The fluorescence of Ag 8 in an argon matrix and in argon droplets is reported. This is the first unambiguous assignment of the fluorescence of a metal cluster larger than the tetramer, indicating that the excited state lifetime is longer than previously thought. It is discussed as a possible result of a matrix cage effect. The excitation spectrum is compared with two-photon-ionization measurements of Ag 8 in helium droplets and to known absorption data. The agreement is excellent. We propose that the excited states relax rapidly through vibrational coupling to a long-lived state, from which the fluorescence occurs. DOI: 10.1103/PhysRevLett.86.2992 Optical spectroscopy has proven to be a powerful tool for understanding the electronic structure of atoms and molecules. Over the past decade these techniques have also been applied very successfully to metal clusters. Small metal clusters have been investigated in free beam experiments, using techniques such as resonant two-photon-ionization (R2PI) spectroscopy [1,2], laser-induced fluorescence [3], and pump-probe [4,5] techniques. Fragmentation becomes, however, more important as the cluster size increases and nondissociative electronic excitation processes have not been observed for free metal clusters larger than trimers [6]. Photodepletion spectroscopy was therefore used to measure optical absorption spectra of free metal clusters larger than three atoms [7][8][9].Also the fluorescence lines of free metal clusters are expected to be observed only for very small clusters. Wöste and co-workers have shown that, in the case of K 3 , the fragmentation of optically excited clusters occurs within a few hundred femtoseconds [10]. This is much shorter than the characteristic time scale necessary for a dipole transition and therefore no fluorescence can be observed in this case.Alternatively, optical absorption measurements of matrix isolated and mass-selected silver clusters have been performed for sizes up to n 39 [11]. No signal decrease has been observed over time, so it is clear that the matrix is effectively preventing the clusters from dissociating. This so-called cage effect is well known for molecules in a gas atmosphere or in a liquid. In the case of I 2 , for example, it has been shown that a single rare gas atom adsorbed on the molecule hinders the photofragmentation and allows the fluorescence [12].This opens the possibility of observing fluorescence for small metallic clusters if the excited state of the particle has a sufficient lifetime for a radiative transition to take place. However, with increasing size the competition between the different possible relaxation processes (vibrations, fluorescence, fragmentation) increases and, due to their shorter characteristic time, the radiationless relaxation processes are expected to quench the fluorescence [13][14][15].In the case of silver, fluorescence of Ag 3 and, more recently, Ag 4 has been unequivocally determined. A number of hitherto unidentified emission bands have been observed in the studies of non-mass-s...

Electron impact ionization of gas phase silver clusters Ag,, n<36 has been achieved in the threshold region. The vertical ionization potentials in this region clearly demonstrate the evidence of shell effects as well as a distinct even-odd oscillation up to n-~ 20. Their general size dependence is somewhat different from that of the alkali metal clusters due to the presence of the delectrons.

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Small noble metal clusters were produced by a gas aggregation technique and were investigated mass spectrometrically using electron ionizing energies ranging from 100 eV down to the threshold region. For energies above approximately 20 eV electronic effects are displayed, that characterize a stable cationic distribution, i.e. enhanced stability is found for odd numbered clusters posessing an even number of electrons, furthermore, shell closing appears at n = 3, 9, 21… On the other hand, the electronic effects found while investigating the threshold energy region have to be attributed to neutral clusters, i.e. even numbered clusters are more stable.–The vertical ionization potentials in this region clearly demonstrate the manifestations of shell effects as well as distinct even‐odd oscillations. For both Ag and Au a dramatical drop in the ionization potential is observed from dimer to trimer. The overall size‐dependence of the ionization energies differs from that of alkali clusters due to the presence of d‐electrons.

The Optical PI .operties of Silver Microcrystallites in Dependence etc.this structure is fast, as demonstrated by the high estimated value of DE above. The system is comparable to the vinylferrocene films investigated by Daum and Murray [36]; in this case the swelling/deswelling transition is caused by the fact that in the oxidized state the film is polar while in the reduced state it is apolar.The kinetics of transition between the deswollen and the swollen state affects the current-voltage curves across the equilibrium potential region. Note the sharp peak at the slow sweep rate of Fig. 3, which originates from the structure change [36]. The rate of deswelling is determined by the rate at which ox is accumulated in the film, cf. the RRDE experiment, Fig. 6. This proceeds at anodic potentials by three mechanisms: (I) ox is produced by oxidation of red which has remained in the film, N:d(diff). (11) ox is formed anodically from red which arrives by diffusion from solution (cf. Zr(t), Fig. 6, with King = +0.393 V); (111) ox diffuses from solution into the film (cf.I,(t), with F', :ing = -0.22 V). Only at the slow sweeps the film remains in the anodic potential range sufficiently long for extensive deswelling to occur. At the fast sweeps of Fig. 3 the matrix apparently remains in the solvated state; the cyclic voltammetry peaks are symmetric and determined by diffusion of the redox-system in the solvated matrix. The diffusion co--efficients of ox and red are likely to be of comparable value in this state, as assumed in the analysis of the fast sweeps of Fig. 3, to determine V , , cussions and support of this project.We thank Professor Heinz Gerischer for continued stimulating dis- References