"Heavy Electron" Photoelectron Spectroscopy: Rotationally Resolved Ion Pair Imaging of CH 3 +

We applied the velocity map imaging technique under high-resolution conditions to study ion pair products of the vacuum ultraviolet photodissociation of methyl chloride. We obtained rotationally resolved kinetic energy release spectra that directly provide vibrational frequencies and rotational constants of the fundamental carbocation, CH3+. The technique is analogous to photoelectron spectroscopy, with the chloride anion playing the role of a "heavy electron." The approach shows promise as a general probe of ionic species not amenable to study by traditional methods.