Preparation of Pure Ion Species

The study of ion-molecule reactions is often highly simplified if only one reactant ion is present in the reaction chamber. The fine energy 

Since pure ion preparations usually require only relatively low photon energy resolution (and hence allow large photon intensities), high ion currents can often be obtained. Mass-resolved ion currents of about 10 -10 sec are not unusual. The threshold behavior of photoionization is very favorable since the cross section for photoionization is often very high (and sometimes even a maximum) just above threshold. This is in contrast to the behavior of the electron-impact ionization cross section which (for direct ionization) is zero at threshold and increases with excess energy above threshold. 

The capability of pure ion preparation extends to the case of gas mixtures as well. Usually, the molecules of lowest ionization energy can be ionized exclusively, even though the difference in ionization potentials is only 0.1 V. Thus, the molecule can be ionized nearly exclusively and with fair intensity in the presence of HD and D2. The exclusive ionization of H2 in a mixture with argon has not been achieved in electron-impact studies but has been accomplished very easily by photoionization.  

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