Elementary Processes in Excited and Ionized Gases - Fundamentals of Ionized Gases - page 105

Figure 2.25 The cross section of the Penning processes involving metastable helium atoms and
atoms of (a) argon; (b) krypton; and (c) xenon as a function of the relative velocity [139].
Figure 2.26 The electron spectrum in the Penning process involving metastable helium atoms
colliding with xenon and mercury atoms [140].
down voltage of helium, and the Penning process (2.97) was discovered [137, 138]
in this manner.
By its nature, the Penning process occurs in the decay of an autoionizing state
A B . At large distances between nuclei the lifetime of this system is long because
of the weak interaction between atomic particles. Therefore, this process occurs in
close collisions, and its cross section is usually smaller than the gas-kinetic cross
section.