Isomer-specific spectroscopy of metal clusters trapped in a matrix:Ag9

Abstract

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]...

Ultraviolet-visible absorption of small silver clusters in neon: Agn (n = 1–9)

“…R2PI measurements of Ag 8 in He droplets 38,40 have also been performed, they reveal a narrow structure centered at 3.96 eV and a shoulder at 3.99 eV corresponding clearly to the main absorption peak observed in neon and argon (FWHM equal to 56 meV, 0.1 eV, 0.2 eV in He, Ne, Ar). 7,41 Ag 9 In the case of Ag 9 in an argon matrix 42 the shape of the spectrum could only be explained by assuming that several isomers coexisted in the matrix. The absorption spectrum in neon is clearly different.…”

Instability of Ag nanoparticles in SiO2at ambient conditions

“…But also in basic research the plasmon absorption and its modelling following Mie theory have proven a versatile tool in understanding cluster size effects, specific quantum mechanical influences [6] and interface effects [7]. Recently it was shown that also very small silver clusters of a few atoms only have intriguing properties and that their fluorescence is not only an ideal tool for fundamental researchers to determine their atomic and electronic structure [8,9] but that it might also be exploited in applications [10]. The aim of our research is to find means to produce samples of well defined metal clusters stabilized in protective matrices and to study their properties [11,12].…”

Matrix effects on the optical response of silver nanoclusters

“…Nanoclusters have interesting and promising properties in domains as diverse as optics, [1][2][3][4] catalysis, 5,6 and magnetism. [7][8][9][10] While some of these properties can arise for the cluster itself and can be studied in the gas phase, in most practical cases the nanoclusters are in interaction with their environment.…”