We perform time-resolved ionization spectroscopy measurements of the excited state dynamics of CH2I2 and CH2IBr following photoexcitation in the deep UV. The fragment ions produced by ionization with a vacuum-ultraviolet probe pulse are measured with velocity map imaging, and the momentum resolved yields are compared with trajectory surface hopping calculations of the measurement observable. Together with recent time-resolved photoelectron spectroscopy measurements of the same dynamics, these results provide a detailed picture of the coupled electronic and nuclear dynamics involved. Our measurements highlight the non-adiabatic coupling between electronic states, which leads to notable differences in the dissociation dynamics for the two molecules.
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While our measurements allow for direct reconstruction of the full 3D momentum of each fragment ion based on the measured (x, y, t), we can also perform an inverse Abel transform on an ensemble of ion measurements, integrating over t values for a given ion species. This can sometimes be advantageous, producing higher momentum resolution than available from the timing information, and is the approach that we took in the measurements presented here.
We also note that there are some modulations in the CH2I+ TKE which are consistent with C-Br stretching, but do not focus on these dynamics in this work.
The populations in panel (c) differ from the previously published ones at later times because now more (86) trajectories are included. Nevertheless, the populations at longer times still have to be taken with a grain of salt, since the electronic structure calculations of CH2I2 are problematic, such that we excluded a few of the crashed trajectories from the analysis.