Low-energy electron-energy-loss spectroscopy of electronic transitions in solid carbon dioxide

Electron photoemission from charged films: Absolute cross section for trapping 0–5eV electrons in condensed CO2

The electron trapping or attachment cross section of carbon dioxide (CO2) condensed as thin films on a spacer of Ar is obtained using a simple model for electron trapping in a molecular film and then charge releasing from the same film by photon absorption. The measurements are presented for different electron exposures and impact energies, film thicknesses, and probing photon energies. The cross section for trapping an electron of incident energy between 0 and 5 eV reveals three different attachment processes characterized by a maximum at about 0.75 eV, a structured feature around 2.25 eV, and a shoulder around 3.75 eV. From the measurement of their dependence with the probing photon energy, the two lowest processes produce traps having a vertical electron binding energy of approximately 3.5 eV, whereas the highest one yields a slightly higher value of approximately 3.7 eV. The 0.75 eV maximum corresponds to the formation of vibrational Feshbach resonances in (CO2)n- anion clusters. The 2.25 eV feature is attributed to the formation of a vibrationally excited 2Piu anion in (CO2)n- clusters, followed by fast decay into its vibrational ground state without undergoing autodetachment. Finally, 3.75 eV shoulder is assigned to the well-known dissociative electron attachment process from 2Piu anion state producing the O- anion in the gas phase and the (CO2)nO- anions in clusters.

Low-energy electron scattering cross section for the production of CO within solid films of carbon dioxide

We report absolute electron scattering cross sections sigma(p) for the production of CO within thin solid film of carbon dioxide (CO(2)) condensed on a solid Ar substrate. The CO fragments, which remain trapped within the bulk of the carbon dioxide film, are detected in situ by recording energy losses to their lowest triplet electronic state a (3)Pi using high-resolution electron-energy-loss spectroscopy. The production of CO is studied as a function of the electron exposure, film thickness, and incident electron energy between 2 and 30 eV, a range within which most of the secondary electrons are created in systems irradiated by high-energy particles. The energy dependence is characterized by a feature around 4 eV with sigma(p)=(7.0+/-4.0)x10(-18) cm(2), a minimum around 7 eV, a strong rise up to a large and broad maximum around 15 eV with sigma(p)=(5.4+/-2.5)x10(-17) cm(2), a decrease to a minimum around 18.5 eV, and finally a monotonous increase up to 30 eV. The CO production is discussed in terms of the formation of electron resonances or transient anion states, which may lead directly to the fragmentation of the molecule via dissociative electron attachment or indirectly by decaying into an entirely repulsive part of the corresponding excited neutral and positive ion states.

VUV spectroscopy and photo-processing of astrochemical ices: an experimental study

In order to understand much of the chemistry that underpins astronomical phenomena (e.g. star and planet formation) it is essential to probe the physico-chemistry of ice surfaces under astronomical conditions. The physical properties and chemical reactivity of such icy surfaces depends upon its morphology. Thus it is necessary to explore how the morphology of astrochemical ices is influenced by their local environment (e.g. temperature and pressure) and the mechanisms by which they are processed. In this paper we report the results of a series of experiments to explore the morphology of a variety of molecular ices using VUV spectroscopy. Spectral signatures are found that may allow the morphology of such ices to be identified.

A multichannel electron energy loss spectrometer for low-temperature condensed films

We describe a wide-gap multichannel cylindrical deflection electron energy analyzer suitable for measuring the weak signals characteristic of electronically inelastic electron energy loss spectra. The analyzer has nearly ideal fringing field termination, and its resolution and energy dispersion were characterized as a function of energy by solving numerically the equation of motion of electrons in an ideal cylindrical electric field. The numerical results for the radial location of the electrons at the detector as a function of the entrance location, angle, and energy are closely approximated by a second order polynomial, and match closely with those observed. The detection efficiency of the analyzer is 100-150 times better than that of an equivalent single-channel instrument, but limited energy transmission of the zoom lens system used in our case reduced it by a factor of about 2. The performance of the new instrument was demonstrated by measuring the (3)E(1u) electronic spectrum of benzene in only 2 min and the spectrum of endo-benzotricyclo[4.2.1.0(2.5)]nonane.