Dissociation of methanol in intense femtosecond laser field

Methanol was irradiated by 80 fs laser pulse, intensity range of 1013-1014 W/cm2. A TOF-mass spectrometer was coupled to the laser system and used to detect the ions produced. The parent ions CH3OH+ appeared firstly at the laser intensity of 1.4×1013 W/cm2. While the laser intensity was gradually increased, the parent ions were dissociated and the primary ions CH2OH+ were given as verified from the irradiation of deuterated methanol (CH3OD) showing the C-H bond cracking firstly. While the laser intensity was further increased to 2.0 ×1013 W/cm2, the C-O bonds of the parent ions also broke to give CH3+. When the laser intensity was higher, smaller fragment ions like CH+, C+, OH+ and O+ also appeared. Among the fragment ions, only H+ ion yield had anisotropic angular distribution dependence on the laser polarization vector in the dissociation of methanol. All the experimental observations show that the dissociation of methanol proceeds through stepwise mechanism but not Coulomb explosion.