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

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 approx.330 down to 780 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 (P/sub O2/<10Torr, P/sub N2/<60 Torr, P/sub CO/<40 Torr). The three-body rate constant of 16O2 decreases from approx.2.4 x 10-30 cm6 molecule-2 sec-1 at 330 0K to about 0.9 x 10-30 cm6 molecule-2 sec-1 at approx. 140 0K but unexpectedly increases again to about 1.7 x 10-30 cm6 molecule-2 sec-1 at 79 0K. Similarly, the three-body rate constant of 18O2 decreases from 5.1 x 10-30 cm6 molecule-2 sec-1 at 300 0K to 1.8 x 10-30 cm6 molecule-2 sec-1 at approx.110 0K but increases to 2.3 x 10-30 cm6 molecule-2 sec-1 at 80 0K. The three-body rate constant of N2 shows a more dramatic monotonic increase from 0.9 x 10-31 cm6 molecule-2 sec-1 at 300 0K to 9.4 x 10-31 cm6 molecule-2 sec-1 at 78 0K. In the case of CO, the three-body rate constant appears to have a very shallow minimum around 170 0K 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, (O2xN2), and (O2xCO) 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