Cyclohexane and 2,3-dimethyl-1,3-butadiene (DMBD) were ionized and fragmented by 0.8 mu m femtosecond pulses with a typical intensity of 4 x 10(13) 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. (c) 2006 Elsevier B.V. All rights reserved.