A method for ion reaction time amplification

In general, ion reaction rates can be observed directly in a time-of-flight spectrometer with a time resolution comparable to that of the system, which is about 10-9 to 10 los. The rate measurement can achieve a much better time resolution by using an ion reaction time amplification method. With this method, very fast ion reactions can be measured with a time resolution much better than the time resolution of the system. It is with this method69 that the field dissociation reaction of 4HeRh2+ was measured with a time resolution of about 20 femtoseconds when the time resolution of the system was still only 1 ns. 

An ion reaction time amplification means that those ion reactions taking place in a very short time period 8r will have their detection stretched over a much longer period 8f so that the ion detector and the system electronics can adequately respond. The amplification factor can be defined as 

The basic idea is to build the time-of-flight mass spectrometer into two electrically well shielded sections, the acceleration-reaction section of very short length / and a field free flight section of very large length L, or 

The ion reaction can be a photo-dissociation reaction or a further field ionization reaction followed by Coulomb repulsive dissociation. For simplicity, the compound ions, (A/ + m)n+, are assumed to be formed right at the surface. Let us assume that an (A/ + m)n+ ion is dissociated by the ion reaction at x where x l. The total flight time of the Mn+ ion formed at x is given by 

This amplification factor is proportional to Lll, and is also dependent on the location where the ion reaction occurs. Maximum amplification occurs at x = /, A(l) = (m 2M)(L/l), or if the reaction occurs right after the parent ion has gained the full energy of the acceleration voltage. Equation (3.33) suggests that whenever possible one should measure the flight time distribution, or the energy distribution, of the ion species with the smaller mass. 

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