Electron detachment, thermal

In order to measure the magnitude of the chemical interactions between various ions and buffer gases, approaches that are based on the measurements of either equilibrium or rate constants for ionic processes can be envisioned. An example of a kinetic method is described in the following. The unimolecular kinetic process known as thermal electron detachment (TED) for negative ions (NT -> M + e), should be particularly sensitive to a chemical effect of the buffer gas. This is because the rate of TED will be given by = constant x where the electron... 

The Kinetics oe Thermal Electron Capture and Thermal Electron Detachment... 

Sahlstrom et al. [60] showed that the thermal detachment of an electron from an anion is observed readily in an ion mobility spectrometer. At an appropriate temperature, anions formed in the source decompose by thermal electron detachment in the drift region. The electrons move rapidly in the electrostatic field to the detector plate and their intensity at arrival time is a measure of the number of anions disappearing at that time. The resulting spectrum, of the form of Figure 13.2d, shows an elevated baseline that has a maximum at zero time, that is, for electron detachment at the shutter where the anion concentration is highest, and terminates at the peak for survivor anions. Examples of the mobility spectra obtained for thermal electron detachment from the azulene anion at different temperatures are shown in Figure 13.9. The Cl" peaks in the spectra are due to background ions formed in the source and do not interfere with the analysis. The exponential decay of the elevated baseline is described by... 

FIGURE 13.9 Thermal electron detachment spectra for the azulene anion (Az ) at different temperatures the Cl ions are background ions formed in the source. (A) 375K, (B) 387 K, (C) 427 K. Drift field 144 V cm , 7 = 740 Torr. (Reproduced from Sahlstrom, K.E. Knighton, W.B. Grimsrud, E.P., Int. J. Mass Spectrom. 1998,179/180, 117-127. With permission from Elsevier.)... 

Arrhenius Parameters for the Thermal Electron Detachment from the Azulene Anion... 

Sahlstrom, K.E. Knighton, W.B. Grimsrud, E.P. Thermal electron detachment of the molecular anion of azulene at elevated pressures by ion mobility spectrometry. Int. J. Mass Spectrom. 1998,179/180, 117-127. 

In a nonattaching gas electron, thermalization occurs via vibrational, rotational, and elastic collisions. In attaching media, competitive scavenging occurs, sometimes accompanied by attachment-detachment equilibrium. In the gas phase, thermalization time is more significant than thermalization distance because of relatively large travel distances, thermalized electrons can be assumed to be homogeneously distributed. The experiments we review can be classified into four categories (1) microwave methods, (2) use of probes, (3) transient conductivity, and (4) recombination luminescence. Further microwave methods can be subdivided into four types (1) cross modulation, (2) resonance frequency shift, (3) absorption, and (4) cavity technique for collision frequency. 

Warman JM, Fessenden RW, Bakale G. (1972) Dissociative attachment of thermal electrons to NjO and subsequent electron detachment. J Chem Phys 57 2702-2711. 

In this chapter the experimental ECD and NIMS procedures for studying the reactions of thermal electrons with molecules and negative ions are described. Gas phase electron affinities and rate constants for thermal electron attachment, electron detachment, anion dissociation, and bond dissociation energies are obtained from ECD and NIMS data. Techniques to test the validity of specific equipment and to identify problems are included. Examples of the data reduction procedure and a method to include other estimates of quantities and their uncertainties in a nonlinear least-squares analysis will be given. The nonlinear least-squares procedure for a simple two-parameter two-variable case is presented in the appendix. 

The complementary techniques for determining rate constants for thermal electron attachment, detachment, and dissociation are the flowing afterglow, the microwave technique, the ion cyclotron resonance procedures, the swarm and beam procedures, the shock tube techniques, the detailed balancing procedures, the measurement of ion formation and decay, and the high-pressure mass spectrometer procedures. In all cases the measurement of an ion or electron concentration is made as a function of time so that kinetic information is obtained. In the determination of lifetimes for ions, a limiting value of the ion decay rate or k is obtained. 

Thermal electron attachment to SF6 has been studied in the ECD and NIMS at atmospheric pressure and in a chemical ionization mass spectrometer at lower pressures. From these data rate constants for thermal electron attachment and detachment to SF6, and the Qan values and electron affinities of SF6 and SF5 have been determined. The Morse potential energy curves for multiple negative-ion states were calculated using electron impact data and electron affinities. 

Grimsrud, E.P. Chowdhury, S. Kebarle, P. Thermal energy electron detachment rate constants. The electron detachment from azulene and the electron affinity of azulene. J. Chem. Phys. 1985, 83(S), 3983-3989. 

Solutions of biphenyl in tetramethylsilane represent an example where the electron mobility is modulated by the formation of temporary negative ions due to electron attachment to the solute molecules. The electron mobility in tetramethylsilane at room temperature is about 100 cm V s The electrons are considered to be quasifree and to move in a conduction band. If biphenyl, Ph2, is dissolved electron attachment and subsequent thermally activated detachment occurs ... 

---