Velocity map imaging of dissociative ionization and coulomb explosion of CH3I induced by a femtosecond laser.

The dissociative ionization and the Coulomb explosion of CH3I irradiated by a 35 fs 800 nm laser with a laser intensity of 4 x 10(13) to 6 x 10(14) W/cm2 was studied. In a relatively weak laser field (about 10(13) W/cm2), the dissociative ionization of CH3I took place. The speed distributions of the CH3+ and I+ fragments were measured and fitted using multiple Gaussian functions. Different product channels were found for CH3+ and I+, respectively. In a strong laser field (about 10(14) W/cm2), the multiply ionized fragment ions of Iq+ (q </= 3), which experienced a Coulomb explosion, were observed. The angular and speed distributions of the I+, I2+, and I3+ fragments were obtained. All of these fragment angular distributions are anisotropic and peaked along the laser polarization direction. The Iq+ ion angular distributions all were of similar widths, which would imply that the geometric alignment dominated the process, as opposed to a dynamic alignment mechanism.