Dissociative ionization and Coulomb explosion of CH₄ in two-color asymmetric intense laser fields

Directional fragment ejection from a tetrahedral molecule CH₄ in linearly polarized two-color (ω and 2ω) asymmetric intense laser fields (50 fs, 1.4 × 10¹⁴ W cm^-2, 800 nm and 400 nm) has been studied by three-dimensional ion coincidence momentum imaging. The H⁺ fragment produced from dissociative ionization, CH₄ → H⁺ + CH₃ + e^-, is preferentially ejected on the larger amplitude side of the laser electric fields. Comparison with theoretical predictions by weak-field asymptotic theory shows that the observed asymmetry can be understood by the orientation selective tunneling ionization from the triply degenerated highest occupied molecular orbital (1t₂) of CH₄. A similar directional ejection of H⁺ was also observed for the low kinetic energy components of the two-body Coulomb explosion, CH₄ → H⁺ + CH₃⁺ + 2e^-. On the other hand, the fragment ejection in the opposite direction were observed for the high energy component, as well as H₂⁺ produced from the Coulomb explosion CH₄ → H₂⁺ + CH₂⁺ + 2e^-. Possible origins of the characteristic fragmentation are discussed.