Laser photoionization and spectroscopy of gas phase silver clusters

Abstract

Silver clusters containing up to 40-50 atoms are produced by laser vaporization in a pulsed-nozzle molecular beam source and studied with laser photoionization mass spectroscopy. A variety of Nd: YAG pumped dye laser and UV excimer laser wavelengths are used to achieve ionization. Ionization dynamics are studied by varying the laser wavelength and fluence. Bracketing experiments under single-photon ionization conditions are used to estimate ionization potentials as a function of cluster size. An even-odd ionization potential alternation is observed with odd-numbered clusters (N= 3, 5, 7 ...) having lower ionization potentials than adjacent even-numbered species. Shell closings at clusters containing 2, 8 20 and 40 electrons are observed consistent with a one-electron shell model picture of cluster electronic structure. Resonance-enhanced ionization produces a vibrationally resolved spectrum for the trimer, Ag 3, yielding an electronic state assignment and excited state vibrational frequencies. Fragmentation in dimer ionization via the E state at 249 nm establishes the dissociation energy of Ag] to be < 2.1 eV.

Structure of small silver clusters and static response to an external electric field

“…[28] favor more the T d structure than the D 2d one, however the assignment of the structural minimum is clearly inverted if we use the experimental IP (6.40 eV) measured in Ref. [31]. Our predicted minimum structure is also supported by a very recent reference where the authors determine the lowest-energy structure of Ag 8 by comparison of the optical spectra provided by time-dependent DFT with the experimental findings [32].…”

Absolute cross sections for electron impact ionization of Ag2

“…From v the measured photoion signals we find that in this regime ~ 3.0 the ratio lz(Ag + )/IL(Ag + ) is independent of Vm [8]. Since "~" two photon ionization of neutral Ag is obviously negli----2.5 gible in this region, we conclude that the observed Ag ÷ is produced by dissociative ionization of neutral Ag 2 "~ 2.0 Ag 2 + 2hv ~Ag + + Ag + e- This agrees well with the corresponding results of [9]. o 0 From (6) …”

“…Hence, the two photon ionization of neutral Ag2 must be strongly resonance enhanced. This result, which has also been observed in [9], is understandable since the photon energy of 5.0 eV closely matches the energy required for the X ~ E transition in Ag2 [10]. As a consequence, in the limit of low laser power density PL -< 106 W/cm2 the role ~" of photon induced fragmentation can be studied.…”

Spectroscopy of extremely cold silver clusters in helium droplets

“…The dimer signal at lower energies documents the photofragmentation of trimers. In the low-energetic part of the trimer absorption, distinct peaks are observed matching R2PI gas-phase data [19,23]. A R2PI spectrum recorded by Wallimann et al using a seeded beam [23] is also shown for comparison.…”

“…The observation of a sharp R2PI resonance for such a large metal cluster as Ag 8 is surprising in itself. It has been common experience with REMPI on gas-phase metal clusters, that with nanosecond laser pulses sharp spectral features are only obtained for the molecule-like dimers and trimers [19], whose excited states have lifetimes on a nanosecond time scale. The shorter lifetimes of the excited states of larger clusters have been invoked to explain the failure of REMPI spectroscopy on species beyond the trimer.…”