Imaging the photodissociation dynamics of neutral metal clusters: copper dimer, Cu2, and copper oxide, CuO.

The spectroscopy and UV photodissociation dynamics of Cu2 and CuO have been studied using a combination of one- and two-colour excitation and velocity map imaging. Resonant excitation of Cu2 via the J ← X (1)Σg(+) transition leads to significant fragmentation which is interpreted in terms of a combination of direct dissociation of Cu2(+ 2)Π produced in the resonant two-photon ionization process and dissociation of excited Cu2 states above the ionization threshold. By fitting of the kinetic energy release spectra obtained from the velocity map images, we determine a value for the dissociation energy of the cation of D0 (Cu2(+), X (2)Σg(+)) of 1.713 ± 0.025 eV, which, when combined with known ionization energies, yields D0 (Cu2, X (1)Σg(+)) = 1.886 ± 0.026 eV. In other experiments, resonant two colour (1 + 1') excitation of CuO via a range of excited states (C, D, F, H), yields unusually simple VMI images indicating fragmentation into a single dissociation channel which has been identified as Cu* (2)D3/2 + O* (1)D. Taken together, this data gives a CuO bond dissociation energy of 3.041 ± 0.030 eV. Finally, the production of Cu2(+) with kinetic energy = 705 ± 75 cm(-1) is tentatively interpreted as photolysis of Cu3 yielding Cu* + Cu2 X (1)Σg(+) from which a dissociation energy of Cu3 of 0.605 ± 0.030 eV is deduced.

Structures of platinum oxide clusters in the gas phase.

The structures of small gas-phase Pt(n)O(2m)(+) (n = 1-6, m = 1, 2) cluster cations have been investigated in a combined infrared multiple photon dissociation (IRMPD) spectroscopy and density functional theory (DFT) study. On the basis of the infrared spectra obtained, it is concluded that in most clusters oxygen is bound dissociatively, preferring 2-fold bridge binding motifs, sometimes combined with singly coordinated terminal binding. Comparison of the oxide cluster structures with those of bare cationic platinum clusters reported previously reveals major structural changes induced in the platinum core upon oxygen binding. For some cluster sizes the presence of the Ar messenger atom(s) is found to induce a significant change in the observed cluster structure.

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.