Chemical reactivity on gas-phase metal clusters driven by blackbody infrared radiation.

We report the observation of chemical reactions in gas-phase Rh(n)(N2O)m(+) complexes driven by absorption of blackbody radiation. The experiments are performed under collision-free conditions in a Fourier transform ion cyclotron resonance mass spectrometer. Mid-infrared absorption by the molecularly adsorbed N2O moieties promotes a small fraction of the cluster distribution sufficiently to drive the N2O decomposition reaction, leading to the production of cluster oxides and the release of molecular nitrogen. N2O decomposition competes with molecular desorption and the branching ratios for the two processes show marked size effects, reflecting variations in the relative barriers. The rate of decay is shown to scale approximately linearly with the number of infrared chromophores. The experimental findings are interpreted in terms of calculated infrared absorption rates assuming a sudden-death limit.

Collisional activation of N2O decomposition and CO oxidation reactions on isolated rhodium clusters.

The reactions of nitrous oxide decorated rhodium clusters, RhnN2O(+) (n = 5, 6), have been studied by Fourier transform ion cyclotron resonance mass spectrometry. Collision induced dissociation with Ar is shown to lead to one of two processes; desorption of the intact N2O moiety (indicating molecular adsorption in the parent cluster) or N2O decomposition liberating molecular nitrogen with the latter becoming increasingly dominant at higher collision energies. Consistent with the results of earlier studies, which employed infrared excitation [Hermes, A. C.; et al. J. Phys. Chem. Lett. 2011, 2, 3053], Rh5ON2O(+) is observed to behave qualitatively differently to Rh5N2O(+) with decomposition of the nitrous oxide dominating the chemistry of the former. In other experiments, the reactivity of RhnN2O(+) clusters with CO has been studied. Chemisorption of (13)CO is calculated to deposit ca. 2 eV into the parent cluster, initiating a range of chemical processes on the cluster surface, which are fit to a simple reaction mechanism. Clear differences are again observed in the reaction branching ratios for Rh5N2O(+) and Rh6N2O(+) parent cluster ions. For the n = 5 cluster, the combined N2O reduction/CO oxidation is the most significant reaction channel, while the n = 6 cluster preferentially is oxidized to Rh6O(+) with loss of N2 and CO. Even larger differences are observed in the reactions of the N2O decorated cluster oxides, RhnON2O(+), for which more reaction possibilities arise. The results of all studies are discussed in relation to infrared driven processes on the same parent cluster species [Hamilton, S. M.; et al. J. Am. Chem. Soc. 2010, 132, 1448; J. Phys. Chem. A, 2011, 115, 2489].

Infrared driven CO oxidation reactions on isolated platinum cluster oxides, Pt(n)O(m)+.

This collaboration has recently shown that infrared excitation can drive decomposition reactions of molecules on the surface of gas-phase transition metal clusters. We describe here a significant extension of this work to the study of bimolecular reactions initiated in a similar manner. Specifically, we have observed the infrared activated CO oxidation reaction (CO(ads) + O(ads) --> CO2(g)) on isolated platinum oxide cations, Pt(n)O(m)+. Small platinum cluster oxides Pt(n)O(m)+ (n = 3-7, m = 2, 4), have been decorated with CO molecules and subjected to multiple photon infrared excitation in the range 400-2200 cm(-1) using the Free Electron Laser for Infrared eXperiments (FELIX). The Pt(n)O(m)CO+ clusters have been characterised by infrared multiple photon dissociation spectroscopy using messenger atom tagging. Evidence is observed for isomers involving both dissociatively and molecularly adsorbed oxygen on the cluster surface. Further information is obtained on the evolution of the cluster structure with number of platinum atoms and CO coverage. In separate experiments, Pt(n)O(m)CO+ clusters have been subjected to infrared heating via the CO stretch around 2100 cm(-1). On all clusters investigated, the CO oxidation reaction, indicated by CO2 loss and production of Pt(n)O(m) = 1+, is found to compete effectively with the CO desorption channel. The experimental observations are compared with the results of preliminary DFT calculations in order to identify both cluster structures and plausible mechanisms for the surface reaction.

Infrared-induced reactivity of N2O on small gas-phase rhodium clusters.

Far- and mid-infrared multiple photon dissociation spectroscopy has been employed to study both the structure and surface reactivity of isolated cationic rhodium clusters with surface-adsorbed nitrous oxide, Rh(n)N(2)O(+) (n = 4-8). Comparison of experimental spectra recorded using the argon atom tagging method with those calculated using density functional theory (DFT) reveals that the nitrous oxide is molecularly bound on the rhodium cluster via the terminal N-atom. Binding is thought to occur exclusively on atop sites with the rhodium clusters adopting close-packed structures. In related, but conceptually different experiments, infrared pumping of the vibrational modes corresponding with the normal modes of the adsorbed N(2)O has been observed to result in the decomposition of the N(2)O moiety and the production of oxide clusters. This cluster surface chemistry is observed for all cluster sizes studied except for n = 5. Plausible N(2)O decomposition mechanisms are given based on DFT calculations using exchange-correlation functionals. Similar experiments pumping the Rh-O stretch in Rh(n)ON(2)O(+) complexes, on which the same chemistry is observed, confirm the thermal nature of this reaction.

Infrared induced reactivity on the surface of isolated size-selected clusters: dissociation of N2O on rhodium clusters.

Multiple photon infrared excitation of size-selected Rh(6)N(2)O(+) clusters drives surface chemistry resulting in partially oxidized clusters.

The electronic spectrum of vanadium monoxide across the visible: new bands and new insight.

Resonance enhanced multiphoton ionization spectra of vanadium monoxide (VO) have been measured in the 16,000-23,300 cm(-1) region. A series of intense peaks, identified as the VO C (4)Sigma(-) (v('))-X (4)Sigma(-) (v(")=0) progression, has been recorded up to v(')=7 and vibrational and rotational parameters have been extracted by simulation of the rotationally resolved spectra. Additional weak transitions in the spectrum are assigned to the 2 (2)Pi-X (4)Sigma(-) spin-forbidden band system allowing the first direct determination of a spin-orbit coupling constant within the doublet spin manifold of VO. Together, the spectra provide absolute energies for several doublet electronic states with respect to the X (4)Sigma(-) ground state. A further vibronic progression observed at 22,000 cm(-1) is assigned as a second spin-forbidden excitation from the X (4)Sigma(-) ground state to a (2)Pi state which has not previously been characterized.