J. Phys. Chem. C 2007, 111, 17725-17729
17725
Two Photon Detachment of d Electrons and Auger Emission in Photoelectron Studies of Hgn- Clusters† Ram Busani and Ori Cheshnovsky* School of Chemistry, the Raymond and BeVerley Sackler Faculty of Exact Sciences, Tel-AViV UniVersity, Tel AViV 69978 Israel ReceiVed: February 22, 2007; In Final Form: NoVember 6, 2007
We report on two-photon photoelectron spectra of negatively charged mercury clusters, Hgn-, excited by 6.43 eV photon energy. The two-photon spectra include narrow bands attributed to inner shell d electron detachment. The position and shape of the d core detachment peaks hardly change over the size range of n ) 9-280. The spectra also include broad bands, consistent with the anticipated kinetic energies of the Auger electrons. To the best of our knowledge, this is the first time that an internal-shell Auger electron has been identified in the photoelectron spectra of free metallic clusters.
1. Introduction Mercury has been considered for many years, to be the prototype material obeying the Wilson-Bloch model for a metal to insulator transition (MIT) in bivalent metal clusters.1 According to the model, bivalent clusters become metallic above some critical size in which full bands formed by s type atomic orbitals overlap empty bands formed by p type atomic orbitals. It is the increasing average number of nearest neighbors with cluster size, which causes the band-broadening and consequently s-p band overlap. We note that for finite-size clusters the term “bands” refers to groups of discrete levels derived from atomic levels, whereas “band gap closure” refers to a situation in which the energy gap between these bands is equal to the average level separation within each group. This is what we are referring to when using in the following the simplifying terms “band”, “band gap”, and “band gap closure.” During the last two decades, many experimental and theoretical studies have focused on the size dependent properties of mercury clusters.2 The first mass selected spectroscopic studies on inner shell autoionization resonances of mercury clusters,3 Hgn, revealed a transition region from van der Waals binding to distinct covalent binding in the size range of n ) 13-20. This observation was further supported by the plasmon excitation studies4 and the cohesive energy studies5 of Haberland et al. For n > 20, a gradual change of the s-p hybridization was observed.6 The ionization size dependence7 as well as the cohesive energy were reported to approach the expected values of finite-size metallic particles according to the liquid drop model. These experimental studies were accompanied by theoretical formulation of the change in the nature of the electronic states in mercury clusters, mainly by Bennemann and co-workers.8 It was primarily the ionization potentials studies and the theoretical studies, which implied that s-p band merging occurs at the size range n ∼ 80-100. In 1998 the concept of band gap closure was directly verified by Cheshnovsky and co-workers in photoelectron spectroscopy (PES) studies of negatively charged mercury clusters, Hgn-, with 7.99 and 6.43 eV10 photon energies. In mercury, the gap between †
Part of the special issue “Richard E. Smalley Memorial Issue”. * Corresponding author. E-mail:
[email protected].
the fully occupied 6s band and the empty 6p band decreases gradually and monotonously from 3.6 eV in Hg3 to 0.25 eV in Hg250. It can be extrapolated to close completely around size n ) 450 ( 50. Recent reports on the PES of Mg11 and Zn12 reveal a considerably more complex size dependent band gap closure. In these two bivalent elements, in which the atomic s-p excitation is substantially smaller than in Hg, the signature of the “free electron” band structure is exhibited already in cluster sizes larger than 18 atoms. Unlike the cases of Mg and Zn, Hg exhibits a monotonic decrease of the band gap with cluster size without any strong indication of irregularities induced by the shell structure. The wide range of band gaps in mercury clusters over the size range n ) 3-400 makes them an interesting model system for studying basic processes in semiconductor clusters. For instance, we have identified Auger electron recombination following 6s f 6p interband excitations in the PES of “semiconductor” mercury clusters.13 In this paper, we report on new photoelectron spectral features, observed at higher intensities of the 6.43 eV ArF laser. These features were found to result from two-photon (2P) detachment processes. We have identified in the spectra narrow 2P peaks originating from the ionization of the core 5d3/2, 5d5/2 levels of Hgn- clusters, as well as broad bands, characterized by low electron kinetic energy, which we assign to subsequent Auger electron emission from the excited neutral mercury clusters. Despite its central role in the analysis of surfaces and molecule Auger spectroscopy played a minor role in cluster research. Only recently, Auger electron emission probed by PES has been reported on neutral argon clusters14 and water clusters.15 To the best of our knowledge, our studies report the first direct observation of Auger electron emission, resulting from core level excitation in free metal clusters. 2. Experimental Procedures Clusters were generated by passing 2.5 bar of argon through a heated mercury-containing reservoir (200-250 °C) of a 15 Hz pulsed nozzle.16 The expansion was intersected by a pulsed electron beam (∼200 eV) generating a wide distribution of negatively and positively charged mercury clusters. The beam
10.1021/jp071505f CCC: $37.00 © 2007 American Chemical Society Published on Web 11/29/2007
17726 J. Phys. Chem. C, Vol. 111, No. 48, 2007
Figure 1. Low intensity (
