Pulsed photodissociation in an ion cyclotron resonance trap: extending the time range for unimolecular dissociation studies of metal clusters

Electron impact ionization/dissociation of size selected gold cluster cations

Abstract Singly charged gold clusters, Au n + in the size range n =12 to 72 have been captured and stored in a Penning trap, size selected and subjected to an electron beam. This interaction leads to further ionization as well as dissociation. The resulting abundance spectra of doubly and triply charged clusters show (a) a lower size limit for the production of multiply charged clusters from an ensemble of hot precursors, which can be understood in terms of the respective decay pathways, (b) an odd/even alternation of singly and doubly charged clusters in the size range below n =30, which inverses sign with change of charge state, and (c) magic numbers, i.e. prominent signals for particular…

Experimental dissociation energies of metal cluster dications and their interpretation in a liquid-drop model with empirical corrections

The dissociation energies of doubly charged silver cluster ions in the size range $9l~nl~25$ are measured by multiple collision induced dissociation. They are compared to the dissociation energies of singly charged clusters. To this end, the latter are used to calculate shell corrections in a macroscopic-microscopic model. Good agreement between the resulting predictions of the dissociation energies of the doubly charged systems and the experimental values is found, which indicates the strong influence of electronic effects on the stability of small silver clusters.

Time resolved photofragmentation of Au $_{n}^{+}$ and Ag $_{n}^{+}$ clusters (n = 9, 21)

Gold and silver cluster ions were produced by laser vaporization and stored in a Penning trap. After mass selection the cluster sizes of interest were illuminated by a laser pulse and electronically excited. Photoabsorption cross sections and fragmentation patterns were measured for photon energies of 2.3 eV to 5.2 eV. Unimolecular dissociation was observed time resolved on a microsecond to millisecond scale. Dissociation energies were deduced from the measured life times.

Fragmentation of gold clusters stored in a penning trap

The collision-induced dissociation of positively charged gold clusters (2 to 23 atoms) stored in a Penning trap has been studied. After collisions with rare gases, excited clusters predominantly decay by emission of one or two atoms. The loss of two atoms occurs most likely through the emission of a dimer rather than a sequential evaporation. The minimum kinetic energies of clusters required to induce dissociation exhibit a pronounced odd-even effect. Clusters with an even number of delocalized electrons are more stable than the odd ones.

The temperature dependence of photoabsorption of V13+

Photoabsorption cross sections of trapped V 13 + clusters were measured for energies between 1.4 eV and 6.1 eV by use of a two-photon fragmentation technique. Using two laser pulses separated by a 100-ns delay period the cross sections of hot clusters (up to 1800 K) could be compared to the values at room temperature. Whereas sodium clusters are known to exhibit pronounced temperature dependence below 300 K, no continuation of these effects to higher temperatures was found for V 13 + .

The dissociation channels of silver clusters Agn+, 3 ≤ n ≤ 20

Abstract The low energy dissociation channels of silver cluster ions Agn+, 3 ≤ n ≤ 20 are determined by collision-induced dissociation (CID) in a Penning trap. While for most cluster sizes the first fragment cluster ion is produced by monomer evaporation, the fragment ion of small odd-sized clusters has two atoms less than their precursors indicating an evaporation of dimers. The results are compared to similar CID studies on gold cluster ions, photofragmentation patterns, abundance spectra for various silver-cluster production techniques and calculated binding energies.

Electronic effects in the production of smali dianionic gold clusters by electron attachment on to stored Au-n, n = 12-28

Abstract Single charged gold clusters Au n -, 12  n  28, are stored in a Penning trap, size selected and transformed into dianions, Au2- n by the application of an electron beam. At the onset of dianion production, that is that range of cluster sizes n where the smallest doubly charged clusters are observed, the measured intensity ratio of the dianions to their precursors is not a continuous function of cluster size. Instead, there is a strong odd-even effect and a comparatively intense signal of Au2-18 The observed structures are very reminiscent of similar phenomena in the abundance spectra of metal clusters as observed by Knight et al. (1984, Phys. Rev. Lett., 52, 2141), which gave ris…

Observation of electronic and geometric shell structures of small silver clusters

Singly charged silver clusters Open image in new window in the size range n = 17 to n = 78 have been captured and stored in a Penning trap, size selected and subjected to an electron beam which leads to further ionization as well as dissociation. The resulting abundance spectra of doubly and triply charged clusters show several features: (1) A critical size for the production of doubly charged clusters which can be understood in terms of the decay pathways as previously investigated by collision induced dissociation, (2) an odd-even alternation in the abundance of singly and doubly charged clusters which inverses sign with change of charge state, (3) prominent signals for particular doubly …

Trapped metal cluster ions

An overview is given of experiments with stored metal cluster ions in a Penning trap system. The setup allows axial injection of clusters produced in an external source and a time-of-flight mass analysis of the reaction products after axial ejection. The system's options include the selection of stored ions, the manipulation of their orbits, addition of reactant and buffer gases and axial optical access for laser spectroscopic studies. As described by various examples, investigations have been made with respect to the development of trapping techniques and the characterization of metal clusters in terms of their physical and chemical properties.

Collision induced dissociation of doubly charged stored metal cluster ions

Fission barriers of doubly charged silver clusters

The monomer evaporation energies and fission barriers of doubly charged silver cluster ions in the size range 9≤n≤25 are measured by collision induced dissociation. They are compared to the dissociation energies of singly charged silver clusters. A macroscopic liquid drop model combined with empirical microscopic corrections successfully describes the measured fission barriers.

Fragmentation pattern of gold clusters collided with xenon atoms

Abstract The dissociation channels of gold cluster ions Au n + (2 ≤ n ≤ 23) have been investigated via collision induced dissociation in a Penning trap. Excited odd cluster ions with n ≤ 15 decay by evaporation of dimers, all others decay by monomer evaporation. Information on the binding energies is deduced from these dissociation channels.

Radiative Cooling of a Small Metal Cluster: The Case ofV13+

Size-selected stored metal cluster ions, ${\mathrm{V}}_{13}^{+}$, have been heated by photoexcitation ( $\ensuremath{\lambda}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}730$ to 229 nm) to well-defined excitation energies corresponding to temperatures between 1000 and 2100 K. A millisecond pump-probe photodissociation technique was applied to measure the time-resolved radiative cooling. The observed decay rates are directly related to the radiative energy loss and are explained quantitatively by the competing processes of photoemission and atom evaporation.

Probing cluster structures with sensor molecules: methanol adsorbed onto gold clusters

Abstract Structural, dynamical and electronic properties of the adducts formed by adsorbing methanol onto size-selected gold clusters are investigated using infrared multiple-photon dissociation spectroscopy of trapped Au n + CH 3 OH, n ⩽15, in conjunction with Car–Parrinello calculations. The C–O stretching vibration of the attached sensor molecule changes discontinuously as a function of cluster size, which is traced back to a change in dimensionality of the cluster structure.

Production and investigation of multiply charged metal clusters in a Penning trap

Singly charged gold cluster ions from a laser-vaporization source are transferred into a Penning trap and subjected to electron bombardment. The charged reaction products are analyzed by time-of-flight mass spectrometry after axial ejection from the trap. They include singly charged cluster fragments, multiply charged clusters of the initial size and multiply charged cluster fragments. The multiply charged clusters are selected and further investigated by collision induced dissociation. Two types of reactions can be distinguished: Dissociation into several charged fragments and evaporation of neutrals. Several features of multiply charged clusters relevant for future investigations are revi…

The Mainz Cluster Trap

When cluster ions are stored by electromagnetic forces they are available in the gas phase for extended preparations and investigations. Over the last decade a Penning trap (Ion Cyclotron Resonance) apparatus has been constructed and further developed with respect to metal cluster research at the Institute of Physics at Mainz. It allows to capture and accumulate ion bunches injected from an external cluster source and to manipulate the ions’ motion, i.e. select and center the clusters of interest. The interactions that have been investigated include those with inert and chemically reactive gases, photons and electrons. Multiple mass spectrometric steps such as fragment ion selection can be …

Decay pathways of stored metal-cluster anions after collisional activation

Size-selected gold clusters, Open image in new window (n ≤ 21), and tungsten clusters, Open image in new window , and Open image in new window (n = 4 – 8 and 12), stored in a Penning trap have been collisionally activated. Neutral monomer and dimer evaporation are observed in the case of gold. While no fragment products have been observed for tungsten clusters, there is evidence of electron emission from the anions.

Low-energy decay pathways of doubly charged silver clusters $Ag_{n}^{2+}$ (n = 9 - 24)

The low-energy dissociation channels of mass selected silver cluster ions Ag n 2+ (n = 9–24) are determined by collision induced dissociation (CID) in a Penning trap. While all clusters of the size n ≥ 17 evaporate neutral monomers, most smaller clusters undergo asymmetric fission of the form Ag n 2+ → Ag −3 + + Ag {3} + . However, Ag 15 2+ and Ag 11 2+ emit monomers which indicates shell or odd-even effects. The observed fragmentation pathways are different from previous reports of measurements with sputtered Ag n 2+ .

First Observation of Doubly Charged Negative Gold Cluster Ions

Singly charged gold cluster anions in the size range n = 16–30 have been captured, stored and size selected in a Penning trap. After application of an electron beam doubly charged gold cluster anions have been observed for 20 ≤ n ≤ 30. To our knowledge this is the first observation of metal cluster dianions. The threshold appearance size is in good agreement with a simple charged sphere model. The application of argon gas pulses simultaneously with the electron beam is found to increase the production rate by an order of magnitude.

Collision induced dissociation of stored gold cluster ions

The stability of gold cluster ions Au + (2≦n≦23) has been investigated via collision induced dissociation in a Penning trap. Threshold energies and dissociation channels have been determined. The cluster stability exhibits a pronounced odd — even alternation: Clusters with an odd number of atoms,n, are more stable than the even-numbered ones. Enhanced stabilities are found for Au 3 + , Au 9 + , and Au 19 + in accordance with the Clemenger-Nilsson and the deformed jellium model of delocalized valence electrons. Excited odd cluster ions withn≦15 predominantly decay by evaporation of dimers; all others decay by monomer evaporation. From the dissociation channels estimates of the binding energi…

Decay pathway determination of even-size dicationic silver clusters: Ag162+ and Ag182+ revisited by pre-precursor selection and sequential decay

Abstract It is demonstrated how by selection of pre-precursor cluster ions Ag 2 n +1 2+ and collision-induced dissociation an ensemble of Ag 2 n 2+ can be produced, which is not contaminated by Ag n + clusters of the same size-to-charge state ratio, n / z . By use of this technique, Ag 16 2+ and Ag 18 2+ precursor ensembles have been prepared for further investigations. They are observed to decay by neutral monomer evaporation, Ag 2 n 2+ →Ag 2 n −1 2+ +Ag, and trimer fission, Ag 2 n 2+ →Ag 2 n −3 + +Ag 3 + , and show no sign of symmetric fission.