Ionization of Methane in Strong and Ultrastrong Relativistic Fields

The photoionization of methane is reported for intensities up to 10¹⁹ W/cm² with linear and circular polarized light. While fragmental ions (e.g., CH₃⁺, CH⁺, C⁺, C²⁺) created from 10¹⁴ W/cm² to 10¹⁵ W/cm² are formed by Coulomb explosion, ionization to form C³⁺ and C⁴⁺ 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 10¹⁹ W/cm² extend up to kinetic energies of 0.6 MeV.