Negative Cluster Ions Research Articles

Formation of small negative and positive cluster ions of gold, silver, and copper by direct laser vaporization

Direct laser vaporization techniques have been employed to produce small cluster cations and anions of gold, silver, and copper in a Fourier transform ion cylotron resonance mass spectrometer using pure metal samples. Aupn (p=+1 and −1, n=1–6), Ag+n (n=1–3), Ag−, Cu+n (n=1–3), and Cu−n (n=1–2) were observed, using two different experimental configurations. In one configuration ions were produced by Nd:YAG laser irradiation directed collinear with the magnetic field in the FTICR mass spectrometer while in the second laser light directed perpendicularly to the magnetic field was used. Relative kinetic energies of the aggregates, the relative intensities of positive vs negative ions, and the nature of the neutrals in the plasma formed by direct laser vaporization are discussed.

Pulsed photoelectron spectroscopy of negative cluster ions: Isolation of three distinguishable forms of N2O−2

Three different ionic species with stoichiometry N2O−2 are generated by varying the neutral precursors in an electron beam ionized free jet expansion. In each case, the ion is isolated by mass spectrometry and then probed using pulsed photoelectron spectroscopy (PES) at 532 and 355 nm (2.330 and 3.495 eV, respectively). The neutral starting materials used in the three preparations are (I) O2 seeded 5% in N2, (II) pure N2O, and (III) NO seeded 10% in Ar. Based on their PES and photofragmentation properties, the three species appear to be best described as (I) O−2 ⋅N2, (II) either O−⋅N2O or more likely a chemically bound species, and (III) NO−⋅NO. It is likely that two of these species are trapped intermediates in the O−+N2O→NO−+NO reaction, suggesting a double minimum potential energy surface. The formation mechanisms of these ions in our source are discussed in the context of previous preparation schemes.

Cluster ions in the laser mass spectra of boron carbide

Cluster ions formed in the laser plasma of a boron carvide target are investigated by means of mass spectroscopy. Positive B n C + m cluster ions could be detected up to m + n = 17 atoms. The negative cluster ions are formed at larger abundances than positive cluster ions. Analogous to the laser mass spectrographic investigations of the boron nitride/graphite system, the odd- and even-numbered cluster ions BC +,− m showed an alternating abundance distribution.

Photodetachment spectroscopy of cluster anions. Photoelectron spectroscopy of H–(NH3)1, H–(NH3)2and the tetrahedral isomer of NH–4

The dominant peaks in the photoelectron spectra of the gas-phase, negative cluster ions H–(NH3)1 and H–(NH3)2 provide evidence for describing them as ion-molecule complexes comprised of intact hydride ions which are solvated by ammonia. Vertical detachment energies ad approximate ion–single-solvent dissociation energies are obtained. Other spectral features reveal the complexation-induced distrotion of the ammonia solvent(s) by their hydride sub-ions. In the photoelectron spectra of H–(NH3)1 and additional peak appears which is small and unusually narrow and which does not shift upon deuteration. Evidence is presented for interpreting this peak as arising due to the photodetachment of a tetrahedral isomer of NH–4. This previously unknown ammonium anion is not a cluster species, and it is described as a NH+4 ion core with two Rydberg-like electrons.

Negative-ion formation from CCl 4 clusters in collision with highly excited Rydberg atoms and slow electrons

Negative ions were produced by impact of either highly excited krypton Rydberg atoms or slow electrons, ≈ 2.5 eV, on the neutral clusters of CCl 4 formed in supersonic nozzle expansion. Negative cluster ions, (CCl 4) n Cl − (0≤ n≤9) and (CCl 4) n − (1 ≤ n≤ 10), were detected by mass spectrometry; (CCl 4) n Cl 2 − and (CCl 4) n CCl 3 − (0≤ n≤9) were also observed by electron impact. An estimation of the excess energy generated by the formation of (CCl 4) n Cl − by Rydberg atom impact indicates that the neutral precursor of (CCl 4) n Cl − is (CCl 4) n+1 . In contrast, evaporation of several CCl 4 units from a precursor cluster ion is expected in the formation of (CCl 4) n −.

Studies in cluster ion formation. Part III. Collision-induced dissociation of the cluster ions of tetraalkylammonium halide salts

The collision-induced fragmentation of the cluster ions of tetraalkylammonium halide salts observed in fast atom bombardment spectrometry is consistent with the elimination of neutral molecules and the degree of fragmentation depends on the salt concentration in the matrix. The collision-induced dissociation (CID) spectra recorded from different concentrations of salt solutions have been compared and the effect of concentration on the stabilities of the cluster ions is discussed. In addition, the influence of the cation and anion size on the CID of both negative and positive cluster ions has been studied and the occurrence of anomalous ion intensity regions explained.

Magnetic time-of-flight photoelectron spectrometer for mass-selected negative cluster ions

Design considerations and initial results are presented for a new type of time-of-flight photoelectron spectrometer which is particularly suited to the study of cold metal and semiconductor cluster anions prepared in a supersonic molecular beam. The desired cluster is extracted from the molecular beam, mass-selected after an initial time-of-flight, and decelerated as it enters the photoelectron spectrometer. Photoelectrons ejected from the cluster by an ArF excimer laser are collected with >98% efficiency in an intense pulsed magnetic field of carefully controlled divergence. This divergent field parallelizes the photoelectron trajectories and maps smoothly onto a low, uniform magnetic field which guides the electrons along a 234-cm flight tube leading to a microchannel-plate detector. The strong magnetic fields and simple, open design provide excellent rejection of stray photoelectrons in a clean, ultrahigh-vacuum environment. The UPS spectrum of Si20− is given as an example.

Photoelectron spectroscopy of the negative cluster ions NO−(N2O)n=1,2

We have recorded the photoelectron (photodetachment) spectra of the gas-phase negative cluster ions NO−(N2 O)1 and NO−(N2 O)2 using 2.540 eV photons. Both spectra exhibit structured photoelectron spectral patterns which strongly resemble that of free NO−, but which are shifted to successively lower electron kinetic energies with their individual peaks broadened. Each of these spectra is interpreted in terms of a largely intact NO−subion which is solvated and stabilized by nitrous oxide. For both NO−(N2 O)1 and NO−(N2 O)2, the ion–solvent dissociation energies for the loss of single N2 O solvent molecules were determined to be ∼0.2 eV. Electron affinities were also determined and found to increase with cluster size. The localization of the cluster ion’s excess negative charge onto its nitric oxide rather than its nitrous oxide subunit is discussed in terms of kinetic factors and a possible barrier between the two forms of the solvated ion.

Ultraviolet photoelectron spectroscopy of semiconductor clusters: Silicon and germanium

Ultraviolet photoelectron spectra (UPS) are reported here for semiconductor clusters prepared in a supersonic beam. Using a new magnetically focused time-of-flight photoelectron spectrometer, UPS spectra were obtained for mass-selected negative cluster ions of silicon and germanium in the 3-12-atom size range. An ArF excimer laser (6.4 eV) was used for photodetachment, enabling the first 3–4 eV of the valence band structure of the clusters to be probed. With few exceptions, the UPS data for corresponding clusters of the two semiconductors were remarkably similar. The spectra suggest that clusters 4, 6, 7, and 10 of silicon and 4, 6 and 7 of germanium are closed-shell species with band gaps of 1 to 1.5 eV.

Reactive etching of positive and negative silicon cluster ions by nitrogen dioxide

Positively and negatively charged silicon cluster ions, Si+1–8 and Si−1–6, react exothermically with NO2. The predominant reaction is loss of a single silicon atom in the form of SiO. This reaction repeats sequentially such that the clusters are ‘‘etched’’ down to monatomic silicon ions. Charge transfer to form NO−2 also occurs for Si−1–4. Previous results have shown that all Si−1–6 exhibit electron transfer to WF6. Together, these observations bracket the electron affinities of the neutral silicon clusters: EA (Si2–4)≤2.6 eV and 2.6 eV≤EA (Si5–6)≤3.7 eV. The reaction rates for the silicon cluster cations decrease slowly from 7.3±0.8×10−10 to 0.4±0.2×10−10 cm3 molecule−1 s−1 with increasing cluster size for Si+1–8. Silicon cluster anion reaction rates show a slight decrease from 15±1×10−10 to 5.0±0.1×10−10 cm3 molecule−1 s−1 with increasing cluster size for Si−1–6. The ionic silicon cluster reactivity is explained by a radical–radical coupling mechanism involving the unpaired electron on NO2 and an unpaired electron on a silicon atom at a charged or trivalent center in the clusters. Involvement of a radical electron on the silicon cluster contrasts with ionic silicon cluster reactivity with CH3SiH3 which requires divalent, silylene reactive centers on the clusters. Previously identified ‘‘magic number’’ clusters, Si+4 and Si+6, do not exhibit atypical reaction rates or products in spite of a theoretically derived 1–2 eV lower reaction exothermicity relative to the other cluster sizes.

Photodetachment and photofragmentation studies of semiconductor cluster anions

Silicon, germanium, and gallium arsenide negative cluster ions are produced by laser vaporization followed by free supersonic expansion. Electron affinities (EA) of the corresponding neutral clusters are roughly bracketed by measuring the fluence dependence for photodetachment from anions at discrete probe laser wavelengths (above the photodetachment threshold the dependence on fluence is linear, below it is quadratic). An even/odd alternation is found in the negative ion distribution with gallium arsenide clusters with an odd number of atoms having higher EA’s than their even neighbors. This suggests that the surfaces of the even clusters are extensively restructured in a way which eliminates dangling chemical bonds. For GaxAsy with x+y constant, EA increases with increasing ratio of y to x. The EA of silicon increases smoothly with cluster size extrapolating towards the EA of bulk silicon. Photofragmentation studies show that, like the corresponding positive ions, silicon and germanium negative ions with 11 to 23 atoms fission into mainly 5 to 10 atom negative ions. Si−10 and Ge−10 are the favorite daughters suggesting the existence of a special structure for ten atoms.

Mass distributions of negative cluster ions of copper, silver, and gold

Mass distributions of negative cluster ions of copper (Cu) n −, silver (Ag) n −, and gold (Au) n − obtained by the bombardment of metal sheets with Xe ions were investigated up to cluster size n = 250 and were compared with those of the positive cluster ions. The mass spectra were obtained under the same conditions as previously employed to study positive cluster ions and the behaviour of the mass distributions of the negative cluster ions were very similar to those of the positive cluster ions. The ion intensity of the clusters decreased with increasing cluster size, n, and two types of anomaly were observed. One was a regular variation of the ion intensity between odd and even values of n, where the intensity of odd- n clusters was greater than that of even- n, which is explained by pairing of the electrons. The other anomaly was a discontinuous variation of ion intensity at specific numbers n = 1, 7, 17, 19, 33, 39, 57, 91, 137, and 197. In the mass spectra of positive cluster ions, these were 3, 9, 19, 21, 35, 41, 59, 93, 139, and 199. It is confirmed from this intensity behaviour that a one-electron shell model can be applied to explain the stability of those metal clusters in which s valence electrons are bound in a spherically symmetric potential well.

Studies in cluster ion formation. Part I. Fast atom bombardment mass spectrometry of tetraalkylammonium halides: Factors influencing cluster ion size and stability

AbstractPositive and negative ion fast atom bombardment mass spectra of tetraalkylammonium halide salts (NR4X, where X = Cl, Br, I and NR4 = NMe4, NEt4) have been studied and intense cluster ion formation has been observed. The cluster ion intensity distributions were found to show enhancements at certain cluster numbers (n). The negative cluster ions of NMe4X salts showed anomalous ion intensity regions, which differed from both the positive cluster ions of all NR4X salts and also the corresponding negative clusters of NEt4X salts. The influence of anion and cation size on cluster ion formation and abundances has been studied and it has been established that smaller anion and cation size favours the formation of larger cluster ions. The possible structures of the cluster ions exhibiting relative increased stabilities are discussed.

Fourier transform ion cyclotron resonance mass spectrometry of trapped carbon cluster ions

Positive and negative carbon cluster ions containing up to 27 carbon atoms have been produced by laser vaporization and detected via FT/ICR mass spectrometry. A periodic pattern of “magic numbers” is found in the positive clusters. The application of this technique to the study of ion-molecule reactions of clusters is demonstrated.

ChemInform Abstract: Formation of Negative Cluster Ions in Collision of SF6 Clusters with Krypton Rydberg Atoms.

AbstractThe formation of (SF6)n‐ (1 ≦ n ≦ 11) by impact of either high‐Rydberg rare gas atoms (RAI) or low‐energy electrons (≅ 2.5 eV) (EI) has been studied using mass spectroscopy.

Photodetachment studies of metal clusters: Electron affinity measurements for Cux

Laser photodetachment studies have been performed on silver and copper cluster negative ion beams extracted from a supersonic metal cluster source. This source involves the use of laser vaporization within a pulsed nozzle to prepare the neutral cluster jet, followed by ArF excimer irradiation of the nozzle orifice as the copper clusters emerge into the expansion. Photoelectrons ejected from the aluminum nozzle by the excimer radiation are entrained in the supersonic flow and attach to the neutral copper clusters, producing negative ions which stabilize and extensively cool in the subsequent supersonic expansion. Laser photolysis of mass-selected negative copper cluster ions extracted from this source reveals efficient photodetachment to produce a free electron in the absence of measurable fragmentation. The laser fluence dependence of the photodetachment process at different photon energy permits a rough experimental determination of the electron affinities of copper metal clusters as a function of cluster size for the first time.

Negative carbon cluster ion beams: New evidence for the special nature of C 60

Cold carbon cluster negative ions are formed by supersonic expansion of a plasma created at the nozzle of a supersonic cluster beam source by an excimer laser pulse. The observed distribution of mass peaks for the C − n ions for n > 40 demon- strates that the evidence previously given for the special stability of neutral C 60 and the existence of spheroidal carbon shells cannot be an artifact of the ionization conditions.

A comparison of positive and negative ion static SIMS spectra of polymer surfaces

AbstractNegative ion static SIMS spectra are presented for several polymers whose positive ion spectra have been well characterized previously. The negative cluster ions present in the spectra of poly‐methyl methacrylate, poly‐hydroxy ethylmethacrylate and poly‐ethylene terephthalate provide detailed information on polymer side chain and back‐bone structure complementing and clarifying the information previously only obtainable from positive ion emission. The detection and optimization of structurally informative negative cluster ions is discussed in relation to experimental parameters and the role of surface potential.

Formation of negative cluster ions in collision of sulfur hexafluoride clusters with krypton Rydberg atoms

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTFormation of negative cluster ions in collision of sulfur hexafluoride clusters with krypton Rydberg atomsKoichiro Mitsuke, Tamotsu Kondow, and Kozo KuchitsuCite this: J. Phys. Chem. 1986, 90, 8, 1552–1556Publication Date (Print):April 1, 1986Publication History Published online1 May 2002Published inissue 1 April 1986https://doi.org/10.1021/j100399a019RIGHTS & PERMISSIONSArticle Views60Altmetric-Citations25LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Get e-Alerts

Detection of mass 16241 ions by Fourier-transform mass spectrometry

An independent investigation is reported of the mass range of FTMS (Fourier-Transform mass spectroscopy) utilizing Cs DI to measure positive and negative cesium iodide cluster ions of masses as high as 16241 daltons, the present low frequency limit of the detection system. 34 references, 1 figure.