Statistical mechanics of Ar-2(+) in an inductively coupled plasma

In the mass spectrum of an argon inductively coupled plasma (ICP), there is a peak due to the presence of the argon dimer ion, Ar-2(+). Using elementary statistical mechanics, an attempt is made to elucidate the mechanism responsible for this ion's presence in the ICP, The assumption of local thermodynamic equilibrium (LTE) in the ICP leads to three possible mechanisms that could be responsible for the presence of the argon dimer ion, however, the results of the calculations show that only one of the mechanisms agrees with experiment. The experimental measurements of the number density ratio of Ar-2(+) to Ar+, against which the theoretical values are compared, were taken using inductively coupled plasma mass spectrometry (ICP-MS).