Linking the gaseous and the condensed phases of matter: The slow electron and its interactions

The interfacing of the gaseous and the condensed phases of matter as effected by interphase and cluster studies on the behavior of key reactions involving slow electrons either as reacting initial particles or as products of the reactions themselves is discussed. Emphasis is placed on the measurement of both the cross sections and the energetics involved, although most of the available information to date is on the latter. The discussion is selectively focussed on electron scattering (especially the role of negative ion states in gases, clusters, and dense matter), ionization, electron attachment and photodetachment. The dominant role of the electric polarization of the medium is emphasized

Pressure-dependent electron attachment and breakdown strengths of unitary gases, and synergism of binary gas mixtures: a relationship

The relationship between the pressure-dependent electron attachment rate constants (k/sub a/) which have been observed in 1-C/sub 3/F/sub 6/ and in several perfluoroalkanes, and the uniform field breakdown strengths (E/N)/sub lim/ in these gases is discussed. Measurements of the pressure dependence of k/sub a/ of OCS in a buffer gas of Ar are presented and the possible pressure dependence of (E/N)/sub lim/ in OCS is discussed. Uniform field breakdown measurements have been performed in C/sub 3/F/sub 8/, n-C/sub 4/F/sub 10/, and SO/sub 2/ over a range of gas pressures (3 less than or equal to P/sub T/ less than or equal to 290 kPa) and are reported. All three molecules have been found to possess pressure-dependent (E/N)/sub lim/ values. The various types of synergistic behavior which have been observed in binary gas dielectric mixtures are summarized and discussed. A new mechanism is outlined which can explain the synergism observed in several gas mixtures where the (E/N)/sub lim/ values of the mixutres are greater than those of the individual gas constituents. Model calculations are presented which support this mechanism, and can be used to explain the pressure-dependent synergistic effects which have been reported in 1-C/sub 3/F/sub 6//SF/sub 6/ gas mixture

Theory Summary and Future Directions

Summary talk at the Lepton-Photon Symposium, Cornell University, Aug. 10-15, 1993.Comment: (Talk presented at the Lepton-Photon Symposium, Cornell University, Aug. 10-15, 1993.) 19 page

Effect of temperature on the uniform field breakdown strength of electronegative gases

In general, the electron attachment rate constant, k/sub a/ (,UPSILON), as a function of the mean electron energy and temperature UPSILON for electronegative gases which attach electrons nondissociatively decreases greatly with UPSILON from room temperature to UPSILON less than or equal to 600K, while that for electronegative gases which attach electrons dissociatively increases with increasing UPSILON. Based on recent studies in our laboratory on k/sub a/ (,UPSILON), we investigated the variation with UPSILON (approx.295-575K) of the uniform field breakdown strength, (E/N)/sub lim/, for three classes of electronegative gases: (a) gases such as c-C/sub 4/F/sub 8/ (and c-C/sub 4/F/sub 6/, 1-C/sub 3/F/sub 6/) which attach strongly low-energy (less than or equal to 1 eV) electrons nondissociatively and for which k/sub a/ (,UPSILON), decreases precipitously with UPSILON above ambient; (b) gases such as C/sub 2/F/sub 6/ and CF/sub 3/Cl which attach electrons exclusively dissociatively and whose k/sub a/ (,UPSILON) increases with UPSILON; and (c) gases such as C/sub 3/F/sub 8/ and n-C/sub 4/F/sub 10/ which attach electrons both nondissociatively and dissociatively over a common low-energy range and whose k/sub a/ (,UPSILON) first decreases and then increases with UPSILON above ambient. The (E/N)/sub lim/(UPSILON) has been found to decrease significantly with UPSILON for (a), to decrease slowly with UPSILON for (c), and to increase slightly with UPSILON for (b). These changes in (E/N)/sub lim/ follow those in k/sub a/ (,UPSILON). A similar behavior is expected for other electronegative gaseous dielectrics in the respective three groups

Effects of temperature on dissociative and nondissociative electron attachment

Results of recent studies on the effects of temperature, T, on the dissociative and nondissociative electron attachment to molecules are presented and discussed. 34 refs., 15 figs

Positron-molecule interactions: resonant attachment, annihilation, and bound states

This article presents an overview of current understanding of the interaction of low-energy positrons with molecules with emphasis on resonances, positron attachment and annihilation. Annihilation rates measured as a function of positron energy reveal the presence of vibrational Feshbach resonances (VFR) for many polyatomic molecules. These resonances lead to strong enhancement of the annihilation rates. They also provide evidence that positrons bind to many molecular species. A quantitative theory of VFR-mediated attachment to small molecules is presented. It is tested successfully for selected molecules (e.g., methyl halides and methanol) where all modes couple to the positron continuum. Combination and overtone resonances are observed and their role is elucidated. In larger molecules, annihilation rates from VFR far exceed those explicable on the basis of single-mode resonances. These enhancements increase rapidly with the number of vibrational degrees of freedom. While the details are as yet unclear, intramolecular vibrational energy redistribution to states that do not couple directly to the positron continuum appears to be responsible for these enhanced annihilation rates. Downshifts of the VFR from the vibrational mode energies have provided binding energies for thirty species. Their dependence upon molecular parameters and their relationship to positron-atom and positron-molecule binding energy calculations are discussed. Feshbach resonances and positron binding to molecules are compared with the analogous electron-molecule (negative ion) cases. The relationship of VFR-mediated annihilation to other phenomena such as Doppler-broadening of the gamma-ray annihilation spectra, annihilation of thermalized positrons in gases, and annihilation-induced fragmentation of molecules is discussed.Comment: 50 pages, 40 figure

CO2 dissociation activated through electron attachment on reduced rutile TiO2(110)-1x1 surface

Converting CO$_2$ to useful compounds through the solar photocatalytic reduction has been one of the most promising strategies for artificial carbon recycling. The highly relevant photocatalytic substrate for CO$_2$ conversion has been the popular TiO$_2$ surfaces. However, the lack of accurate fundamental parameters that determine the CO$_2$ reduction on TiO$_2$ has limited our ability to control these complicated photocatalysis processes. We have systematically studied the reduction of CO2 at specific sites of the rutile TiO$_2$(110)-1x1 surface using scanning tunneling microscopy at 80 K. The dissociation of CO2 molecules is found to be activated by one electron attachment process and its energy threshold, corresponding to the CO$_2^{\dot-}$/CO$_2$ redox potential, is unambiguously determined to be 2.3 eV higher than the onset of the TiO$_2$ conduction band. The dissociation rate as a function of electron injection energy is also provided. Such information can be used as practical guidelines for the design of effective catalysts for CO$_2$ photoreduction

Photoelectric Emission from Interstellar Dust: Grain Charging and Gas Heating

We model the photoelectric emission from and charging of interstellar dust and obtain photoelectric gas heating efficiencies as a function of grain size and the relevant ambient conditions. Using realistic grain size distributions, we evaluate the net gas heating rate for various interstellar environments, and find less heating for dense regions characterized by R_V=5.5 than for diffuse regions with R_V=3.1. We provide fitting functions which reproduce our numerical results for photoelectric heating and recombination cooling for a wide range of interstellar conditions. In a separate paper we will examine the implications of these results for the thermal structure of the interstellar medium. Finally, we investigate the potential importance of photoelectric heating in H II regions, including the warm ionized medium. We find that photoelectric heating could be comparable to or exceed heating due to photoionization of H for high ratios of the radiation intensity to the gas density. We also find that photoelectric heating by dust can account for the observed variation of temperature with distance from the galactic midplane in the warm ionized medium.Comment: 50 pages, including 18 figures; corrected title and abstract field