Structure and Reactivity of Metal Clusters

Figure 1. The four most stable isomeric forms of Rh6

Figure 2. Cluster ablation source

Figure 3. Loss of weakly-bound atom as a signature of IR absorption

Small clusters of atoms (up to 20 atoms) - particularly those of transition metal atoms - exhibit many remarkable size-dependent physical properties which are quite unlike those of the isolated atoms or the bulk. Indeed in many ways these clusters represent a unique state-of-matter of their own. The structures of small metal clusters remain, however, largely a mystery and very little is known of the complex relationship between geometrical structure and reactivity.

We take several parallel approaches to these problems. Firstly we have studied the reactivity of small clusters by means of Fourier transform ion cyclotron mass spectrometry (FT-ICR) looking in particular for clusters which show unusual kinetics which may indicate the presence of multiple isomeric forms. Secondly we have developed a range of spectroscopic techniques both in order to determine the structures of these clusters but also as a means of generating clusters in known isomeric forms. Thirdly, we use Furthermore, we employ computational methods such as density functional theory to calculate energetically low-lying structures and their interconversion and reaction dynamics.

Action Spectroscopy

Action spectroscopy, involving sensitive mass spectrometric-based measurements, allows us to investigate the structure of bare and complexed metal clusters. Photon absorption induces the dissociation of a weak bond within the cluster (Figure 3), depleting the signal from the parent cluster and enhancing the signal of the fragments. This change is detected in our linear time-of-flight mass spectrometer as a function of wavelength, allowing us to simultaneously measure electronic and vibrational absorption spectra of a whole range of cluster sizes (Figure 4). In collaboration with the Fielicke and Meijer group in Berlin, we can probe the underlying structure of the metal cluster by using far-infrared multiple photon dissociation to excite the vibrational modes of the cluster as well as adsorbed species. For this we use a similar instrument, comprised of an ablation source and ToF-MS, and a free electron laser (FELIX in the Netherlands) as the infrared source. Spectroscopic measurements are backed up by density functional theory calculations performed on a national supercomputing cluster.

Figure 4. Two colour action spectrum of Cu+ and Cu2+ recorded by varying the visible pump laser wavelength while the UV probe laser is tuned to the Cu2+ J-X (0-0) resonance at 37446 cm-1.

Velocity Map Imaging

We also study dissociation events in weakly bound complexes using a technique known as velocity map imaging (VMI). Essentially, this technique employs time-of-flight mass spectrometry and position sensitive detection to measure photofragment velocities following photodissociation events. Conservation of momentum and conservation of energy arguments then allows us to determine a number of molecular parameters such as dissociation energies and dissociation pathways.

Figure 5. Clusters are produced in an ablation source (top left). After photodissociation and photoionisation, they are accelerated towards a position-sensitive detector. Clusters with the same kinetic energy are mapped to the same point on the detector, building up an image on successive shots.

The photodissociation of Xe2 clusters was also investigated.

Figure 6. (Left) Potential energy curves and bound state probability distribution functions involved in the dissociative ionization of Xe2. (Right) The fKER spectra recorded following excitation of Xe2 6p2[1/2]0 0g+ and subsequent dissociative ionization. (Inset) A symmetrised quadrant of the reconstructed momentum space image recorded from the v=3 level.

We have undertaken such studies with diatomic gold and gold - rare gas (RG) molecules. Following production via the laser ablation method, these species were injected into our VMI source where they were interrogated using a 157 nm F2 laser (Au2) or a frequency doubled dye laser (Au-RG; RG = Ar, Kr, and Xe). In the case of Au2, seven dissociative product channels were observed, each corresponding to a different ring in the Au image formed following dissociation (see Figure 7). The results of this study were published as an editor's choice article in Chemical Physics Letters. The Au-RG study has led to the first direct measurement of the dissociation values for the Au-Ar, Au-Kr, and Au-Xe ground state dissociation energies. It has also uncovered a peculiar variation in the excited state dynamics associated with the Au-RG series. The work has been published in the Journal of Chemical Physics.

Figure 7. VMI of Au2 photodissociation fragments. Left: TKER spectrum of Au2. Right: recorded velocity map image of particle positions (left half) and reconstructed image (right half).

Figure 8. Au-RG photodissociation dynamics.

Figure 9: (Upper) UV R2PI recorded in the Cu2+ and Cu+ channels simultaneously. (Lower)Total kinetic energy release spectra of Cu formed as a result of one-colour Cu2 photofragmentation with a UV laser tuned to the (0-0) component of the Cu2 J-X transition.

Recently, we have used VMI in combination with action spectroscopy (see above) to unravel the the photodissociation dynamics of copper dimer, Cu2 and Copper oxide, CuO using our our frequency doubled dye laser (UV) and our optical parametric oscillator (visible) as well as a combination of the two (pump-probe experiment). A range of different Cu2 fragmentation processes were observed (see Figure 9) chiefly corresponding to dissociation of doubly excited Cu2 as well as dissociation out of the 2Π state of the Cu2 cation. On the other hand, the CuO images recorded in pump-probe experiments reveal a strong preference for a single dissociation channel, forming O 1D and Cu 2D3/2. The data has also led to mproved values for the dissociation energies of CuO and Cu2 (see Figure 10). These findings have been submitted for the special edition of PCCP on photodissociation dynamics.

Figure 10: (left) Total kinetic energy release spectrum of Cu formed as a result of CuO fragmentation, with the visible pump laser resonant with several CuO F-X (v-0) transitions. (right) Determination of CuO dissociation energy by extrapolation from the observed dissociation channels.