Photoionization Mass Spectrometry and Valence Photoelectron‐Photoion Coincidence Spectroscopy of Isolated Clusters in a Molecular Beam

A new and versatile photoelectron‐photoion coincidence spectrometer has been developed and advanced to facilitate for the first time the investigation of the valence electronic structure of isolated metal atom clusters up to a mass range of approximately 40000 amu. The neutral cluster distribution, which is synthesized in a doubly skimmed seeded supersonic molecular beam, is probed 130 mm down‐stream by the pulsed and monochromatized output of a newly developed sub‐nanosecond vacuum‐UV/UV‐flashlamp in the photon energy range between 3 and 11.8 eV. Single photoelectrons are energy analyzed and time‐correlated detected either in a field‐free or in a magnetic type time‐of‐flight spectrometer. The corresponding cluster cations are mass analyzed and counted in coincidence in a modified Wiley‐McLaren time‐of‐flight mass spectrometer. The data acquisition electronics is capable of recording and storing simultaneously photoion‐photoelectron coincidence spectra of all those neutral clusters that have sufficient intensity in the molecular beam. First photoelectron spectroscopic results for mass selected mercury clusters up to Hg 78 are used to discuss the performance of this new technique. — A photoionization mass spectrometry study of sodium cluster beams has revealed shell closings at the sizes being predicted by the cluster shell model. For the case of large mercury clusters a gradual transition from van der Waals type bonding to metallic cohesion occurs in the size range between 20 and 70 atoms. The occurrence of this transition, which has been observed by energy resolved mass spectrometry, is strongly confirmed by photoelectron‐photoion coincidence spectroscopy. Moreover, the photoelectron spectra clearly indicate that the evolution of the valence band structure toward the metallic state is almost fully accomplished for clusters larger than Hg 70 . The gradually size dependent transition is found to be induced by the rapid increase of the averaged number of nearest neighbours. Hence, the transition can be explained semi‐quantitatively in the framework of the tight‐binding approximation.