Thermal electron attachment to oxygen and van der Waals molecules containing oxygen

Abstract

Thermal electron attachment to O2 has been studied for pure O2 (16O2 and 18O2), O2–N2, O2–CO, and O2–n-C4H10 (16O2 and 18O2) systems at temperatures from ∼330 down to 78° K using pulse radiolysis and microwave conductivity. For pure O2, O2–N2, and O2–CO mixtures, the electron attachment rates showed three-body pressure dependences at all temperatures over the pressure range studied (PO2<10Torr, PN2<60 Torr, PCO<40 Torr). The three-body rate constant of 16O2 decreases from ∼2.4×10−30 cm6 molecule−2 sec−1 at 330 °K to about 0.9×10−30 cm6 molecule−2 sec−1 at ∼ 140 °K but unexpectedly increases again to about 1.7×10−30 cm6 molecule−2 sec−1 at 79 °K. Similarly, the three-body rate constant of 18O2 decreases from 5.1×10−30 cm6 molecule−2 sec−1 at 300 °K to 1.8×10−30 cm6 molecule−2 sec−1 at ∼110 °K but increases to 2.3×10−30 cm6 molecule−2 sec−1 at 80 °K. The three-body rate constant of N2 shows a more dramatic monotonic increase from 0.9×10−31 cm6 molecule−2 sec−1 at 300 °K to 9.4×10−31 cm6 molecule−2 sec−1 at 78 °K. In the case of CO, the three-body rate constant appears to have a very shallow minimum around 170 °K and again increases with further decrease of temperature. Since theory predicts a simple decrease in rate constant with reduced temperature, an extra contribution to the rate constant which increases with lowered temperature is evident. Electron attachment to the van der Waals molecules (O2)2, (O2⋅N2), and (O2⋅CO) is proposed to account for this behavior. It has been found that the dependence of the excess rate on temperature follows rather closely the predicted concentration of van der Waals molecules. Qualitatively, this observation suggests that the rate constant for electron attachment to the van der Waals molecules is only weakly dependent on temperature. The estimated rate constants for this attachment are nearly two orders of magnitude larger than for O2 itself. A discussion of possible reasons for this large increase is given. Analysis of the data for O2–n-C4H10 mixtures suggests that van der Waals molecules contribute significantly only at higher pressures in this system.