A photoelectron-photoion coincidence imaging apparatus for femtosecond time-resolved molecular dynamics with electron time-of-flight resolution of σ=18ps and energy resolution ΔE∕E=3.5%

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Abstract
We report on the construction and performance of a novel photoelectron-photoion coincidence machine in our laboratory in Amsterdam to measure the full three-dimensional momentum distribution of correlated electrons and ions in femtosecond time-resolved molecular beam experiments. We implemented sets of open electron and ion lenses to time stretch and velocity map the charged particles. Time switched voltages are operated on the particle lenses to enable optimal electric field strengths for velocity map focusing conditions of electrons and ions separately. The position and time sensitive detectors employ microchannel plates (MCPs) in front of delay line detectors. A special effort was made to obtain the time-of-flight (TOF) of the electrons at high temporal resolution using small pore (5μm) MCPs and implementing fast timing electronics. We measured the TOF distribution of the electrons under our typical coincidence field strengths with a temporal resolution down to σ=18ps. We observed that our electron coincidence detector has a timing resolution better than σ=16ps, which is mainly determined by the residual transit time spread of the MCPs. The typical electron energy resolution appears to be nearly laser bandwidth limited with a relative resolution of ΔEFWHM∕E=3.5% for electrons with kinetic energy near 2eV. The mass resolution of the ion detector for ions measured in coincidence with electrons is about ΔmFWHM∕m=1∕4150. The velocity map focusing of our extended source volume of particles, due to the overlap of the molecular beam with the laser beams, results in a parent ion spot on our detector focused down to σ=115μm.

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