Production of SF−5by dissociative attachment of very low energy electrons to SF6is known from previously reported work to be strongly enhanced by increasing the gas temperature. Data on this effect is presented and analyzed to give an activation energy of &egr;a=0.2 eV for the reaction. The expectation that this effect can be produced by direct optical excitation of the &ngr;3vibrational mode is confirmed by using a tunable cw CO2laser focused collinearly with an electron beam inside a collision chamber. The product ions are monitored using a quadrupole mass filter. By chopping the laser beam and monitoring ion signals and electron current during the laser on, and laser off, periods it is possible to isolate the desired signals from the interfering effects of heating of the collision chamber and the electron gun filament, caused by the laser beam. The observed enhanced of the SF−5signal by the radiation is strongly dependent on the laser wavelength, and is confined to the attachment peak at very low (0.1 eV) electron energy. This is consistent with the thermal excitation data. The tuning curves for the production of32SF−5and34SF−5are well resolved and are separated by the known isotope shift of the &ngr;3SF6absorption. Both peaks, however, are red shifted by 8 cm−1from their respective room temperature small‐signal absorption peaks. Possible reasons for this shift are discussed. They suggest that efficient promotion of the (SF−6) * dissociative decay channel requires a total of two or more vibrational quanta to be present in the SF6. The peak enhancement of32SF−5was found to occur at theP(28) CO2laser line (936.85 cm−1). At this wavelength the enhancement effect was found to be linearly dependent on laser intensity. The interpretation that this implies single‐photon absorption is rejected on the grounds that the laser fluence levels are too high for such conditions to prevail. The linearity remains unexplained, in common with similar observations by others on absorption effects in SF6at similar fluence levels. Future measurements of the present type, in particular, of the dependence on laser fluence at other wavelengths should provide additional insight to this general problem of understanding the mechanisms contributing to the absorption of the first few photons in any multiple photon absorption process.
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