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

摘要

Thermal electron attachment to O2has been studied for pure O2(16O2and18O2), O2ndash;N2, O2ndash;CO, and O2ndash;nhyphen;C4H10(16O2and18O2) systems at temperatures from sim;330 down to 78deg; K using pulse radiolysis and microwave conductivity. For pure O2, O2ndash;N2, and O2ndash;CO mixtures, the electron attachment rates showed threehyphen;body pressure dependences at all temperatures over the pressure range studied (PO210Torr,PN260 Torr,PCO40 Torr). The threehyphen;body rate constant of16O2decreases from sim;2.4times;10minus;30cm6moleculeminus;2thinsp;secminus;1at 330thinsp;deg;K to about 0.9times;10minus;30cm6thinsp; moleculeminus;2thinsp;secminus;1at sim; 140thinsp;deg;K but unexpectedly increases again to about 1.7times;10minus;30cm6thinsp;moleculeminus;2thinsp; secminus;1at 79thinsp;deg;K. Similarly, the threehyphen;body rate constant of18O2decreases from 5.1times;10minus;30cm6thinsp; moleculeminus;2thinsp;secminus;1at 300thinsp;deg;K to 1.8times;10minus;30cm6thinsp;moleculeminus;2thinsp;secminus;1at sim;110thinsp;deg;K but increases to 2.3times;10minus;30cm6thinsp;moleculeminus;2thinsp;secminus;1at 80thinsp;deg;K. The threehyphen;body rate constant of N2shows a more dramatic monotonic increase from 0.9times;10minus;31cm6thinsp;moleculeminus;2thinsp;secminus;1at 300thinsp;deg;K to 9.4times;10minus;31cm6thinsp;moleculeminus;2thinsp;secminus;1at 78thinsp;deg;K. In the case of CO, the threehyphen;body rate constant appears to have a very shallow minimum around 170thinsp;deg;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, (O2sdot;N2), and (O2sdot;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 O2itself. A discussion of possible reasons for this large increase is given. Analysis of the data for O2ndash;nhyphen;C4H10mixtures suggests that van der Waals molecules contribute significantly only at higher pressures in this system.