Strong-field ionization of CH₃Cl: proton migration and association

Strong-field ionization of CH₃Cl using femtosecond laser pulses, and the subsequent two-body dissociation of CH₃Cl²⁺ along H⁺ (n = 1-3) and HCl⁺ forming pathways, have been experimentally studied in a home-built COLTRIMS (cold target recoil ion momentum spectrometer) setup. The single ionization rate of CH₃Cl was obtained experimentally by varying the laser intensity from 1.6 × 10¹³ W cm^-2 to 2.4 × 10¹⁴ W cm^-2 and fitted with the rate obtained using the MO-ADK model. Additionally, the yield of H⁺ ions resulting from the dissociation of all charge states of CH₃Cl was determined as a function of intensity and pulse duration (and chirp). Next, we identified four two-body breakup pathways of CH₃Cl²⁺, which are H⁺ + CH₂Cl⁺, H₂⁺ + CHCl⁺, H₃⁺ + CCl⁺, and CH₂⁺ + HCl⁺, using photoion-photoion coincidence. The yields of the four pathways were found to decrease on increasing the intensity from I = 4.2 × 10¹³ W cm^-2 to 2I = 8.5 × 10¹³ W cm^-2, which was attributed to enhanced ionization of the dication before it can dissociate. As a function of pulse duration (and chirp), the H⁺ forming pathways were suppressed, while the HCl⁺ forming pathway was enhanced. To understand the excited state dynamics of the CH₃Cl dication, which controls the outcome of dissociation, we obtained the total kinetic energy release distributions of the pathways and the two-dimensional coincidence momentum images and angular distributions of the fragments. We inferred that the H⁺ forming pathways originate from the dissociation of CH₃Cl dications from weakly attractive metastable excited states having a long dissociation time, while for the HCl⁺ forming pathway, the dication dissociates from repulsive states and therefore, undergoes rapid dissociation. Finally, quantum chemical calculations have been performed to understand the intramolecular proton migration and dissociation of the CH₃Cl dication along the pathways mentioned above. Our study explains the mechanism of H⁺ and HCl⁺ formation and confirms that intensity and pulse duration can serve as parameters to influence the excited state dynamics and hence, the outcome of the two-body dissociation of CH₃Cl²⁺.