Tamotsu Noda

Magic Numbers in a Mass Spectrum of Lithium Clusters Emitted from a Liquid Metal Ion Source

Lithium ion clusters Lin+ with n ranging up to 19 have been observed from liquid lithium ion sources by using a magnetic mass analyzer. The ion intensity as a function of cluster size showed local maxima at n=7, 9, 13, and 19. Only n=9 and 19 can be explained by the electronic shell model, which successfully predicts magic numbers of other alkali metal (Na, K) clusters. On the other hand, the geometrical atom packing model explains the observed magic numbers except for n=9.

Formation and Ionization Potentials of Lead Clusters

Clusters of lead atoms have been produced by an adiabatic expansion of the vapour in a high temperature crucible through a small hole into a vacuum. These clusters have been ionized by electron bombardment and analysed with a time-of-flight mass spectrometer. Clusters of up to ten atoms have been observed, and Pb7, with the highest intensity. The relative abundance of the clusters is higher at higher crucible temperatures and/or with larger hole diameters. Ionization potentials of the clusters of up to seven atoms have also been measured. The change in the ionization potentials shows the same trend as that in an effective work function of a continuum metal drop.

Oxygen effects on ion-induced secondary ion and photon emissions from GaAs and Ga surfaces

Abstract Secondary ion and photon emissions from GaAs or Ga metal surfaces during oxygen flooding were simultaneously measured by the SIMS-SCANIIR technique. The oxygen coverage was sequentially monitored by ISS, utilizing a double focusing mass spectrometer of a SIMS instrument. With increasing oxygen coverage, the Ga+ intensity from the GaAs surface decreased slightly, whereas that from the Ga metal surface increased. The photon intensity due to excited Ga atoms sputtered from the surface, whether GaAs or Ga metal, was in proportion to the oxygen coverage. This indicates that Ga+ intensity depends on the Ga-O bonding configuration, as well as the oxygen coverage, whereas the photon intensity depends only on the oxygen coverage, implying that the mechanism of electronic excitation of Ga atoms is different from its ionization.

Cluster ions ejected from an Li-Mg alloy liquid metal ion source: Observation of MG 2 2+ and MG 3 2+

Ions ejected from a liquid metal ion source of an Li-Mg (10 atom %) alloy have been investigated by using a magnetic mass analyzer. In addition to singly charged homonuclear Lin+ (n ≤ 9) and Mgn+ (n ≤ 4) and heteronuclear MgmLin+ (m, n ≤ 2) clusters, doubly charged diatomic and triatomic Mg clusters are observed. Discussion is focused on the observability and the formation mechanism of the doubly charged small Mg clusters. A postionization process is suggested for the formation of the doubly charged clusters.

Abundance of Na cluster ions ejected from a liquid metal ion source

Sodium cluster ions Na+n withn ranging up to 25 have been observed from a liquid sodium ion source by using a magnetic mass analyzer. Ion intensity as a function of cluster size showed distinct steps and local maxima atn=3, 5, 11, 13 and 19 (magic numbers), and a pronounced odd-even alternation. The features in the ion abundance curve are attributed to the relative stability of cluster ions. The observed magic numbers are only partially explained by the electronic shell model, indicating need to include a consideration of atomic structure in a cluster.

Formation of Lead Clusters in Supersonic Nozzle Expansion: Effect of Nozzle Geometry

Atom-clusters of lead have been produced by adiabatic expansion of the vapor through small nozzles. Effect of nozzle geometry on the formation of clusters has been studied by time-of-flight mass spectrometry following the electron impact ionization. Three kinds of nozzle forms, converging-diverging, converging, and straight nozzles, have been investigated. Of the three, the converging-diverging nozzle produces clusters most effectively. Throat diameter also affects the cluster formation; the mean cluster size becomes larger for the larger throat among three different diameters, 0.3, 0.5 and 0.7 mm.

Binary Compound Clusters Formed by Free Jet Expansion

Atom-clusters of NaCl, KCl, PbS and PbSe have been produced by adiabatic expansion of the vapor through a small nozzle. These clusters have been ionized by electron bombardment and analyzed by time-of-flight mass spectrometry. Formation of compound clusters from MX to (MX)4 has been observed for all the samples, where M represents a cation and X an anion. Alkali halide clusters are considered to keep their stoichiometry. On the other hand, for lead chalcogenides, clusters having non-stoichiometric compositions as well as those having stoichiometric compositions are formed, though the former clusters are less abundant than the latter.

Analysis of Mechanical Aberrations in the Eleclrostatic Deflector

A third-order aberration formula of mechanical aberrations due to various errors in assembling or machining of the electrostatic deflector is derived from a second-order perturbation theory. Mechanical aberrations in a magnetic focusing and electrostatic deflection system are analyzed by utilizing this new formula. The results show that the electrostatic deflector suffers, in general, more serious effects from errors in machining than in assembling if the error magnitudes are almost the same, and also that the edge of the deflection electrodes where the electric field concentrates exerts a greater influence upon occurrence of mechanical aberrations than other portions with less electric field. It is also ascertained that the octopole deflector is much superior to the quadrupole regarding misalignment through numerical comparison between both deflectors.

Study on Fragmentation of Ionized Atom-Clusters by Time-of-Flight Mass Spectrometry

Atom-clusters of Pb and PbS produced by nozzle beam expansion have been analyzed by time-of-flight mass spectrometry following the electron impact ionization. The dependence of the ion intensity on ionizing energy has shown occurrence of the fragmentation by electron impact. However, an ion retarding analysis has revealed that both Pb and Pb Scluster tons do not break during their flight in a drift tube. Therefore, the fragmentation has already finished prior to full acceleration of ions.

Cluster cations ejected from liquid metal ion source: alkali metals (Li, Na) and group IV elements (Si, Ge, Sn, Pb)

Cluster abundance of Li+ n (n ≤ 19), Na+ n (n ≤ 25), Si z+ n (n ≤ 8 for z = 1,3 ≤n ≤ 7 for z = 2), Ge z+ n (n≤11 for z=l,3 ≤ n≤9 for z = 2, n = 4 for z = 3), Sn z+ n (n≤7 for z=l, 3 ≤ n ≤ 9 for z = 2, n = 4 for z = 3) and Pb z+ n (n ≤ 6 for z = 1, 5 ≤ n ≤ 7 for z = 2) ejected from a liquid metal ion source has been investigated by mass spectrometry. The abundance spectra of alkali metal clusters showed distinct maxima and steps at n = 3,7,9,13 and 19 for Li, and at n = 3,5,11,13 and 19 for Na. Mass spectra of Si, Ge and Sn clusters were very similar each other, showing intensity drops after n = 4 and 6 (and also n=10 for Ge) for singly charged clusters. The magic numbers observed are discussed in terms of stability of charged clusters.