We report on the low-energy electron-induced production of aldehydes within thin solid films of tetrahydrofuran (THF) condensed on a solid Kr substrate. The aldehyde fragments, which remain trapped within the bulk of the THF film, are detected in situ via their 3,1(n-->pi*) and 3(pi-->pi*) electronic transitions and vibrational excitations in the ground state using high-resolution electron-energy-loss spectroscopy. The production of aldehyde is studied as a function of the electron exposure, film thickness, and incident electron energy between 1 and 18.5 eV. The aldehyde production is calibrated in terms of an electron scattering cross section, which is found to be typically 6-7 x 10(-17) cm(2) between 11 and 19 eV. Its energy dependence is characterized by a small feature around 3 eV, a strong rise from 6 eV up to a maximum at 12.5 eV, followed by two structures centered around 15 and 18 eV. The aldehyde production is discussed in terms of the formation of electron resonances or transient anion states, which may lead to the fragmentation of the molecule and explain the structures seen in the energy dependence of the measured cross section.

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Chemical reactions have been induced in condensed acetaldehyde by exposure to low-energy electrons as demonstrated by use of high-resolution electron energy loss (HREEL) spectroscopy and thermal desorption spectroscopy (TDS). The HREEL spectra show that the acetaldehyde film is modified significantly when the incident electron energy is increased above 10 eV. Release of CO upon exposure to electrons at an incident energy of 14 eV is observed by electronic HREEL spectra. The changes in the vibrational spectra upon exposure of the molecular film to electrons at an incident energy of 15.5 eV are analyzed in detail using reference spectra for a number of potential product molecules. TDS data reveal clearly the decomposition of acetaldehyde and formation of CO and CH 4 as major products under electron exposure. The combined results also give evidence for the formation of propionaldehyde. Other less prominent products are acetone and possibly an alcoholic species. A reaction mechanism is suggested that rationalizes the formation of larger products. It involves fragmentation of the molecule releasing CO, abstraction of an H atom from an adjacent acetaldehyde by the H radical fragment, and, finally, recombination of the resulting radical with the remaining CH 3 fragment.

Electron-stimulated reactions in solid films of tetrahydrofuran (THF), condensed on Kr spacers deposited on a Pt substrate, or directly onto the substrate, were induced and monitored simultaneously with use of high-resolution electron-energy-loss spectroscopy in the ranges of vibrational and electronic excitations. The spectra of the molecular films obtained after a certain time of exposure to electrons at incident energies of 14 and 15.5 eV were analyzed and different products were identified. Besides an aldehyde, which is the main product, olefins, conjugated olefins, as well as CO were identified. Closer investigation of the reactions of propionaldehyde, as a model aldehyde, demonstrates that CO appears in THF as a secondary product (i.e., from the intermediate aldehyde). On the basis of the cross sections for the formation of an aldehyde from THF, of CO from propionaldehyde, and for the loss of propionaldehyde under electron impact, the reaction sequences were evaluated with the help of a kinetic model. This analysis suggests that some CO could also be formed directly from THF (i.e., without involvement of an intermediate aldehyde).