Roman Rabinovitch

Amino-acid and water molecules adsorbed on water clusters in a beam

Water clusters (H2On and (D2On (n<or=15) are produced by supersonic expansion and then pick up an additional heavy or light water molecule, respectively, or an amino-acid molecule (glycine or tryptophan). The products are analyzed by electron bombardment ionization mass spectrometry. Ionization proceeds via the well-known loss of an OH or OD group, but these turn out to have a strong predilection to come from the guest, rather than the host, molecule: between 30% and 60% of the time the loss originates in the picked-up molecule, even for large n. In fact, the magnitude of this fraction depends on the guest, but is largely insensitive to the cluster size. The observations suggest that the host clusters are frozen into compact annealed shapes, and the adducts reside on the surface and form an inhomogeneity where dissociative ionization tends to localize. It is also notable that no significant amino-acid fragmentation is observed beyond the hydroxyl group loss, in contrast to other measurements employing electron-impact ionization.

Pick-up cell for cluster beam experiments

In experiments in which a cluster beam picks up guest molecules by passing through a cell, the detection efficiency may be limited because of cluster beam scattering by the molecular vapor. We present a modified pick-up cell design that reduces cluster deflections and thereby improves the detection of mixed clusters. Its performance is illustrated with the help of a beam of water clusters picking up amino-acid or heavy-water molecules.

Slow Electron Attachment as a Probe of Cluster Evaporation Processes

Neutral alkali clusters efficiently capture low-energy electrons with the aid of long-range polarization attraction. Upon attachment, the electron affinity and kinetic energy are dissipated into vibrations, heating the cluster and triggering evaporation of atoms and dimers. This process offers a novel means to explore nanocluster bonding and evaporation kinetics. The present work investigates the formation of Na(N)(-). A crossed-beam experiment reveals that relative anion abundances become strongly and nontrivially restructured with respect to the neutral precursor beam. This restructuring is explained in quantitative detail by an analysis of evaporative cascades initiated by the attachment. The analysis thus furnishes a complete description of the electron attachment process, from initial attraction to final rearrangement of the cluster population. In addition, the paper describes a systematic derivation of cluster evaporation kinetics and internal temperature distributions; a new relation between the dissociation energies of cationic, neutral, and anionic metal clusters; and a scenario for inferring the neutral cluster population in the supersonic beam from the cationic mass spectrum.

Evaporative attachment of slow electrons to alkali-metal nanoclusters

The abundance spectrum of Na^-_{n~7-140} anions formed by low energy electron attachment to free nanoclusters is measured to be strongly and nontrivially restructured with respect to the neutral precursor beam. This restructuring is explained in quantitative detail by a general framework of evaporative attachment: an electron is captured by the long-range polarization potential, its energy is transferred into thermal vibrations, and dissipated by evaporative cooling. The data also affirm a formulated relation between the binding energies of cationic, neutral, and anionic clusters, and an adjustment to the prior values of dimer evaporation energies.

Loss of chlorine in mass spectra of DCl picked up by water clusters in a beam

Water clusters (H(2)O)(n), n<or=16, are produced by supersonic expansion of water vapor into vacuum, and then pick up a DCl molecule. The resulting mixed clusters are analyzed by electron bombardment ionization mass spectrometry. In all cases observed, the chlorine atom is lost in the ionization process, producing a deuterated water cluster cation [(H(2)O)(n)D(+)]. This suggests that the chlorine atom stays on the surface and has a weaker bond to the host cluster.