Optical absorption spectra for mass-selected Hgn"^ and Hgn"*"^ clusters show an abrupt transition to a collective, plasmonlike absorption as a function of increasing cluster size. The position of the single observed plasmon maximum is independent of the charge state and nearly independent of cluster size. The peak positions agree well with those of the classical Mie plasmon calculated from the experimental dielectric function. The width of the plasmon resonance is discussed and a strong temperature effect is conjectured.PACS numbers: 71.45. Gm, 21.10.Re, The Hg atom is a fascinating object in cluster physics. Because of its bd^^ds'^ closed shell electronic structure it is dominantly van der Waals bound for the diatomic molecule and small clusters [1,2]. The widths of the occupied 6s band [3] and the empty 6p band increase roughly proportional to the number of nearest neighbors. Thus the "band gap" A (compare Fig. 1) decreases for increasing cluster size. In the bulk, the 6s and 6p bands overlap giving bulk Hg its metallic character. The chemical bonding changes from dominantly van der Waals, to covalent, and then to metallic [1, 2]. One thus has a metal-tononmetal (MNM) transition as a function of cluster size.Little is known on the optical properties of Hg clusters. The cluster size dependence of the bd^^6s'^ -> bd^6s'^6p autoionizing transition has been measured [4]. An absorption spectrum of a neutral, not-mass-selected Hg cluster beam was published [5]. For the bulk, the optical constants of Hg have been measured [6] and calculated [7]. The absorption spectrum of "expanded" mercury has been studied in the context of the MNM transition [8].The aim of this experiment was to study how the changes in chemical bonding and the MNM transition influence the optical properties of Hg clusters. The apparatus is an improved version of the one used earlier to measure optical spectra for Ar^"^ and Xcn"^ clusters [9, cluster atom /////// 0 5p 6s 6p 6s •5d FIG. 1. Electronic configurations of the Hg atom, cluster, and bulk. A metal-to-nonmetal transition occurs as function of the cluster size, when the 6s and 6p bands start to overlap.'/////// 10]. Briefly, an Ar-seeded supersonic expansion generates the clusters. They are ionized by electron impact, and mass separated by a refiectron time-of-flight (TOF) mass spectrometer. Clusters of a single size are irradiated by photons. The electronic excitation is rapidly converted into vibrational energy, leading to ejection of atoms. A second TOF measures the fragment spectrum. Standard "depletion spectroscopy" is used to record the optical spectra [11][12][13][14][15][16][17].The cluster temperature can be estimated from the Klots equation ksT ^ D+/24, where ks is Boltzmann's constant, and T and D^ are the temperature and dissociation energy of the cluster ion, respectively [18]. The D^j; values have been determined experimentally up to n -25 [2]; they agree well with recent calculations [1]. For n = 10 one finds Df^ ^ 0.2 eV. For n = 100, an educated guess (from Fig. 9 of Ref.[2])...

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