Electron–ion–ion coincidence experiments for photofragmentation of polyatomic molecules using pulsed electric fields: Treatment of random coincidences

In molecular photofragmentation processes by soft X-rays, a number of ionic fragments can be produced, each having a different abundance and correlation with the kinetic energy of the emitted electron. For investigating these fragmentation processes, electron–ion and electron–ion–ion coincidence experiments, in which the kinetic energy of electrons are analyzed using an electrostatic analyzer while the mass of the ions is analyzed using a pulsed electric field, are very powerful. For such measurements, however, the contribution of random coincidences is substantial and affects the data in a non-trivial way. Simple intuitive subtraction methods cannot be applied. In the present paper, we describe these electron–ion and electron–ion–ion coincidence experiments together with a subtraction method for the contribution from random coincidences. We provide a comprehensive set of equations for the data treatment, including equations for the calculation of error-bars. We demonstrate the method by applying it to the fragmentation of free CF3SF5CF3SF5 molecules.