Formation of even-numbered hydrogen cluster cations in ultracold helium droplets

Neutral hydrogen clusters are grown in ultracold helium nanodroplets by successive pickup of hydrogen molecules. Even-numbered hydrogen cluster cations are observed upon electron-impact ionization with and without attached helium atoms and in addition to the familiar odd-numbered H(n)(+). The helium matrix affects the fragmentation dynamics that usually lead to the formation of overwhelmingly odd-numbered H(n)(+). The use of high-resolution mass spectrometry allows the unambiguous identification of even-numbered H(n)(+) up to n congruent to 120 by their mass excess that distinguishes them from He(n)(+), mixed He(m)H(n)(+), and background ions. The large range in size of these hydrogen cluster ions is unprecedented, as is the accuracy of their definition. Apart from the previously observed magic number n = 6, pronounced drops in the abundance of even-numbered cluster ions are seen at n = 30 and 114, which suggest icosahedral shell closures at H(6)(+)(H(2))(12) and H(6)(+)(H(2))(54). Possible isomers of H(6)(+) are identified at the quadratic configuration interaction with inclusion of single and double excitations (QCISD)/aug-cc-pVTZ level of theory (C) 2008 American Institute of Physics. [DOI: 10.1063/1.3035833

Multiply charged clusters

We review progress made in understanding Coulomb explosion of multiply charged atomic clusters. Their collision with highly charged atomic ions leads to clusters in charge states as high as z = 10 with little vibrational excess energy; these systems approach the Rayleigh limit. Phase transitions become evident at higher excess energies. Numerous studies have been devoted to C-60(z+) like collisions with surfaces, multi-coincidence fragmentation analysis and gas-phase reactions. Stability and decay of highly charged micrometer-sized droplets and of metal di- and trianions have been monitored in ion traps. Excitation by fermosecond laser pulses allows to unravel properties of highly charged transient cluster ions. (C) 2002 Academie des sciences/Editions scientifiques et medicales Elsevier SAS

Electron attachment to doped helium droplets: C(60)(-), (C(60))(2)(-), and C(60)D(2)O(-) anions

Helium nanodroplets, formed in a supersonic expansion, are doped with C(60) in a pickup cell. In some experiments, they are co-doped with water. Electrons are attached to the doped droplets; the yield of anions is recorded as a function of electron energy. The C(60)(-) yield extends to much higher energies than ill experiments involving isolated, hot fullerences; we attribute the difference to the low temperature of the neutral precursors and the efficient. cooling of the nascent anions by the helium droplet, which quench thermally activated autodetachment. The yields of (C(60))(2)(-) and C(60)D(2)O(-) anions reveal another important factor, namely depletion of the anion signal by dissociation which is energetically more facile than autodetachment

High-resolution mass spectrometric study of pure helium droplets, and droplets doped with krypton

Mass spectra of doped and undoped helium droplets are presented. The high resolution of the time-of-flight spectrometer (m/Delta m a parts per thousand... 5000) makes it possible to fully resolve small helium cluster ions from impurities and to unambiguously identify abundance anomalies in the size distribution of He (n) (+). The yield of He(4) (+) shows the well-known enhancement relative to other small cluster ions when the expansion changes from sub- to supercritical, provided the electron energy exceeds a value of 40 +/- A 1 eV, the threshold for formation of electronically excited ions. Upon doping with krypton, pure Kr (n) (+) cluster ions containing up to 41 Kr atoms are observed. The spectra exhibit abundance anomalies at 13, 16, 19, 22 & 23, 26 and 29, in agreement with spectra obtained by ionization of bare krypton clusters that are formed in neat supersonic beams. Mixed clusters He (m) Kr(+) indicate closure of a solvation shell at m = 12

Absolute cross sections and kinetic energy release for doubly and triply charged fragments produced by electron impact on CO+

Absolute cross sections for electron impact ionization of CO+ leading to the formation of doubly and triply charged products (CO2+, C2+ and O2+, C3+ and O3+) are reported in the energy range from their respective thresholds to 2500 eV. Around the maximum, cross section values are found to be ( 13.36 +/- 0.56) x 10(-18) cm(2), ( 5.58 +/- 0.55) x 10(-18) cm(2) and (1.37 +/- 0.14) x 10(-18) cm(2) for CO2+, C2+ and O2+, respectively, and (28.3 +/- 7.0) x 10(-21) cm(2) and (2.4 +/- 0.7) x 10(-21) cm(2) for C3+ and O3+, respectively. The analysis of ionic product velocity distributions, obtained by means of a crossed electron-ion beam set-up, allows the determination of the kinetic energy release distributions. They are seen to extend from 0 to 50 eV both for C2+ and O2+. The mean kinetic energy releases for C3+ and O3+ are found to be 33 +/- 5 eV and 39 +/- 9 eV, respectively. The cross sections are seen to depend exponentially on the potential energy of each dissociated ion pair. The total single CO+ ionization cross section calculated by application of the Deutsch-Mark ( DM) formalism is found to be in good agreement with the experimental results

Isotope effects in dissociative electron attachment to the DNA base thymine

Dissociative electron attachment to thymine and two partially deuterated derivatives are studied with high electron energy resolution. For all fragment anions involving hydrogen loss, a dramatically reduced anion yield is measured for loss of D compared to loss of H. For the low-energy vibrational Feshbach resonances this isotope effect reaches a value of 40. Narrow features in the anion yields that previously were assigned to H loss from the N1 site of thymine and concomitant excitation of specific stretching modes are shifted to lower electron energies when deuterium is involved. (C) 2008 Elsevier B.V. All rights reserved

Die Jsrael. Kultusgemeinde Bamberg von 1803 - 1853 : Festschrift zur Einweihung der neuen Synagoge in Bamberg ...

von A. Eckstein. Mit e. Beitr. von Jos. Werner u. Joh. Kronfus

Submersion of potassium clusters in helium nanodroplets

Small alkali clusters do not submerge in liquid helium nanodroplets but instead survive predominantly in high spin states that reside on the surface of the nanodroplet. However, a recent theoretical prediction by Stark and Kresin [Phys. Rev. B 81, 085401 (2010)], based on a classical description of the energetics of bubble formation for a fully submerged alkali cluster, suggests that the alkali clusters can submerge on energetic grounds when they exceed a critical size. Following recent work on sodium clusters, where ion yield data from electron impact mass spectrometry was used to obtain the first experimental evidence for alkali cluster submersion, we report here on similar experiments for potassium clusters. Evidence is presented for full cluster submersion at n>80 for Kn clusters, which is in good agreement with the recent theoretical prediction. In an additional observation, we report “magic number” sizes for both Kn+ and Kn2+ ions derived from helium droplets, which are found to be consistent with the jellium model