Spectroscopy of extremely cold silver clusters in helium droplets

Abstract

The first highly resolved electronic spectra of small non-alkali metal clusters embedded in nanoscopic helium droplets (N ≈ 20 000) are presented. The helium droplets serve as an extremely cold liquid matrix with a temperature of 0.37 K. Resonant two-photon-ionization is used for size-selective spectroscopy on a silver cluster distribution (N ≤ 10). Results are reported for Ag 2 , Ag 3 and for Ag 8 . For the latter, a very narrow absorption spectrum is resolved corresponding to the Ag 8 plasmon resonance. The linewidth of the observed resonance is a factor of two smaller than theoretical zero-temperature predictions for the plasmon line width of closed-shell metal clusters. In contrast, the lifetime of the Ag 8 resonance is estimated to be of the order of nanoseconds, which is inconsistent with the plasmon picture and typical for molecular transitions.PACS. 36.40.Mr Spectroscopy of clusters -67.40.Yv Impurities and other defects in 4 He

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

“…It is in excellent agreement with the measured spectrum in an argon matrix, whose corresponding peaks are situated at 3.15, 3.57, and 3.82-3.90 eV. 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.…”

“…Ag 3 Contrary to the case of Ag 3 in an argon matrix grown at 10 K 5, 36, 37 which shows essentially three resolved absorption lines at 2.52, 3.21, and 3.86 eV, the Ag 3 /Ne spectrum is formed of a large number of narrow transitions, including two major peaks at 3.51 and 3.71 eV, that we all attribute to the trimer absorption, since there is no sign of fragmentation during deposition. Existing measurements of the trimer in helium droplets by Federmann et al 38 reveal distinct peaks in good agreement with the two major peaks if we consider a blueshift of 0.07 eV in neon compared to helium. The large difference between the neon and argon matrix absorption spectra is surprising.…”

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

“…As already mentioned above, our TDDFT calculations also confirm the authors' feeling of Ref. [21] that the asymmetry of the peak involves more than one transition. On the other hand, the comparison between the excitation spectrum in Ar matrix [22] and the calculated D 2d isomer spectrum allow us to measure the interaction of the Ar matrix with the Ag 8 cluster.…”

“…In this respect, as it is shown in Fig. 2, the calculated spectrum for D 2d symmetry is in excellent agreement with the resonant two-photon ionization spectrum reported by F. Federmann et al [21] and the excitation spectrum reported by C. Félix et al [22], whereas the T d -TT calculated spectrum only shows one resonant peak and it is about 0.31 eV blue-shifted with respect to the experimental measurements. Thus, the LCGTO-KSDFM predicted structure is confirmed by the TDDFT calculations of the optical response when it is compared to the experimental evidences.…”

“…2 for D 2d symmetry, the TDDFT calculations predict four electronic transitions starting from the twofold degenerate HOMO and HOMO-1 states to two excited states separated 0.07 eV in energy and it gives rise to two peaks because of the nearness in energy of the HOMO-1 and HOMO states. In the case of the T d symmetry, six transitions are predicted giving rise to only one peak because Open circles correspond to the resonant two-photonionization (R2PI) spectroscopy on Ag8 clusters in He droplets [21] while the solid triangles are for the excitation spectrum of Ag8 excited with monochromatic Xe light in an Ar matrix [22]. The D 2d -DD isomer spectrum is in excellent agreement with the experiment whereas the T d -TT isomer spectrum is around 0.31 eV blue-shifted compared to the R2PI spectrum.…”