Slow fragmentation of hydrocarbons after ultrafast laser interaction

We experimentally and theoretically investigated the deprotonation process on nanosecond to microsecond timescale in ethylene and acetylene molecules, following their double ionization by a strong femtosecond laser field. In our experiments we utilized coincidence detection with the reaction microscope technique, and found that both the mean lifetime of the ethylene dication leading to the "slow" deprotonation and the relative channel strength of the slow deprotonation compared to the fast one have no evident dependence on the laser pulse duration and the laser peak intensity. Furthermore, quantum chemical simulations suggest that such slow fragmentation originates from the tunneling of near-dissociation-threshold vibrational states through a dissociation barrier on an electronic dication state. Such vibrational states can be populated through strong field double ionization induced vibrational excitation on an electronically excited state in the case of ethylene, and through intersystem processes from electronically excited states to the electronic ground state in the case of acetylene.