Generation of multiple charged ions: Photoemission electron impact ionization

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Detection of trace concentrations of helium and argon in gas mixtures by laser-induced breakdown spectroscopy

We report what we believe to be the first demonstration of the detection of trace quantities of helium and argon in binary and ternary gas mixtures with nitrogen by laser-induced breakdown spectroscopy (LIBS). Although significant quenching of helium transitions due to collisional deactivation of excited species was observed, it was found that losses in analytical sensitivity could be minimized by increasing the laser irradiance and decreasing the pressure at which the analyses were performed. In consequence, limits of detection of parts-per-million and tens of parts-per-million and linear dynamic ranges of several orders of magnitude in analyte concentration were obtained. The results of this study suggest that LIBS may have potential applications in the detection of other noble gases at trace concentrations.     

TPEPICO studies near ionization threshold of argon and krypton clusters

Single photon ionization of argon and krypton clusters has been studied in the region between threshold and the ionization potential of the corresponding atom. Synchrotron radiation from the electron storage ring BESSY is used to ionize the clusters; threshold-photo-electron-photoion-coincidence (TPEPICO)-time-of-flight technique is used to detect ions correlated with the emission of zero-kinetic-energy-electrons. The spectra of the clusters in the range ofn=2 to 15 are discussed in view of the extensive fragmentation taking place in these systems. In order to characterize the properties of the clusters a method using scaling laws is applied. The principles and the deduction of Hagena's scaling parameter Γ* are briefly reviewed. Using Γ* an experimentally derived mean cluster size for molecular beams can be assigned. This allows one to clearly demonstrate the systematic variations of the measured spectra due to cluster fragmentation. As a general feature it is observed that, in the range studied, the peak in the measured ionization rate for a cluster ion (fragment) of a given size shifts to higher photon energies as the mean cluster size is increased.     

Electric dipole,polarizability and structure of cesium chloride clusters with one-excess electron

The measurement of the electric dipole of gas phase one-excess electron Cs_nCl_n−1 clusters is reported together with a theoretical ab initio prediction of stable structures, dipole moments and electronic polarizabilities for these species in their ground state. Results are in agreement with NaCl cubic structures.     

Threshold electron spectra of semiconductor cluster ions

A simple threshold electron analyzer was constructed and laser photodetachment threshold electron spectra of Si^– and Si ₄ ^– were measured.     

Fragmentation dynamics of size-selected pyrrole clusters prepared by electron impact ionization: forming a solvated dimer ion core

Structure and dynamics of size-selected charged pyrrole clusters have been studied by means of molecular beam scattering experiments and ab initio calculations. Small neutral Pyn clusters were produced in Py/He mixture expansions, and the scattering experiment with a secondary beam of He-atoms was exploited to select the neutral clusters of different sizes. The complete size-selected fragmentation patterns for the neutral dimer to the tetramer after an electron impact ionization at 70 eV were obtained from the measurements of the angular and velocity distributions at different fragment masses. All the investigated cluster sizes decay mainly to the monomer ions Py+1 (from 60 to 80% of the corresponding neutral size) and to the dimer ion Py+2 (20-30%). The trimer ions Py+3 are generated to less than 10% from the neutral trimer and tetramer. To explain the observed results, we have calculated the structures and energetics of pyrrole clusters up to the trimer for the neutral and the ionic state using DFT and PMP2 methods. The ab initio calculations show that ionized pyrrole clusters are formed with a dimeric core that is solvated by neutral pyrrole molecules. In addition, the ground and ionic state of Py-Ar complexes were calculated at CCSD(T) level with extended basis in relevance to the mixed clusters produced in supersonic expansions of Py seeded in Ar. The calculated dissociation energies of the Py-Ar and (Py-Ar)+ complexes indicate that Ar atoms are able to rapidly evaporate after ionization. The combined analysis of the fragmentation probabilities, and calculations allowed us to estimate the distribution of energy deposited in the clusters after the electron impact, which peaks above 1 eV and has a tail up to 5 eV.     

Theoretical modeling of ionization energies of argon clusters: nuclear delocalization effects

Temperature dependence of vertical ionization energies is modeled for small argon clusters (N ≤ 13) using classical parallel-tempering Monte Carlo methods and extended interaction models based on the diatomics-in-molecules approach. Quantum effects at the zero temperature are also discussed in terms of zero-point nuclear vibrations, either at the harmonic approximation level or at the fully anharmonic level using the diffusion Monte Carlo calculations. Both approaches lead to a considerable improvement of the theoretical predictions of argon clusters ionization energies and represent a realistic way of modeling of ionization energies for weakly bound and floppy complexes in general. A thorough comparison with a recent electron-impact experiment [O. Echt et al., J. Chem. Phys. 123, 084313 (2005)] is presented and a novel interpretation of the experimental data is proposed.     

Size dependent fragmentation of argon clusters in the soft x-ray ionization regime

Photofragmentation of argon clusters of average size ranging from 10 up to 1000 atoms is studied using soft x-ray radiation below the 2p threshold and multicoincidence mass spectroscopy technique. For small clusters (=10), ionization induces fast fragmentation with neutral emission imparting a large amount of energy. While the primary dissociation takes place on a picosecond time scale, the fragments undergo slow degradation in the spectrometer on a microsecond time scale. For larger clusters ( >or=100) we believe that we observe the fragmentation pattern of multiply charged species on a time-scale which lasts a few hundred nanoseconds. The reason for these slower processes is the large number of neutral atoms which act as an efficient cooling bath where the excess energy ("heat") dissipates among all degrees of freedom. Further degradation of the photoionic cluster in spectrometer then takes place on the microsecond time scale, similar to small clusters.     

Electron impact ionization of small silver and copper clusters

Using a quadrupole mass spectrometer, relative cross sections for electron impact ionization of neutral Ag _n and Cu _n clusters withn=1 ... 4 have been measured for electron energies between threshold and 125 eV. From the results, the following ionization energies were obtained: Ag₂: 7.26±0.1 eV, Ag₃: 6.19±0.2 eV, Ag₄: 6.33±0.3 eV, Cu₂: 7.46±0.15 eV, Cu₃: 6.14±1.0 eV, Cu₄: 7.00±0.6 eV. With only two exceptions, these values agree with other data published for Ag₂, Cu₂, Cu₃ and Cu₄.     

Laser-induced electron impact ionization in a reflectron time-of-flight mass spectrometer

Some details of the generation of electrons by impinging a laser beam on a metal surface are described. It is shown that highly efficient electron generation is observed only during the laser pulse. Therefore, this technique delivers intense pulses of electrons. The process is investigated and different ion source set-ups are discussed. In conjunction with a time-of-flight mass spectrometer this technique can be used to produce mass spectra of different samples ranging from simple organic molecules to peptides.     

On the role of dissociative ionization in the formation of argon dimer ions

The formation of Ar ₂ ⁺ ions has been investigated by means of the threshold photoelectron photoion coincidence (TPEPICO) technique. Two pathways for the formation of Ar ₂ ⁺ ions are important. One is a direct path via excitation of Rydberg states of Ar₂ with consecutive autoionization. The other path is dissociative ionization of larger argon clusters, in this case argon trimers. These two pathways lead to Ar ₂ ⁺ ions with different internal energy. The pathways are easily distinguished in the TPEPICO-TOF spectra by the kinetic energy released (KER) in the dissociative ionization. The KER for the reaction Ar ₃ ⁺ → Ar ₂ ⁺ + Ar was measured as a function of the photon energy and compared to the KER expected from statistical theory. The agreement is satisfying and confirms that Ar ₃ ⁺ ions do indeed dissociate at the thermochemical threshold. At higher photon energy the excited²Π(3/2)g state of Ar ₃ ⁺ is also detected from a second component in the KER. By applying a kinetic energy discrimination it is possible to measure cluster ion spectra in the presence of larger clusters but essentially without interference from the latter.     

Novel cluster ions from magnesium and zinc acetates in filament heated in-beam electron impact mass spectrometry

Mass spectral peaks of cluster ions believed to be due to penta-μ-acetato-μ₄-oxo-tetramagnesium and tri-μ-acetato-μ₃-oxo-trimagnesium, or the corresponding zinc compounds, were detected when magnesium acetate or zinc acetate was heated on a tungsten filament and impacted at the same time by an electron beam. Structures of these non-transition metal complexes are analogues of the so-called ‘electron-sponge’ complexes, [Ru₃O(OAc)₆L₃]. Previously reported new cluster ions from lithium acetate are formulated by similar structures.     

Composition of water cluster ions under ionization conditions at atmospheric pressure

Russian Chemical Bulletin -     

Electron impact ionization efficiency curves of van der Waals clusters

The study of van der Waals clusters is an area of growing interest and is being widely studied for a number of reasons. The measurement of the ionization efficiency (IE) curves have yielded a wealth of information by enabling ionization and appearance energies of ions to be determined which are essential for the calculation of thermochemical data. In the case of van der Waals clusters, the measurement ofIE curves enables one to determine the qualitative trends in the ionization potentials as a function of cluster size. In additionIE curves have also offered valuable insight into ionization related processes occurring in clusters. This paper will cover some of the more recent studies of Penning ionization, exciton induced decay and Coulomb explosion in van der Waals clusters through the use of electron impactIE curves.     

Colloidal silver nanoparticles stabilized by a water-soluble triosmium cluster

Silver nanoparticles stabilized by the water soluble triosmium cluster Os₃(μ-H)(CO)₁₀S(CH₂)₁₀COO]Na were prepared by both photochemical and chemical reduction of silver nitrate. The silver nanoparticles were characterized by UV-Vis spectroscopy and high resolution TEM. The particles obtained by chemical reduction showed remarkable stability.     

Hydrocarbon clusters from a foil diffusion source

Rydberg states and clusters of Rydberg states have been reported in several cases from high temperature sources, so-called diffusion sources. The present study uses the same technique as the one used for the study of excited Cs clusters (Åmanet al., 1990), and is aimed at highly excited Rydberg cluster formation from hydrogen containing molecules. Ionized clusters from a diffusion source with the graphite foil emitter at 1400 K are studied by time-of-flight mass spectrometry. The excited clusters are shown to be ionized by field ionization. The best results are found using ethylene in the source, and the flux from the source probably contains both hydrogen and hydrocarbon species. Typical clusters have a mass of 10,000–200,000 a.u., assuming singly charged clusters. The formation and stabilization (cooling) of such large clusters under the present conditions is only possible since excited states can form a condensed phase, of so called Rydberg matter (Manykinet al., 1981, 1982, Petterssonet al., 1992, Svenssonet al., 1991, 1992). The importance of excited hydrogen containing clusters for the chemistry and physics of interstellar space is pointed out.