Strong-field induced bond rearrangement and hydrogen migration in small hydrocarbons

Imaging and control schemes for photo-induced structural rearrangement dynamics (isomerization, proton migration, H$_{2}$/H$_{3}$ elimination etc.) are of particular interest to ultrafast photochemistry. The strong-field regime offers a variety of possibilities to map these reactions (e.g., employing coincident momentum-resolved ion spectroscopy), and to control them by exploiting field-modified or field-induced potentials. Here we report on a series of experiments that study bond rearrangement in small hydrocarbons (CH$_{4,\, }$C$_{2}$H$_{2}$, C$_{2}$H$_{4})$ irradiated by intense 800 nm laser pulses. We disentangle different fragmentation pathways and identify the isomerization channels by measuring coincident ion momentum patterns for two- or three-body breakup channels. For C$_{2}$H$_{2}$ and C$_{2}$H$_{4}$ isomerization, we observe the evolution of kinetic energy release spectra with increasing laser pulse duration, which allows us to distinguish the isomerization pathways active within or after the pulse. We demonstrate that a significant (up to an order of magnitude) enhancement of C$_{2}$H$_{4}$ isomerization yield and H$^{3+}$ elimination from CH$_{4}$ for a given light intensity can be achieved with an increase in pulse duration from 25 to 200 fs.