Ionization of Methane in Strong and Ultrastrong Fields

The photoionization of methane is reported for intensities up to 10$^{19}$ W/cm$^{2}$ with linear and circular polarized light. While fragmental ions (e.g. CH$_{3}^{+}$, CH$^{+}$, C$^{+}$, C$^{+2})$ created from 10$^{14}$ W/cm$^{2}$ to 10$^{15}$ W/cm$^{2}$ are formed by Coulomb explosion, ionization to form C$^{+3}$ and C$^{+4}$ involves Coulomb explosion and tunneling ionization. In ultrastrong fields, removal of a carbon K-shell electron from methane proceeds via tunneling and rescattering ionization, without the influence of molecular channels. Photoelectrons from methane at 10$^{19}$ W/cm$^{2}$ extend up to kinetic energies of 0.6 MeV.