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Low-energy vibrational and electronic electron-energy-loss (EEL) spectra of pyrimidine condensed on a thin film of solid argon held at 18 K are reported for the incident-energy range of 2-12 eV. Sensitivity to symmetry and spin forbidden transitions as well as correlations to the triplet states of benzene make it possible to ascribe the main features, below 7 eV in the electronic part of the EEL spectrum, to triplet transitions. The lowest EEL feature with an energy onset at 3.5 eV is attributed to a transition to the (3)B(1)(n-->pi(*)) valence electronic state and the next triplet n-->pi(*) transition to a (3)A(2) state located around 4.5 eV. The remaining EEL features at 4.3, 5.2, 5.8, and 6.5 eV are all assigned to pi-->pi(*) transitions to states of symmetry (3)B(2), (3)A(1), (3)B(2), and (3)B(2)+(3)A(1), respectively. The most intense maximum at 7.6 eV is found to correspond to both (1)B(2) and (1)A(1) transitions, as in the vacuum ultraviolet spectra. Absolute inelastic cross sections per scatterer are derived from a single collision treatment described herein. Their values are found to lie within the 10(-17) cm(2) range for both the electronic and the vibrational excitations. Features in the energy dependence of the cross sections are discussed, whenever possible, by comparison with data and mechanisms found in the gas phase. A maximum over the 4-5 eV range is attributed to a B (2)B(1) shape resonance and another one observed in the 6-7 eV range is ascribed to either or both sigma(*) shape resonances of (2)A(1) and (2)B(2) symmetries.

An elementary introduction on direct and resonant low-energy (0-20 eV) electron scattering from molecules condensed on surfaces is given with reference to concepts developed from gas-phase investigations. It is shown that while elastic scattering can be described in terms of band-structure parameters, inelastic processes are often governed by intramolecular mechanisms. Following a brief description of the experimental techniques, elastic scattering and resonant vibrational excitation of physisorbed and chemisorbed molecules are reviewed. The results of electron impact experiments leading to stable anion formation are also reviewed. Typical data are presented to illustrate the main characteristics of the processes involved in electron scattering from molecules condensed in submonolayer, single and multilayer coverages of various solid surfaces.

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Low energy (0-10 eV) electron driven reactions in the halogenated organic acids CCl3COOH, CClF2COOH, and CF3CHNH2COOH (trifluoroalanine)