Pulsed afterglow method

One of the simplest and most accurate techniques for investigating the collision processes involving metastable atoms is the pulsed afterglow method. In this technique a sample of metastable atoms is created in a cylindrical or spherical cell by means of a pulsed discharge and the decay of the metastable density is observed following the termination of the excitation pulse. By measuring the decay rate as a function of gas pressure it is often possible to obtain enough information to determine the individual contributions to the observed decay rate made by each of the processes listed above. 

Most of the studies in this decade were carried out with conventional single source mass spectrometers, which limited the kind and accuracy of the information. During the next decade, however, various sophisticated techniques for the study of ion—molecule reactions, such as tandem mass spectrometers, photoionization sources, pulsed sources, flowing afterglow and drift tube methods, crossed and merging beams and ion cyclotron resonance, have been developed. Much detailed information on various aspects of ion—molecule reactions has accumulated, and this has consequently stimulated the theoretical studies as well. This decade was, so to speak, the second epoch in the history of ion—molecule studies. 

Rather limited and sporadic use has been made of the stationary afterglow technique. Historically, it played an important role as the first method to produce data on reactions of aeronomic interest. Typically, a gaseous sample is subjected to a pulse of radiation or excitationf and the subsequent history of the decaying plasma is followed by sampling from it into a mass spectrometer. The phenomena governing this history, such as ambipolar diffusion of the ions and electron-ion recombination, are complex, and ion-neutral reactions constitute only one part of this. As always, the whole must be understood before reliable quantitative information can be obtained for a part, and the deciphering of the history to yield quantitative information on reaction rates is thus difficult. 

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