Separation of neutral versus cationic dissociation processes in an ultracompact double time-of-flight spectrometer: first results on CH 3I

Abstract

We present a simple experimental setup, which allows simultaneous detection and analysis of photoions and photoelectrons. In short, the apparatus is a hybrid of a time-of-flight mass spectrometer and a time-of-flight magnetic bottle photoelectron spectrometer. For ions and electrons the same field-free drift region as well as the same multichannel plate detector is used. Mass resolution m/Δ m is about 1000. Electron energy resolution Δ E decreases with absolute energy and is 50 meV at 0.5 eV. For lower electron energies the resolution is limited by the bandwidth of the 200 fs laser pulse. The apparatus is tested at the well-known à state dynamics of methyl iodide by femtosecond pump–probe excitation and ionization. The observed fragment ions can have two different origins. They can be formed (1) either by parent cation photodissociation or (2) by ultrafast neutral fragmentation with subsequent fragment ionization by the intense probe laser. Because of the different ionization energies of CH 3I and iodine, neutral, and cationic dissociation pathways can be distinguished by means of electron energy analysis. This tool is very important for interpretation of mass and femtosecond laser pump–probe spectra of short-lived molecular systems. Because of the comparable detection probability for ions and electrons the apparatus would allow electron–ion coincidence measurements at kilohertz laser repetition rates.