On the fragment ion angular distributions arising from the tetrahedral molecule CH3I

The mass spectra for both horizontal and vertical polarizations and the angular distributions of fragment ions arising from Coulomb explosion of tetrahedral methyl iodide (CH3I) ions, obtained at a laser intensity of 10(16) W cm(-2) are presented. All fragment ion distributions are peaked along the direction corresponding to collinearity of the,laser electric field with the time-of-flight mass spectrometer axis. The In+ ion (n less than or equal to 7) angular distributions from the dissociation of the parent ions are all of similar widths, which would imply a geometric, as opposed to dynamic, alignment. Additionally, the lower-charged I ions have an isotropic component that decreases as the charge state increases. Measurements of the CHm+ (m less than or equal to 3), Cp+ (p less than or equal to 4) and H+ ion distributions show that these are also maximal along the polarization direction. Furthermore, there is also a CH22+ ion peak present in the CH, group, which has a distribution similar to those measured for the other ions., This mass peak is the prominent multi-charged ion in this group. As the CH3I molecule is initially tetrahedral, these results suggest that the molecular structure undergoes a change such that the H-C and C-I bonds tend to lie along the field. Several authors have described work which first aligned CH3I molecules. with a nanosecond laser and then photodissociated with a femtosecond laser,, to produce fragment ion distributions. This is the first time that the angular distributions from a tetrahedral molecule have been presented using femtosecond laser pulses only and in the case of CH3I, for fragments other than CH3+ and I+. The fragment energetics from the single CH3I molecule have been compared with those from recent work dealing with the Coulomb explosion of CH3I clusters.