An ion optical system for the detection of charged fragments with high acceptance in a time-of-flight mass spectrometer
The ion optical system, described in this paper, has been designed to obtained a high transmission (close to unity) through a time-of-flight mass spectrometer, which analyses the charged fragments resulting from ion-molecule collisions with high time resolution. An ion optical conduction without losses is an indispensable condition to measure correlations between several charged fragments produced in one ion-molecule collision directly via time-of-flight analysis. If the transmission is not affected by the initial kinetic energy of the fragments (several eV are released if the dissociation occurs from an excited dissociative state), and the initial velocity distribution is isotropic, line shape analysis can now be used to evaluate the initial energy distribution of fragments originating from a specific decay channel, which can be selected just by their correlation. Furthermore, the realized detection method, which is based on counting the secondary electrons released by the fragment striking an ion-converter dynode, can be extended to deliver further information, such as the fragment's charge state. Various measurements, using methyl chloride as the target gas and analysing initial energy as well as correlation phenomena, prove that the realized system meets all the requirements high-resolution time-of-flight analysis poses while representing an essential experimental progress, which was mainly due to the discovery of an ion optical element consisting of concentric electrodes that we call "Doppelring".