The laser pulse width effect on the dissociation probability of CH 4 + irradiated by an ultrafast laser has been investigated experimentally and theoretically. The femtosecond laser at 800 nm with an intensity of 8.0 × 10 13 W/cm 2 was used. The observed relative yield of the primary fragment ion CH 3 + increases with increasing pulse width and tends to saturate when the pulse width is longer than 120 fs. The field-assisted dissociation (FAD) model and quasi-classical trajectory (QCT) calculation were applied to predicting the dissociation probability of CH 4 + . The calculated probability is corrected with the molecular orientation effect and the spatial distribution of laser intensity. The modified results show that the dissociation requires at least 23 fs and saturates with long pulse widths (≥100 fs). The result is approximately consistent with the experimental observation.

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