Photoelectron photoion coincidence imaging of ultrafast control in multichannel molecular dynamics

Abstract

The control of multichannel ionic fragmentation dynamics in CF₃I is studied by femtosecond pulse shaping and velocity map photoelectron photoion coincidence imaging. When CF₃I is photoexcited with femtosecond laser pulses around 540 nm there are two major ions observed in the time-of-flight mass spectrum, the parent CF₃I⁺ ion and the CF₃⁺ fragment ion. In this first study we focussed on the influence of LCD-shaped laser pulses on the molecular dynamics. The three-dimensional recoil distribution of electrons and ions were imaged in coincidence using a single time-of-flight delay line detector. By fast switching of the voltages on the various velocity map ion lenses after detection of the electron, both the electron and the coincident ion are measured with the same imaging detector. These results demonstrate that a significant simplification of a photoelectron-photoion coincidence imaging apparatus is in principle possible using switched lens voltages. It is observed that shaped laser fields like chirped pulses, double pulses, and multiple pulses can enhance the CF₃⁺/CF₃I⁺ ratio by up to 100%. The total energetics of the dynamics is revealed by analysis of the coincident photoelectron spectra and the kinetic energy of the CF₃⁺ and I fragments. Both the parent CF₃I⁺ and the CF₃⁺ fragment result from a five-photon excitation process. The fragments are formed with very low kinetic energy. The photoelectron spectra and CF₃⁺/CF₃I⁺ ratio vary with the center wavelength of the shaped laser pulses. An optimal enhancement of the CF₃⁺/CF₃I⁺ ratio by about 60% is observed for the double pulse excitation when the pulses are spaced 60 fs apart. We propose that the control mechanism is determined by dynamics on neutral excited states and we discuss the results in relation to the location of electronically excited (Rydberg) states of CF₃I.