The design and performance of a threshold-photoelectron–photoion coincidence spectrometer for the study of unimolecular decomposition in polyatomic ions

Abstract

The design, construction and performance of a threshold-photoelectron–photoion coincidence (TPEPICO) spectrometer for the study of unimolecular decomposition in polyatomic ions is described. The spectrometer incorporates a hemispherical electrostatic energy analyser and a time-of-flight (TOF) mass spectrometer. The entrance lens to the hemispherical analyser has been designed to have a high collection efficiency for low energy electrons but to discriminate strongly against energetic electrons. This arrangement has resulted in a resolution of about 3.5 meV being achieved for the threshold electron peak recorded at the krypton 2P 3/2 ionisation limit. A pulsed electric field is used to extract the ions from the interaction region and propel them towards the TOF analyser. Computer modelling has been used to trace the electron and ion trajectories through their respective analysers. These simulations have enabled the effective interaction volume to be defined, and this has allowed the transmission efficiency of energetic fragment ions, formed through a process which also yielded a threshold electron, to be quantified. The ion TOF peak shape has been examined as a function of initial kinetic energy and as a function of ion residence time. The contribution of energetic fragments, having specific initial spatial and directional properties, to the TOF peak shape has been determined by tracing the paths of individual ions. The actual performance of the spectrometer is illustrated by a TPEPICO spectrum of the krypton isotopes. Experimental breakdown curves for furan are presented as an example of the use of the apparatus to study unimolecular decomposition in polyatomic ions. By introducing a delay between the detection of the threshold electron and the application of the ion extraction field, breakdown curves can be recorded as a function of ion residence time in the source region. The procedure for analysing the data is described, and the experimental factors that need to be taken into account to obtain a meaningful comparison with theoretical predictions are discussed.