Photoelectron Spectroscopy on Size-Selected Cluster Cations
We are developing a new experimental technique to record ultraviolet and x-ray photoelectron spectra of size-selected clusters and nanoparticles using synchrotron radiation. The new setup is located at a sidebranch of the VUV beamline.
The properties of clusters and nanoparticles may vary with each additonal atom. For meaningful results, size-selection is required prior to the experiment. The particles need to be charged in order to mass separate them. As a result, the sample in numerous cluster experiments is an ion beam with a target density several orders of magnitudes lower than in a gas jet. For photoelectron spectroscopy, the low target density is usually compensated by the high light intensity of a laser. With commercial lasers, intense radiation is available at photon energies up to 7.9 eV. This restricts photoelectron spectroscopy to the study of the uppermost occupied valence orbitals.
Some metal clusters have higher catalytic activity than the surfaces of the corresponding bulk metals. The mechanism responsible for this size-dependence are still a matter of research. The first step in a catalytic reaction is the chemisoprtion of a molecule to the catalyst. In the case of CO, the chemisoprtion bond is formed by the interaction of the CO orbitals with the metal orbitals. These orbitals can be studied with valence band photoelectron spectroscopy. However, the binding energies of these orbitals are above 10eV. Higher photon energies are required to study these orbitals with photoelectron spectroscopy.
A synchrotron is the perfect light source for photoelectron spectroscopy. With a combination of an intense cluster ion source and an electron spectrometer with a high collection Efficiency, it is possible to carry out photoelectron spectroscopy of size-seletced clusters with synchrotron radiation. A magnetron sputter source is able to deliver several nanoamperes of size-selected clusters. A velocity map imaging spectrometer collects pratically all photoelectrons. In addition, in the weak electrostatic field of the VMI positively charged clusters turn around. At the turn around point, the target density is highest, and suffcicient for photoelectron spectroscopy.