Photodissociation of Iron Oxide Cluster Cations

Abstract

Iron oxide cluster cations, Fe(n)O(m)(+), are produced by laser vaporization in a pulsed nozzle cluster source and detected with time-of-flight mass spectrometry. The mass spectrum exhibits a limited number of stoichiometries for each value of n, where m > or = n. The cluster cations are mass selected and photodissociated using the second (532 nm) or third (355 nm) harmonic of a Nd:YAG laser. At either wavelength, multiple photon absorption is required to dissociate these clusters, which is consistent with their expected strong bonding. Cluster dissociation occurs via elimination of molecular oxygen, or by fission processes producing stable cation species. For clusters with n < 6, oxygen elimination proceeds until a terminal stoichiometry of n = m is reached. Clusters with this 1:1 stoichiometry do not eliminate oxygen, but rather undergo fission, producing smaller (FeO)n(+) species. The decomposition of larger clusters produces a variety of product cations, but those with the 1:1 stoichiometry are always the most prominent and these same species are produced repeatedly from different parent ions. These combined results establish that species of the form (FeO)n(+) have the greatest stability throughout these small iron oxide clusters.

The challenge of generating iron-sulfur clusters

“…This finding is in favorable agreement with previous dissociation patterns of Fe x O y + clusters. Photodissociation, collision induced dissociation as well as temperature-programmed desorption experiments revealed the preferred formation of stoichiometric fragments for Fe x O y + (x > 2) while the smaller clusters tend to fragment into Fe + and FeO 2 + [25][26][27][28]. Using bare iron powder changes the color of the targets to a shiny gray (cf.…”

Mass spectrometry and its role in advancing cluster science

“…Collision-induced photodissociation experiments can further establish the relative stability of mass-selected metal-oxide cluster ions. [47,48]. Contrary to the conventional understanding that the structure of atomic clusters usually differ from the bulk constituents of which they are comprised, Sun et al [51] showed that even a dimer of tungsten oxide (WO 3 ) 2 possesses bulk-like features and geometry of a small cluster containing only 4 tungsten and 12 oxygen atoms bears the hallmarks of crystalline tungsten oxide, Fig.…”

“…Seminal studies by Castleman and coworkers [44][45][46] and Duncan's [47][48][49][50] groups on oxide clusters of V, Fe, Co, Zr, Nb, etc. have shown dramatic patterns with specific stoichiometries throughout the mass range and compositions not found in bulk solids.…”

High irradiance laser ionization mass spectrometry for direct speciation of iron oxides

“…Second, as the laser irradiation in our work was high and reached 9 ϫ 10 10 W/cm 2 , the central plasma temperature was estimated to be 9 eV at 10 ns and 4.5 eV at 100 ns after laser radiation [35][36][37]. Most of the neutral clusters were likely to be dissociated than be ionized at the high temperature (e.g., bond energies for the loss of O and O 2 from iron oxide clusters were between 3 and 5 eV [38], while ionization potentials were 7 to 8 eV for the clusters [39]). As a result, particles at the central part of the expansion plasma Ϫ123…”