Intact molecular ion formation of cyclohexane and 2,3-dimethyl-1,3-butadiene by excitation with a short, intense femtosecond laser pulse

Cyclohexane and 2,3-dimethyl-1,3-butadiene (DMBD) were ionized and fragmented by 0.8 μm femtosecond pulses with a typical intensity of 4 × 1013 W cm−2, which were resonant with the cation absorption of cyclohexane and DMBD. Their intact molecular ions were dominant at a pulse duration of 15 fs, whereas the molecules were heavily fragmented by excitation at 205–210 fs. Possible reasons for the formation of intact molecular ions by a short pulse are discussed in terms of the vibrational periods of excited precursor states.