Helium metastable atoms collision processes

If the electronic excitation energy of a metastable atom A exceeds the ionization potential of another atom B, their collision can lead to an act of ionization, the so-called Penning ionization. The Penning ionization usually proceeds through the intermediate formation of an unstable excited quasi molecule in the state of auto-ionization cross sections of the process can be very high. Cross sections for the Peniung ionization of N2, CO2, Xe, and Ar by metastable helium atoms He(2 S) with an excitation energy of 19.8 eV reach gas-kinetic... 

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