<i>Ab initio</i> study of the absorption spectra of Agn (<i>n</i>=5–8) clusters

Bonačić‐Koutecký, Vlasta; Veyret, Vincent; Mitrić, Roland

doi:10.1063/1.1415077

Cited by 204 publications

(215 citation statements)

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“…This method gives reliable transition energies and oscillator strengths for electronic states dominated by singly excited configurations, which is usually the case for optically allowed transitions with large intensities. We have verified this by comparing RPA results and equation of motion coupled cluster (EOM-CCSD) calculations in our previous work [22]. In order to study the fluorescence of Ag 9 , geometry optimization in the excited state has been performed by using numerically calculated gradients combined with the conjugated gradient method.…”

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confidence: 95%

“…In our previous work on optical properties of silver clusters [22] it has been shown that an 11-electron relativistic effective core potential (11e-RECP) with ͑6s5p5d͒ AO basis set for silver allows one to accurately determine transition energies and oscillator strengths in the range up to 5 eV [22,23]. Structures of the isomeric forms of Ag 9 cluster have been fully optimized in the framework of the density functional (DFT) method, with S-VWN and Becke-Perdew parametrization (BP86) [24] using gradientbased optimization methods.…”

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

et al. 2004

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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]...

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

et al. 2004

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“…It was reported in Refs. [5,8] that the D 2d geometry is favored energetically over T d symmetry when explicit correlation treatments for 5s electrons are included, but since the calculated energy difference between T d and D 2d isomers is very small, the predicted theoretical ordering is uncertain. One way of solving this vexing problem comes from the hand of the time-dependent density functional theory (TDDFT) [11] that is a generalization of traditional ground stationary state DFT to treat the dynamic response of the charge density to a time-dependent perturbation.…”

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confidence: 99%

“…After a review of the literature on silver clusters [4,5,6,7,8], we have decided to optimize, as a good candidate to the structural minimum of the octamer, the following isomers of Ag 8 : a D 2d dodecahedron (D 2d -DD), which can also be viewed as a distorted bicapped octahedron, a T d tetracapped tetrahedron (T d -TT) and a C s 1-pentagonal bipyramid (C s -PBP) in Fournier's notation [4]. The main results (density of states (DOS), optimized structures, polarizabilities, ground state energies, .…”

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“…A review of the literature reveals that there are two competing geometries in eight-atom clusters of s-electron elements, having T d and D 2d symmetry. In fact, different levels of theories favor different geometries: DFT in its local density approximation (LDA) [4], multireference configuration interaction method [5], a tight-binding approach [6], and the many-body perturbation theory-based calculations [7] give the D 2d geometrical shape as the lowest energy structure of Ag 8 whereas the equation-of-motion coupled cluster method [8], time-dependent DFT only at LDA level [9] and molecular-dynamics simulations [10] predict a T d structure as the structural minimum. It was reported in Refs.…”

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Determination of the lowest-energy structure ofAg8from first-principles calculations

Pereiro

Baldomir

2005

Phys. Rev. A

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The ground-state electronic and structural properties, and the electronic excitations of the lowest energy isomers of the Ag8 cluster are calculated using density functional theory (DFT) and timedependent DFT (TDDFT) in real time and real space scheme, respectively. The optical spectra provided by TDDFT predict that the D 2d dodecahedron isomer is the structural minimum of Ag8 cluster. Indeed, it is borne out by the experimental findings.PACS numbers: 36.40.Mr In latter years, a new field has emerged from the understanding, control and manipulation of objects at nanoscale level (nano-objects). It is commonly known as nanoscience. This field involves physics, chemistry and even engineering, and addresses a huge number of important issues starting from basic science and ending in a large variety of technological applications [1]. Among the nano-objects, the small clusters or nanoclusters play a very important role, since they are the bricks of nanoscience. Therefore, the study of small clusters deserves a special attention. In this respect, the steps to follow for a complete description of a cluster can be summarized in the following three questions: what is the lowest energy structure?, what is the effect of increasing or decreasing the temperature on the structural properties of a cluster? and the last step deals with the kinetic effects in the formation of the nanocluster. Here, we are only concerned about the first question for the silver octamer, leaving the other two questions open for future investigations.From the theoretical point of view, first-principles methods give an enormous advantage for understanding, projecting and inventing new materials that is reflected in the huge number of articles published in the field of materials science. Likewise, density functional theory (DFT) has emerged as a new and promising tool for ab initio electronic structure calculations and gives valuable information about the geometry of nanoscale systems [2] but unfortunately it not always predict the correct structure of the cluster under consideration. In this regard, the silver octamer belongs to the group of the controversial systems for which the lowest energy structure is unresolved by DFT.Recently, P. Radcliffe et al.[3] have proposed Ag 8 clusters embedded in helium droplets as a suitable system for light amplification based on an optically accessible long-living excited state (E*) and thereby, from the theoretical point of view, the determination of the structure becomes a key point as the first step for identifying and controlling the levels that populate E*. Up to now, it has not been possible to make a reliable theoretical prediction of the most stable structure of Ag 8 . A review of the literature reveals that there are two competing geometries in eight-atom clusters of s-electron elements, having T d and D 2d symmetry. In fact, different levels of theories favor different geometries: DFT in its local density approximation (LDA) [4], multireference configuration interaction method [5], a tight-binding appr...

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Chemical Effects and the SERS Spectrum

Aroca

2006

Surface‐Enhanced Vibrational Spectroscopy

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After the local electromagnetic field enhancement (EM mechanism) of the Raman scattering intensity has been discussed, the effect of the electronic interactions between the metal and the adsorbate should be considered in order to tackle the interpretation of the observed Raman spectra. Recently, Otto wrote [1], "For sure, without the EM mechanism there would be no signal. But the chemical mechanism determines what is observed." This comment captures the fundamental fact that the observed SERS spectrum contains the information about the adsorbate and its environment, in particular its interaction with the enhancing nanoparticle, its spatial orientation and the polarization properties of the local electric field.In this chapter, we examine the Raman vibrational spectrum of admolecules, molecules adsorbed on an active SERS nanostructure. The factors affecting the rate of adsorption are not discussed here (see Chapter 6), nor are the effects induced by the photon flux (photodesorption and photoreactions), so we examine the Raman spectrum of an adsorbate that does not degenerate during illumination. The electronic cloud of the adsorbate represented by the volume α(Q, ω) can be distorted by the interaction with the solid surface of the nanostructure. When the energy of this interaction (enthalpy of adsorption) is more positive than ∼ −25 kJ mol −1 , the interaction is classified as physisorption, and the Surface-Enhanced Vibrational SpectroscopyR. Aroca

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Ab initio study of the absorption spectra of Agn (n=5–8) clusters

Cited by 204 publications

References 47 publications

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

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

Determination of the lowest-energy structure ofAg8from first-principles calculations

Chemical Effects and the SERS Spectrum

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