Dissociative ionization and Coulomb explosion of CH 4 in two-color asymmetric intense laser fields.

Directional fragment ejection from a tetrahedral molecule CH 4 in linearly polarized two-color ( ω and 2 ω ) asymmetric intense laser fields (50 fs, 1.4 × 10 14 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, CH 4 → H + + CH 3 + 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 (1 t 2 ) of CH 4 . A similar directional ejection of H + was also observed for the low kinetic energy components of the two-body Coulomb explosion, CH 4 → H + + CH 3 + + 2e - . On the other hand, the fragment ejection in the opposite direction were observed for the high energy component, as well as H 2 + produced from the Coulomb explosion CH 4 → H 2 + + CH 2 + + 2e - . Possible origins of the characteristic fragmentation are discussed.