Isomer-specific spectroscopy of metal clusters trapped in a matrix: Ag 9 Christoph Sieber, Jean Buttet, Wolfgang Harbich, and Christian Félix Institut de Physique des Nanostructures, École Polytechnique Fédérale (EPFL), CH-1015 Lausanne, Switzerland Roland Mitrić and Vlasta Bonačić-Koutecký Institut für Chemie, Humboldt Universität zu Berlin, Brook-Taylor-Strasse 2, 12489 Berlin, Germany (Received 21 August 2003; published 12 October 2004) 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. DOI: 10.1103/PhysRevA.70.041201 PACS number(s): 36.40.Mr, 36.40.Vz, 61.46.+w, 73.22.-f Recent experiments by fluorescence microscopy of nano- scale silver oxide [1] have demonstrated that strong photo- activated emission can be generated by uv excitation. The individual luminescent species are thought to be silver nano- clusters 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 in- vestigations, indicates that silver nanoclusters and, more gen- erally, metal clusters could be useful in optoelectronics as storage devices [1], full quantum logic elements [2], or pos- sibly as lasing media. It is thus important to understand bet- ter the emissive properties of supported metal clusters. In the gas phase, small metal clusters have been investi- gated using optical spectroscopy techniques such as resonant two-photon ionization (R2PI), laser-induced fluorescence [3,4], and pump-probe techniques. However, as the size in- creases, fragmentation becomes a dominant process and non- dissociative electronic excitation processes have not been ob- served 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 ef- fect, 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 par- ticular, recently shown that neutral Ag 4 [7] and Ag 8 [8] clus- ters 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 particu- larly 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 struc- ture [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]. However, it is not clear that the embedded clusters all correspond to the minimum-energy configuration: the different isomers present in the beam and the deposition process itself could lead to the trapping of different isomers. It is thus important to use isomer-specific experimental techniques, which allow one to extract out of a sample formed of several isomers a signature characteristic of a single isomer. We present in this Rapid Communication the observation of an intense and very narrow fluorescence line for Ag 9 clusters trapped in an argon matrix. The narrowness of the line and the observation of a well-defined vibrational structure are strong indications that this fluorescence is asso- ciated with a single isomer. The excitation spectrum, mea- sured by changing the excitation wavelength while recording the fluorescence intensity at the position of the fluorescence line, is thus an indirect measurement of the transition prob- abilities between the electronic states of this single isomer. We can exclude energy transfer between different clusters due the high dilution ratio of the clusters in the matrix. In- deed, although the absorption spectrum previously measured [14] shows broad features due to the contribution of different isomers, the excitation spectrum that we measure has a well- defined four peaks structure. Isomer-specific experimental techniques have been ap- plied to molecules and clusters in a beam. For gas-phase organic molecules, e.g., uv-hole-burning spectroscopy coupled with R2PI spectroscopy has been used to differenti- ate the absorption spectra of different isomers [15,16]. An- other promising route is to separate a gas-phase isomeric ion mixture prior to a spectroscopic probe. This has been re- cently achieved [17] in the case of carbon clusters by cou- pling photoelectron spectroscopy to ion mobility/mass spec- trometry. Excitation spectroscopy by detecting the fluorescence in a narrow band has previously been used to characterize, e.g., small matrix isolated carbon clusters [18], organic molecules [19], or metal dimers [20] placed in dif- ferent environments or at different sites. This paper reports on isomer-specific spectroscopy measurement of metal clus- ters in a matrix. PHYSICAL REVIEW A 70, 041201(R)(2004) RAPID COMMUNICATIONS 1050-2947/2004/70(4)/041201(4)/$22.50 ©2004 The American Physical Society 70 041201-1