Photon-enhanced dissociative electron attachment in SF6 and its isotopic selectivity

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Abstract
Production of SF−5 by dissociative attachment of very low energy electrons to SF6 is 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 εa=0.2 eV for the reaction. The expectation that this effect can be produced by direct optical excitation of the ν3 vibrational mode is confirmed by using a tunable cw CO2 laser 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−5 signal by the radiation is strongly dependent on the laser wavelength, and is confined to the attachment peak at very low (32SF−5 and 34SF−5 are well resolved and are separated by the known isotope shift of the ν3 SF6 absorption. Both peaks, however, are red shifted by 8 cm−1 from 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 of 32SF−5 was found to occur at the P (28) CO2 laser 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 SF6 at 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.