Calculation of Cross Sections for Zero Activation Energy Processes by Simple Collision Models with Emphasis on the Penning Effect
Recent cross sections obtained for the destruction of He(2 3S) metastable atoms by ionization of Ar, Kr, Xe, H2, and N2 are analyzed by a simple collision picture. It is found that the ratio of the experimental to momentum transfer cross sections is the same order as the efficiency of ejection of electrons from metal surfaces by He(2 3S) atoms, namely 0.2-0.3. The momentum transfer cross sections agree with the experimental values better than some of the published experimental values agree among themselves and in addition there is a great paucity of data in this area so that an order-of-magnitude estimate is often useful. In addition, it is shown that calculated momentum transfer cross sections for the reactions, He(2 3S)+He(2 3S)-->He++He(1 1S)+e-, He2++Ne-->Ne++2 He He(1 1S)+He(n 1P)-->He(1 1S)+He(nf), He(2 1S)+Ar-->He(1 1S)+Ar++e-, agree very well with literature values of cross sections for the processes. It appears that for a broad class of atomic reactions requiring zero activation energy, a momentum transfer collision has a high probability of reaction. Momentum transfer cross sections are dominated by the long-range attractive forces and can easily be calculated, thereby providing a useful estimate of the actual reaction cross section.