Abstract: Methane molecules were irradiated by a laser beam in the intensity range of 7.6 ×1013～1.4×1014 W•cm－2 (800 nm, 160 fs). A time of flight mass spectrometer was coupled to the laser system. The parent ions can be seen at low laser intensity of 7.6×1013 W•cm－2. When the laser intensity increases to 8 .0×1013 W•cm－2, CH3＋ ions appear. The appearance of the CH2＋, CH＋, C＋ ions are at the laser intensities of 1.0×1014 W•cm－2, 1.4×1014 W•cm－2 and 1.4×1014 W•cm－2, respectively. The facts show that dissociation of methane is a step-wise process. Coulomb explosion does not happen during the dissociation because no multi-electron ions are found in the mass spectra. Only H＋ ion yield has anisotropic angular distribution when methane is irradiated by a linearly polarized laser. This fact implies that the chemical bonds in the molecule are pulled off by the laser field and that the H＋ ion flies along the direction of the laser electronic field. The quasi-diatomic molecule model we proposed can explain the experimental results satisfactorily.

Key words: Femtosecond laser, Intense laser field, Methane, TOFMS, Dissociation