Trochoidal Electron Monochromator Research Articles

The working principle of the trochoidal electron monochromator revisited

On the basis of a number of different experiments employing various principles, we have demonstrated that the energy resolution of the trochoidal electron monochromator used in our laboratory the past 5 yr is not independent from the electron energy used, that is, the very high nominal-energy-resolution close-to-zero electron energy in the range of several meV deteriorates quickly with increasing electron energy reaching values of up to 100 meV at ∼1-eV electron energy. Carrying out extensive electron trajectory calculations with the Simion program, we were able to show that our variant of a trochoidal monochromator does not only operate on the trochoidal dispersion principle but also involves a retarding field component right after the dispersion region in achieving this high-energy-resolution close-to-zero energy. This retarding field is, however, weakened at higher electron energies (caused by the influence of the electron-acceleration field) leading to the decrease in energy resolution with increasing electron energy. On the basis of further simulations, we have designed and constructed a new monochromator avoiding this and other deficiencies. This new monochromator currently has an energy resolution of ∼45 meV independent of the electron energy. Further improvements are under consideration.

A crossed beam high resolution study of dissociative electron attachment to CCl 4

Using a crossed electron/molecule beam ion source in combination with a quadrupole mass spectrometer we have studied the electron energy dependence of the dissociative attachment process CCl 4 + e → Cl − + CCl 3 (at electron energies between threshold and 2 eV) in a target gas temperature range of 300–500 K. With the available high energy resolution of about 11 meV FWHM from an improved trochoidal electron monochromator it was possible to investigate in a single collision crossed beam experiment the true electron energy dependence of the cross-section of this s-wave electron capture process down to the limiting E −1/2 behavior. At energies above approximately 20 meV the cross-section decreases more strongly towards higher energies, exhibiting around 100 meV the typical E −1 behavior predicted for the s-wave de Broglie cross-section. Additional structure above 200 meV is interpreted in terms of autodetachment and a peak at around 0.8 eV (with a peak height of approximately 0.3% of that of the zero energy peak) is attributed to the triply degenerate 2T 2 excited state resonance. No temperature dependence of the measured cross-section function could be observed in the investigated temperature range of 300–500 K.

Analysis of organophosphate pesticides by a trochoidal electron monochromator mass spectrometer system.

An electron monochromator mass spectrometer was used to study the resonant electron energies versus negative ion masses of the organophosphate insecticides dicapthion, EPN, ethion, fenitrothion, leptophos, leptophosoxon, paraoxon, and parathion. Each compound yielded a unique two-dimensional electron energy/mass spectrum. The most abundant ions are produced with electrons of energies between 0.03 and 1 eV, but ions result also from capture of electrons with energies ranging to 8.5 eV. Both resonance electron capture ions and dissociative electron capture ions are produced with electrons of energies ranging from 0.03 to 8.5 eV, and ions may have as many as three observable resonance states from which they are formed. Substituted thiophenoxide ions are postulated to arise by rearrangement of the parent thiophosphate ions through a spiro intermediate. Most fragment ions can be rationalized as arising through simple homolytic cleavage of the parent radical anions.

Time-of-flight apparatus for studies of pulsed neutral van-der-Waals-cluster production and of the interaction between electrons and van der Waals clusters at high electron energy resolution

A time-of-flight apparatus has been built to study pulsed neutral van-der-Waals (vdW)-cluster production and the interaction between free electrons and neutral vdW-clusters at high electron energy resolution. The experimental setup (consisting of a piezoelectrically pulsed supersonic nozzle source, a time-of-flight section, a trochoidal electron monochromator and a quadrupole mass analyzer) and first experimental results concerning the velocity distribution of monomers and vdW-clusters will be described.

Application of a trochoidal electron monochromator/mass spectrometer system to the study of environmental chemicals.

A trochoidal electron monochromator has been interfaced to a mass spectrometer to perform electron capture negative ion mass spectrometric (ECNIMS) analyses of environmentally relevant chemicals. The kinetic energy of the electron beam can be varied from 0.025 to 30 eV under computer control. No reagent gas is used to moderate the electron energies. An electron energy spread of +/- 0.1 to +/- 0.4 eV full width at half-maximum (fwhm) can readily be obtained at a transmitted current of 2 x 10(-6) A, improving to +/- 0.07 eV at 5 x 10(-7) A. Comparisons of ECNI results from the electron monochromator/mass spectrometer system with those from a standard instrument that uses a moderating gas show similar spectra for heptachlor but not for the s-triazine herbicides, as for example, atrazine. This compound shows numerous adduct ions by standard ECNIMS that are eliminated by using the electron monochromator to generate the mass spectra. Isomeric tetrachlorodibenzo-p-dioxins show distinct differences in the electron energies needed to produce the maximum amount of parent and fragment anions. Multiple resonance states resulting in stable radical anions (M.-) are easily observed for nitrobenzene and for polycyclic aromatic hydrocarbons. Ionic products of dissociative electron capture invariably occur from several resonance states.

Negative ion formation in chloroacetyl chloride, trichloroacetyl chloride, and chloroacetaldehyde under low-energy electron impact

Negative ion formation from ClCH{sub 2}COCl, CCl{sub 3}CoCl, and ClCH{sub 2}CHO under low-energy (0-10 eV) and monochromatic (0.2 eV) electron impact is studied by using a quadrupole mass filter and a trochoidal electron monochromator. Most of the ions produced are due to a dissociative attachment process and show resonance structure as the energy of the electron is varied. Cl{sup {minus}} ion is the dominant ion and is produced near 0.0 eV. Other ions, resulting from cleavage of C-Cl or C-H bonds, are also observed. Appearance potentials of RCO{sup {minus}} ions resulting from cleavage of a C-Cl bond are 1.0, 0.4, and 0.2 eV for acetyl chloride, chloroacetyl chloride, and trichloroacetyl chloride, respectively.

An apparatus for low energy atomphysical experiments and the first results

An experimental apparatus for the investigation, of the energy dependence of the excitation cross-section of atomic states by electron impact is described. The exciting electron beam is produced by a trochoidal electron monochromator. The best energy homogeneity of the beam is characterized by 40 meV (FWHM) even below 1 eV mean energy. The cross-section is studied by setting up the optical excitation function. The possibilities of the apparatus and method are compared with other experimental possibilities. New experimental information about the excitation of Cd is discussed in case of negative resonance, and the effect of post-collision interaction on the optical excitation function of Cd has been observed for the first time.

Low-energy electron dissociative attachment processes in HgBr2

Br- formation in HgBr2 has been studied by means of two cross-beam electron impact spectrometers, one equipped with a trochoidal electron monochromator and a magnetic mass analyser and the other one equipped with two 127 degrees electrostatic energy filters and a quadrupole mass analyser. It is found that the Br- cross section in the energy range 0-7 eV exhibits two peaks, at zero energy and 4.15 eV respectively. The energy distributions of Br- ions are obtained in the energy range of the second process, different interpretations are discussed.

Electronic excitation (5–9 eV) in ethylene and some haloethylenes by threshold electron-impact spectroscopy with an improved energy resolution

An improvement of a conventional trochoidal electron monochromator is presented. In trapped-electron spectra the fwhm is now reduced to 100 meV, even in the presence of reactive gases. Electron excitation and resonance processes have been studied in ethylene, fluoroethylene, 1,1-difluoroethylene and chlorotrifluoroethylene. Singlet and triplet levels of the first Rydberg state can be resolved in the threshold spectrum, due to resonance excitation and the improved energy resolution. Vibronic structure is reported in the transition to the 1B 1g state in ethylene. Some optically forbidden valence shell transitions have been discovered around 8.5 eV in 1,1-difluoroethylene and chlorotrifluoroethylene.

Mass spectrometric study of the ionization and fragmentation of carbon disulphide by monoenergetic electron impact

The ionization and fragmentation of carbon disulphide under electron impact is studied mass spectrometrically. An ion source with a trochoidal electron monochromator is used, producing electron beams with an energy spread of 80 meV (FWHM). The ionization efficiency curves of CS 2 +, CS + and S + ions are measured and the appearance potentials of various electronic states of these ions determined. Diffuse peaks at m/z values of 13.5 and 25.5 u are observed and assigned to a decomposition of CS 2 + ions, to produce S + and CS + ions, respectively, in the field-free region of the mass spectrometer. The results are discussed in terms of possible fragmentation mechanisms, and the heat of formation of the CS molecule is calculated.

Optimization of the trochoidal electron monochromator

The trochoidal electron monochromator employs an axial magnetic field and a transverse electric field. Many aspects of its performance can be described by electron trajectory equations which include the voltage drop across the entrance aperture. A beam shear is predicted, which is a function of a parameter tanϑ. Optimum performance is achieved by matching the exit aperture to the sheared beam. tanϑ establishes a relationship between the field magnitudes and the dimensions of the monochromator. Estimates of appropriate values for these parameters can be made, with the final optimum values being determined by a combination of practical and empirical considerations. Finally, the use of the instrument as an energy analyzer is described.

Dissociative Electron Attachment to CCl4, CHCl3, CH2Cl2 and CH3Cl

AbstractResonance dissociative attachment of monoenergetic electrons in the energy range from 0.0 eV to 10 eV to CCl4, CHCl3, CH2Cl2 and CH3Cl has been studied using a trochoidal electron monochromator and a quadrupole mass filter. The following fragment negative ions as a function of electron impact energy could be observed: Cl−, Cl2−, CCl2− and CCl3− from CCl4; Cl−, HCl2 and CCl2− from CHCl3; Cl−, Cl2− and CCl2− from CH2Cl2 and Cl− and CH2Cl− from CH3Cl. — The attachment cross section decreases rapidly as H replaces Cl. Appearance potentials for the negative fragments and for CCl3 and CCl2 electron affinities in the limit of the excess energy are given.

Characteristics of the 180 degrees backscattering trochoidal spectrometer

The characteristics are investigated by calculation of the energy distribution of the electrons whose trajectories pass through the spectrometer. This work is an extension of previous calculations on the trochoidal electron monochromator on which the backscattering spectrometer is based. A comparison between certain of the predicted characteristics and those determined by experiment is made.