is consistent with production of metal atoms and carbon monoxide as final products for Ni(CO)₄, Fe(CO)₅, Cr(CO)₆, and Mo(CO)₆. This extensive decarbonylation along the ground state surface is consistent with recent studies of the photochemistry of these molecules from excited electronic states.