The time-resolved decay of silver clusters Ag n ϩ (nϭ8Ϫ21) has been observed after excitation by photons with energies 1.5-4 eV. Clusters were found to decay by emission of neutral atoms or dimers with lifetimes in the range 100 s to 15 ms. Separation energies were calculated from the lifetimes assuming a statistical unimolecular decay. As a function of cluster size, the resulting values increase towards the bulk cohesive energy of silver. They show a pronounced odd-even alternation and an indication of a shell closure at nϭ9. The separation energies for nϭ8,9 are in good agreement with configuration-interaction ab initio calculations.
A Penning trap has been applied to the study of photoinduced unimolecular dissociation of Ag 10 + . The cluster decays by the evaporation of a neutral atom. At photon energies of 1.75 eV and 1.59 eV lifetimes of 330 ± 70 µs and 11.1 ± 2.9 ms, respectively, have been measured. By the use of statistical models the dissociation energy is determined from the observed lifetimes and their corresponding excitation energies. The values from both measurements agree, indicating the reliability of free cluster decay studies at lifetimes in the 10 ms range. By the investigation of the unimolecular decay of free clusters, information is derived about their stability which leads to suggestions of their internal structure, in particular when compared to neighboring cluster sizes. 1The decay may be studied in beam experiments employing a time-of-flight spectrometer, e.g. by signal broadening for decay processes within the acceleration region, or by the application of appropriate electric potentials in the drift region for later dissociation events. The passage of clusters through a multipole ion guide combined with pulsed laser excitation is another example.2 However, the beam experiments are limited by the drift period of the ions from the source (or ionization site) to the detector. If reactions are to be studied on a longer time scale, it is necessary to stop and store the clusters at some point in space (or, as an alternative at high kinetic energies, to use a storage ring 3,4 ). A series of time-resolved photodissociation experiments has been performed on various gold and silver clusters stored in an ion cyclotron resonance (ICR or Penning) trap. [5][6][7] In the following, we describe the measurements on Ag 10 + where the decay has been followed for several tens of milliseconds. Its lifetimes, at the two particular excitation energies under study, are about 0.3 ms and > 10 ms, respectively. EXPERIMENTAL SETUP AND PROCEDUREThe setup and earlier experiments have been presented in detail elsewhere. 8,9 The clusters are produced by laser vaporization of a silver wire in a helium gas pulse which is subsequently expanded into the vacuum. 10 The cations are transferred by ion optical elements through differential pumping stages to the UHV section of a Penning trap where they are captured in flight 11 and the ions at the cluster size of interest are selected by radial stored-waveform inverse Fourier transform (SWIFT) 12 ejection of all unwanted ions. The clusters are axialized by buffer-gas-assisted cyclotron excitation 13,14 in order to increase the spatial overlap between the cluster ensemble and the pulsed laser beam which is applied during the next step of the experimental sequence. A Nd:YAG pumped dye laser and an optical parametric oscillator have been used for the measurements reported below. Both the laser pulse energy and beam shape are monitored on-line by a beam profiling system. After a delay period, which is variable, the resulting cluster-ion size distribution is analysed by time-of-flight (TOF) mass spectrometry aft...
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.