Positive Ion Desorption Research Articles

Surface enhanced photodissociation of physisorbed molecules.

We have investigated the photon stimulated desorption of ions from physisorbed layers of CO and ${\mathrm{O}}_{2}$ on graphite using VUV synchrotron radiation in the range of 13 to 35 eV. In both systems we find a significant enhancement in the monolayer regime of the desorption of ${\mathrm{O}}^{\ensuremath{-}}$ ions via resonant states which correspond to direct molecular photoabsorption. This enhancement, which is accompanied by suppression of positive ion desorption, is attributed to surface electron attachment following molecular photoexcitation.

Electron-stimulated desorption of positive and negative oxygen ions from YBa2Cu3O7 surfaces

Electron-stimulated desorption of positive and negative oxygen ions from YBa2Cu3O7 surfaces has been studied as a function of electron kinetic energy in the 70–600 eV range. It has been found that the threshold electron kinetic energy needed to induce positive oxygen ion desorption occurs at 260±5 eV, whereas that associated with negative oxygen ion desorption was found to be 100±5 eV. These electron kinetic energies correspond to Cu, Y, and Ba core-level binding energies. These results suggest that negative and positive oxygen ion desorption may be initiated via a primary core level ionization.

Electron stimulated desorption of F + ions from MBE grown [formula omitted

We have studied the detailed dynamics of electron stimulated desorption (ESD) of F + ions by low energy electrons from MBE grown Ca 0.6 Sr 0.4 F 2 GaAs(111) . A threshold for F + emission at an incident electron energy of 24 eV was observed, corresponding to the ejection of an electron from the Sr 4p core level. The kinetic energy distribution of the desorbed ions was found to peak at ~ 1 eV and to be independent of the incident electron energy. Furthermore, the angular profile of desorbed F + ions exhibited a maximum normal to the surface with a full width at half maximum of 30°. These results are consistent with the Knotek-Feibelman mechanism for positive ion desorption.

Characterization of fullerenes by laser-based mass spectrometry

Mass characterization of C 60 related high-mass carbon clusters, called fullerenes, and their compounds is of increasing interest as the unique properties of these materials become known. We are using laser desorption time-of-flight mass spectrometers to investigate direct emission of positive and negative ions and also neutral desorption. We show results on the laser desorption process, namely, desorption thresholds for positive ion, negative ion and neutral desorption and velocity distributions of desorbed neutral particles. Furthermore, post-ionization with the wavelengths 118, 212.8, 266, 355, and 532 nm is investigated.

Positive and negative ion desorption induced by photon impact (10–100 eV) on condensed gases

Singly and doubly charged ion desorption from condensed argon is studied by photon excitation in the 10–100 eV energy range. The ion yield curve and threshold energy measurements indicate that multiple excitation or ionization processes, such as formation of satellite states and double or triple photoionization, are the dominant primary electronic processes. Negative ion (O −) desorption from condensed oxygen as produced by photon impact in the 17–38 eV range is also reported. The observation of resonant processes suggests that molecular Rydberg states of adsorbed O 2 molecules may contribute to the desorption mechanism.

Massenspektroskopie von Thiogibberelllnen

Mass Spectroscopy of Thiogibberellins.The results of the positive ion (electron impact, desorption electron impact, low voltage electron impact, positive ion desorption chemical ionization, — EI, DEI, LEI, PIDCI), as well as negative ion (electron attachment — EA) mass spectra of 4 different types of thio gibberellins 6–9 are compared, and the fragmentation pathways are discussed.

Auger-induced valence processes in alkali halides following alkali core-hole creation.

Ab initio calculated values are presented for the interatomic Auger and radiative decay rates for initial (2p${)}^{2}$ and 2p hole states of the Na ion in NaF. A comparison of the rates shows that these hole states decay overwhelmingly by the nonradiative Auger process. The knowledge of the rates makes possible a clarification of the mechanism of photon-stimulated positive-ion desorption presented by Parks et al. [Phys. Rev. B 28, 4793 (1983)]. Following the creation of a Na(1s) hole in NaF, the ensuing Auger cascade results in the creation of four to six valence holes, or one ${\mathrm{F}}^{+}$ ion and two to four valence holes, on the neighboring negative fluorine ions within an elapsed time of a few times ${10}^{\mathrm{\ensuremath{-}}14}$ s. The majority of the valence holes are produced as hot holes, and are likely to diffuse several tens to hundreds of angstroms in times of the order of picoseconds before self-trapping to form ${V}_{K}$ centers. The fast diffusion of these holes greatly reduces their chance of participating in ion desorption processes. The possibility that a fraction of the holes self-trap immediately and assist in the desorption of positive ions is considered. It is found that positive-ion desorption can occur if enough holes self-trap immediately. However, the required configurations are so special that they may occur with too small a probability to account for the observed positive-ion current. The fast hole diffusion also makes it unlikely that the ${V}_{K}$ centers from the cascade will exhibit the clustering effects which would have been expected had the holes self-trapped in place. An increase in the yield of electron-hole pairs of about 6% is predicted to occur at the Na K edge.

Molecular-dynamics simulation of positive-ion and neutral halogen desorption following Na K-shell Auger cascades in the NaF crystal.

A Na K-shell Auger cascade in NaF can produce either four to six neighboring valence holes, or two to four neighboring valence holes and a ${\mathrm{F}}^{+}$ ion. If the holes remain localized as neutral atoms where they are created, they will assist the ${\mathrm{F}}^{+}$ ion to desorb, or assist in the desorption of ${\mathrm{Na}}^{+}$ ions. The neutral atoms themselves may also desorb. We use molecular-dynamics calculations to investigate whether the ${\mathrm{F}}^{+}$ ion and neutral F atoms produced by the Na K-shell Auger cascade in NaF can in fact lead to the desorption of positive ions and/or neutral F atoms. Simple fits to the Gordon-Kim potentials are used for the ionic interactions. Ab initio two-body potentials are developed to describe the interaction between a neutral F atom just outside of the crystal surface and a Na or F ion in the surface plane. The potentials exhibit a repulsive inner wall similar to that found for the diatomic molecules, and a shallow well. The integration time required for a satisfactory simulation of positive-ion desorption by the Knotek-Feibelman mechanism is shown to be 1--2 ps. Using 2-ps simulations, it is shown that only when the ${\mathrm{F}}^{+}$ ion is accompanied by four neighboring F atoms from the same Auger cascade does positive-ion desorption occur from the perfect (100) surface. Analogous results are obtained in simulations designed to relate to the imperfect (100) surface. It is concluded that the neutral atoms assist positive ions to desorb only in configurations which appear to be too rare to be of experimental significance. It is shown that crystal temperature, near room temperature, is not a factor in determining whether positive-ion desorption occurs by the Knotek-Feibelman mechanism. This work extends that of Walkup and Avouris on NaF to the case where the Auger-produced ${\mathrm{F}}^{+}$ ion is accompanied by neutral F atoms.

Systematic analysis of desorption chemical ionization (DCI) mass spectra of nucleic acids--7.

The positive ion and negative ion pyrolysis mass spectra of the herring sperm DNA have been studied using desorption chemical ionization. The positive ion desorption chemical ionization spectra have been produced with CH4, i-C4H10, NH3, HCl and Cl2; the negative ones with N2O/CH4, N2O/i-C4H10, Cl2, CCl4, HCl and via electron capture. These spectra have been compared with the electron impact ionization spectra. We have observed an important increase of sensitivity when negative ionization has replaced the positive ionization mode. The series of diagnostic ions resulting from direct chemical ionization belong to the family of base + reagent ion X [BH + X] and base + X - HX ion [B]. Their abundance has increased considerably compared to the electron impact spectra. The application of these new diagnostic ions in nucleic acid studies is interesting especially for the much higher abundance of the usually weak dG fragment ion obtained in the negative ionization mode. The dG-base segment of the DNA is the most nucleophilic centre of the whole nucleic acid and is implicated in numerous important biochemical reactions involving, for example, proteins.

Positive and negative ion desorption chemical ionization of some penicillins

The ammonia positive ion desorption chemical ionization and methane negative ion desorption chemical ionization mass spectra of five commercially available penicillins have been studied. In every case ions characteristic of the molecular weight were obtained together with diagnostically significant fragment ions. As well as the known fragmentation pathways observed under electron impact conditions a significantly different fragmentation pathway was observed using negative ion desorption ionization. The reproducibility of the spectra obtained contrasts markedly with previous reports on the mass spectra of penicillin free acids.

Positive Ion Contamination on Metal Surfaces

Alkali metal, positive ion, contamination on metal surfaces contained in borosilicate glass envelopes has been studied for various gas atmospheres up to 100 torr. Positive ion desorption phenomena have been observed in ambient gases which are essentially similar to those obtained in the in-vacuo and sputtering experiments of Bills and Evett. Mass spectrometer examination confirmed the results of Bills and Evett, Riddock and Leck and of Bradley that the dominant ion was K+ with lesser amounts of Na+. Particularly heavy contamination by K+ was observed in the presence of O2. In the extreme case, direct vapor phase condensation of alkali metal contamination may be observed. The results differ from those of Riddock and Leck in that the amount of alkali metal ions desorbed from a W surface after extended gas discharge saturated at 0.03 monolayer coverage.