An electron monochromator for use in an axial magnetic field is described. Electrons are injected parallel to the magnetic field and an electric field is applied in a perpendicular direction. The electrons thus describe trochoids and drift in a direction perpendicular to both the electric and magnetic fields and disperse according to their initial velocities. An electron energy width at half-maximum of 0.020 eV can be obtained, with a transmitted current of about 10−9 A.

Absolute partial and total electron impact ionization cross sections for CF4 from threshold up to 180 eV

No abstract

Negative ion formation following low energy (0–10 eV) electron attachment to free and bound CF2Cl2 molecules is studied in (1) a molecular beam experiment (single molecules, homogeneous clusters, and mixed CF2Cl2/NH3 clusters) and (2) a UHV surface experiment where desorption of negative ions from condensed CF2Cl2 is observed. From single gas phase CF2Cl2 molecules we observe Cl− and F− generated via dissociative electron attachment from a resonance near 0 eV and 3 eV, respectively, as the most abundant ions. From homogeneous clusters (CF2Cl2)n, we additionally detect undissociated complexes of the form (M)n−(M=CF2Cl2) including the stabilized monomer CF2Cl2− and also “solvated fragment ions” of the form Mn⋅X−(X=Cl, F). Their relative abundance vs size (n) of the final product varies in a significant different way between (M)n− and Mn⋅X− reflecting the different relaxation probabilities in the initial cluster. In the desorption spectra, the dominant low energy Cl− gas phase resonance is strongly suppressed in favor of a significant resonant feature appearing near 8 eV. These last results are discussed in light of previously reported giant enhancements of electron induced desorption of Cl− and F− from CF2Cl2 on Ru coadsorbed with water or ammonia ices under 250 eV electron impact [Q. B. Lu and T. E. Madey, Phys. Rev. Lett. 82, 4122 (1999); J. Chem. Phys. 111, 2861 (1999)].

Differential cross sections for vibrational excitation of ground state C 2 H 4 and C 2 D 4 by electron impact have been studied with a crossed-beam apparatus for electron energies between I and II eV. The scattering is dominated by two resonance regions in which the vibrational cross sections of totally symmetric modes are preferentially enhanced to the order of 10 16 cm 2 The first resonance region is centered near 1.8 eV. Here all energy-loss spectra and energy and angular dependences of cross sections can be accounted for by a 'B" shape resonance of an intermediate lifetime. The second resonance region centered near 7.5 eV is very broad. The dominant vibrational modes and the corresponding angular distributions are distinctly different from those in the lower region. We interpret this second region in terms of short-lived shape resonances, the dominant one being a 'Ag compound state comprising the target molecule plus an electron in the 4a g orbital. 5532

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 CHCl3+e→Cl− at electron energies from about 0 to 2 eV and in a target gas temperature range of about 300–430 K. The energy resolution and working conditions of this newly constructed crossed beams machine have been characterized using CCl4 as a test and calbrant gas. Utilizing the improved energy resolution of the present experimental setup (which allows measurements with FWHM energy spreads down to below 5 meV) it was possible to determine the accurate shape and magnitude of the cross section function in the low-energy range. This leads to the conclusion that between an electron energy of about 20 and 130 meV the reaction proceeds via deBroglie s-wave capture, whereas at higher energy (above about 0.4 eV) autodetachment plays a significant role. Moreover, the present measurements allow us to clarify previously reported differences in the absolute cross section, the number of peaks and in the energy position of these peaks. Finally, by analyzing the measured strong temperature dependence of the cross section close to zero electron energy the activation barrier for this dissociative attachment was determined to be 110±20 meV in good agreement with thermochemical data from swarm experiments performed under thermal equilibrium. Taking into account the present results it is also possible to discuss the mechanism for the existence of the second peak.

An improved technique for calculating the resolution of trochoidal electron monochromators.

We report absolute partial cross sections for the formation of selected positive and negative ions resulting from electron interactions with uracil. Absolute calibration of the measured partial cross sections for the formation of the three most intense positive ions, the parent C 4 H 4 N 2 O + 2 ion and the C 3 H 3 NO + and OCN + fragment ions, was achieved by normalization of the total single uracil ionization cross section (obtained as the sum of all measured partial single ionization cross sections) to a calculated cross section based on the semi-classical Deutsch-Märk formalism at 100 eV. Subsequently, we used the OCN + cross section in conjunction with the known sensitivity ratio for positive and negative ion detection in our apparatus (obtained from the well-known cross sections for SF + 4 and SF − 4 formation from SF 6 ) to determine the dissociative attachment cross section for OCN − formation from uracil. This cross section was found to be roughly an order of magnitude smaller, about 5 × 10 −22 m 2 at 6.5 eV, compared to our previously reported preliminary value. We attribute this discrepancy to the difficult determination of the uracil target density in the earlier work. Using a reliably calculated cross section for normalization purposes avoids this complication.