We present a joint theoretical and experimental investigation of the absorption spectra of silver clusters Ag n ͑4 Յ n Յ 22͒. The experimental spectra of clusters isolated in an Ar matrix are compared with the calculated ones in the framework of the time-dependent density functional theory. The analysis of the molecular transitions indicates that the s-electrons are responsible for the optical response of small clusters ͑n Յ 8͒ while the d-electrons play a crucial role in the optical excitations for larger n values.

Variable-temperature scanning tunneling microscopy was used to study the effect of kinetic cluster energy and rare-gas buffer layers on the deposition process of size-selected silver nanoclusters on a platinum(111) surface. Clusters with impact energies of

We present optical absorption and fluorescence spectra in the UV-visible range of size selected neutral Ag n clusters (n = 1-9) in solid neon. Rich and detailed optical spectra are found with linewidths as small as 50 meV. These spectra are compared to time dependent density functional theory implemented in the TURBOMOLE package. Excellent agreement between theory and experiment is achieved in particular for the dominant spectroscopic features at photon energies below 4.5 eV. This allows a clear attribution of the observed electronic transitions to specific isomers. Optical transitions associated to the s-electrons are concentrated in the energy range between 3 and 4 eV and well separated from transitions of the d-electrons. This is in contrast to the other coinage metals (Au and Cu) which show a strong coupling of the d-electrons.

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...

We present optical absorption spectra in the UV-visible range (1.6 eV <¯ω < 5.5 eV) of mass selected neutral copper clusters Cu n (n = 1-9) embedded in a solid neon matrix at 7 K. The atom and the dimer have already been measured in neon matrices, while the absorption spectra for sizes between Cu 3 and Cu 9 are entirely (n = 6-9) or in great part new. They show a higher complexity and a larger number of transitions distributed over the whole energy range compared to similar sizes of silver clusters. The experimental spectra are compared to the time dependent density functional theory (TD-DFT) implemented in the TURBOMOLE package. The analysis indicates that for energies larger than 3 eV the transitions are mainly issued from d-type states; however, the TD-DFT scheme does not reproduce well the detailed structure of the absorption spectra. Below 3 eV the agreement for transitions issued from s-type states is better.

We present optical absorption spectra in the UV-visible range (1.5 eV < E < 6 eV) for mass selected neutral gold clusters Au n (n = 1-5 and 7-9) embedded in solid Ne at 7 K. The experimental spectra are compared with time-dependent density functional calculations. Electronic transitions are distributed over the whole energy range without any concentration of the oscillator strength in a small energy window, characteristic for the more s-like metals such as the alkalis or silver. Contrary to the case of silver and partly copper clusters, transitions issued from mainly d-type states are significantly involved in low energy transitions. The measured integrated cross section is smaller (<20%) than expected from a free-electron system, manifesting the strong screening of the s electrons due to the proximity of the s and d levels in gold.

The surface plasmon polariton-enhanced Raman spectra of size-selected C 16 , C 18 , and C 20 clusters isolated in nitrogen matrices are presented along with the calculated vibrational frequencies for the ring and linear chain isomers. The Raman spectra, recorded at a range of excitation wavelengths from 457.9 to 670 nm, show strong resonance enhancement for the three clusters. The calculated vibrational frequencies for ring and linear chain isomers and the cage and bowl structures for C 20 are compared to the experimental frequencies. Systematic shifts in the series of peaks in the 200 cm Ϫ1 region for C 16 , C 18 , and C 20 suggest that the observed isomers have the same geometry, thereby ruling out the bowl and cage isomers for C 20 . The measured spectra appear to be most consistent with the linear chain isomer. This high-energy isomer may be produced during neutralization of the deposited cluster ions.

Clusters of metal atoms at a fixed size can assume different structural arrangements, known as isomers, which may have nearly the same energy. Therefore, at given experimental conditions distribution of these isomers can be present. While the size selection is a relatively common technique, the isomer selection is not; it is therefore more difficult to obtain information about a single isomer. We report here on isomer-specific spectroscopy of Ag 9 clusters together with ab initio calculations allowing to identify the isomer responsible for the measured excitation pattern and fluorescence. Recent experiments by fluorescence microscopy of nanoscale silver oxide [1] have demonstrated that strong photoactivated emission can be generated by uv excitation. The individual luminescent species are thought to be silver nanoclusters that are photochemically generated from the oxide. The color of individual emissive sites changes as a function of time, and the authors relate it to the changing charge and size of the Ag clusters. This work, together with further investigations, indicates that silver nanoclusters and, more generally, metal clusters could be useful in optoelectronics as storage devices [1], full quantum logic elements [2], or possibly as lasing media. It is thus important to understand better the emissive properties of supported metal clusters.In the gas phase, small metal clusters have been investigated using optical spectroscopy techniques such as resonant two-photon ionization (R2PI), laser-induced fluorescence [3,4], and pump-probe techniques. However, as the size increases, fragmentation becomes a dominant process and nondissociative electronic excitation processes have not been observed for gas-phase metal clusters larger than the trimer, unless very short pulses are used [5]. The optical absorption spectra of larger metal clusters have thus been obtained using photodepletion spectroscopy [6].The situation is different in a matrix due to the cage effect, which effectively prevents dissociation. This opens the possibility to observe the fluorescence of particles larger than the trimer, if the excited state of the particle has a sufficient lifetime for radiative transition to take place. It was, in particular, recently shown that neutral Ag 4 [7] and Ag 8 [8] clusters embedded in an argon matrix have a strong fluorescence signal.However, a major difficulty, both in the gas phase and supported cluster experiments, is that in general molecular beams are not formed from a single isomer. This is particularly true for metal clusters, in which the delocalized nature of the valence electrons leads, for a given size, to isomers which may be quasidegenerate with the lowest energy structure [9,10]. This can lead to ambiguities in the assignment of measured features to a given isomer and to enlargement of the peaks. There are, however, exceptions, especially at low temperature [11]. For size-selected clusters deposited in a rare-gas matrix, it has been shown that conditions can be found to minimize fragmentation [14]...