International Journal of Mass Spectrometry and Ion Physics最新文献

Penning ionization of organic molecules has been used as a convenient, clean ion source for flowing afterglow studies of organic positive ion/molecule reactions. Ionization reactions of 36 compounds were investigated, including alkanes, alkenes, ketones, ethers, chlorides and silanes. In many cases the parent ion is produced solely, but a few compounds fragment to give a daughter ion as the only ion. Some of these ions were then studied in reactions with their parent neutral molecules and in hetero-ion/molecule reactions. The combination of mild ionization and rapid thermalization in helium buffer gas circumvents complications from excited reactants. The reactions between ethylene radical ion and ethylene were examined in detail and the results are compared with both low- and high-pressure results in the literature. Rate constants and branching ratios were measured and mechanistic implications are discussed. It was determined that the loss of metastable argon by diffusion and destruction at the walls is considerably slower than the comparable loss of ions and this must be taken into account in kinetic measurements. Other reactions, including proton transfer equilibria, were also studied briefly.

Hydrogen duster ions of 800 keV were passed through an H₂O gas target. The production and destruction of charged fragments was followed as a function of the target thickness. The comparison of the experimental results with those of a model calculation shows that the dominant process for fragment production is most probably the evaporation of H₂ molecules. Indications of the luminescence of clusters after electron impact are found from a background signal.

An improved design for a tandem mass spectrometer-ion-cyclotron-resonance spectrometer (TICR) is presented. The TICR consists of an ion source, a 180° mass filter, and a separate reaction/detection region, consisting of an ICR cell. Since the ion formation and reaction regions are separate, determinations of kinetics and product distributions are greatly simplified.

The design presented includes two major changes from that of Smith and Futrell [4]: (1) a velocity filter at the entrance of the ICR cell, to remove translationally hot ions; and (2) the use of a second ICR cell as an ion source. This allows ICR double-resonance techniques to be used in the ion formation region.

Data on performance are presented, as well as examples of several novel experiments which are possible. The question of reactant-ion kinetic energy is discussed, and data are presented which indicate that the ions entering the ICR cell have kinetic energies of ∼ 0.5 eV (lab). Examples are presented illustrating the use of ICR double resonance to determine the effects of kinetic energy on reaction rate constants and product distributions.

The distribution of products in negative chemical-ionization spectra of alcohols are shown to remain unaffected (<1%) by pyrolytic reactions. On the other hand, it is shown that slow ion/molecule reactions can be obscured by fast processes implying impurities formed by pyrolysis. The importance of pumping speed in ion cyclotron resonance (ICR) spectrometers is also discussed.

Metastable peak characteristics have been applied to study the fragmentation behaviour of the molecular ions and the high intensity fragment ions of the ortho- and meta-xylene isomers after electron impact. Metastable peaks for the fragmentation of ions generated by field ionization have also been studied.

The kinetic energy released (T) is calculated for the decomposition of molecular and fragment ions of the two isomers in the first and second field-free regions. It is also found that the T-values are life-time independent.

This paper describes the experimental technique of an in situ mass spectrometric method proved to be suitable for studying the processes taking place during heat-treatments of contacts between metals and compound semiconductors. This method was combined with an in situ electrical measurement of the voltage drop across the sample during the heat-treatment.

In the case of gallium arsenide, the contacts were exposed to an arsenic molecular beam for replacing the arsenic evaporated out of the contact system due to high-temperature annealing.

The three-body ion/molecule association reactions N₂⁺ + N₂ + He →$\text{k}{}_1$ N₄⁺ + He and O₂⁺ + O₂ + He →$\text{k}{}_1$ O₄⁺ + He have been studied in a variable-temperature selected-ion flow tube (SIFT) and a variable-temperature ion drift tube (DT). The rate coefficients k₁ and k₂ were determined using both apparatuses over uniquely wide temperature ranges, i.e., 45–520 K for k₁ and 48–200 K for k₂. Accurate measurements of k₂ at higher temperatures could not be made because of thermal break-up of the weakly bonded O₄⁺ ion. The magnitudes and temperature dependences of k₁ and k₂ determined in the SIFT and DT experiments are in very good agreement, and can be represented by the analytical expressions k₁ = 1.8 × 10⁻²⁹(300/T)^2.3±0.2 cm⁶ s⁻¹k₂ = 5.5 × 10⁻³¹(300/T)^2.7±0.3 cm⁶ s⁻¹ However, noticeable deviations from these power-law dependences occur at the lowest temperatures. The results are compared with those obtained from previous studies of these reactions and for the analogous association reactions involving N₂ and O₂ third bodies.

Field desorption mass spectra have been obtained for several low-molecular weight hydrocarbon polymers—polybutadiene, polyisoprene, polyethylene and polystyrene. The principal oligomer ions corresponded to [M]^+· in every case, i.e., there was little tendency to form ions via proton attachment. Molecular weight averages ($\text{M}$_n and $\text{M}$_w) derived from FDMS data agree well with values obtained by conventional techniques.

The detection of 5 metastable decay channels in the laser-induced multi-photon ionization and fragmentation pathway of C₆H₆ is reported. The mass of the precursor ions and the product ions (C₄H₄⁺, C₄H₃⁺, C₄H₂⁺, C₃H₃⁺) is determined in a reflectron time-of-flight (TOF) mass spectrometer, the latter with the reflecting field acting as an energy analyzer. The metastable decay channels are explained in terms of a switching of photon absorption from the neutral molecule to the parent ions and to the fragment ions (ladder switching) as well as by the heat of formations of the ions under consideration.

An ion source is described in which molecules having an ionization potential less than 11.8 eV are selectively photoionized. It is shown that this method considerably improves the air contaminant detection limit of a standard quadrupole mass spectrometer and has great potential in the analysis of complex mixtures of trace gases.