Complex sentence: As a group of primary electrons, coming from the arrival of a group of initial photons, move toward the first dynode, they are accelerated by the electric field and arrive each with the ≈ 100 eV kinetic energy imparted by the potential difference.
Split into simpler sentences: As a group of primary electrons, created by the arrival of a group of initial photons, move toward the first dynode they are accelerated by the electric field. They each arrive with ≈ 100 eV kinetic energy imparted by the potential difference.