Phase-sensitive molecular ionization induced by a phase-controlled two-color laser field in methyl halides

We have investigated the phase-sensitive photoionization process of molecules induced by phase-controlled two-color laser pulses consisting of a fundamental light and a second-harmonic light with a pulse duration of 130 fs and an intensity of 10{sup 12}-10{sup 13} W/cm{sup 2}. We observed a phase-sensitive forward-backward asymmetry of photofragment emission along the laser polarization direction in the dissociative ionization of four methyl halide molecules (CH{sub 3}X; X=F,Cl,Br,I). The directional asymmetries observed in the halogen ions and the counter methyl cations were out of phase with each other. This result indicates that detected molecules are oriented with discrimination of their head-and-tail order. We show that under our experimental conditions, selective ionization of oriented molecules based on the molecular Ammosov-Delone-Krainov model is the main process rather than dynamical molecular orientation for the phase-sensitive ionization process in methyl halides.