Argon metastable level

Accordingly, an over-population of the argon metastable levels would explain both the over-ionization as well as the high electron number density in the ICP. Indeed, it could be accepted that argon metastables act both as ionizing species and at the same time are easily ionized [385]. This could explain the fairly low interferences caused by easily ionized elements and the fact that ion lines are excited very effi-... 

PENNING EFFECT. An increase in the effective ionization rate of a gas due to the presence of a small number of foreign metastable atoms. For instance, a neon atom has a metastable level at 16.6 volts and if there are a few neon atoms in a gas of argon which has an ionization potential of 15.7 volts, a collision between the neon metastable atom with an argon atom may lead to ionization of the argon. Thus, the energy which is stored in the metastable atom can be used to increase the ionization rate. Other gases where this effect is used are helium, with a metastable level at 19.8 volts, and mercury, with an ionization level at 10.4 volts. 

This rule is responsible for the metastability of the lowest Pq levels in mercury, cadmium, zinc, neon, argon, etc. 

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