Ag8Fluorescence in Argon

Abstract

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...

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