Reactions of excited atoms and molecules with atoms and molecules

From a He-beam excited by electron impact we eliminated the He(2¹ S) component to better than 0.5% by irradiating light from a He discharge. The quenching process ishv(2¹ P→2¹ S)+He(2¹ S)→He(2¹ P)→He(1¹ S +hv) (2¹ P 1)¹ S. By measuring the ions produced in collisions of the He-metastables with various target gases in a mass spectrometer, singlet to triplet Penning-cross section ratios were obtained. These ratios are without exception close to one, which is taken as evidence for the previously proposed electron exchange mechanism of the Penning ionization. In the case that more ions are produced in the collision of He (2¹ S) and He(2³ S) with a target gas, separate relative production cross sections are obtained for the two metastables. For the rare gases the measurements are performed at two temperatures of the He-beam, 320 and 90 °K. It is found that the cross section ratio of associative — to Penning ionization increases considerably as the temperature is decreased for both, He(2¹ S) and He(2³ S), the effect being much more pronounced for He(2¹ S). The results of this work are found to confirm conclusions drawn from measured energy distributions of the electrons ejected in the Penning process.