Fragmentation dynamics of methane by few-cycle femtosecond laser pulses.

The fragmentation pattern of CH4 was experimentally studied at an intensity of approximately 10(14) W/cm2 with laser durations varying from 8 to 110 fs. When the laser duration was 8 fs, only the primarily fragmental CH3+ ion was observed in addition to the parent CH4+ ion. When the laser duration was 30 fs, small fragmental CH2+ and H+ ions appeared. When the laser duration was 110 fs, some doubly charged ions were also observed in addition to the abundant singly charged ions. The large mass spectra difference demonstrated that the pulse duration had a strong effect on the fragmentation of the parent ion produced in the single ionization. The effect of laser intensity on the fragmentation of CH4+ was also studied for few-cycle femtosecond laser pulses. The results demonstrated that the first-return recollision between the rescattered electron and the parent ion played a significant role in the fragmentation dynamics of the parent ion. Depending on the ion-electron impact energy, the recollision excited the parent ion to a dissociated state or doubly charged state. The experimentally observed singly charged fragmental ions resulted from the recollision-induced dissociation of CH4+ or the Coulomb explosion of CH(4)2+.