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

The mass spectra for both horizontal and vertical polarizations and the ang ular distributions of fragment ions arising from Coulomb explosion of tetra hedral 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 th e 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 al l of similar widths, which would imply a geometric, as opposed to dynamic, alignment. Additionally, the lower-charged I ions have an isotropic compone nt 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 dist ributions show that these are also maximal along the polarization direction . Furthermore, there is also a CH22+ ion peak present in the CH, group, whi ch has a distribution similar to those measured for the other ions., This m ass peak is the prominent multi-charged ion in this group. As the CH3I mole cule is initially tetrahedral, these results suggest that the molecular str ucture 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 mo lecules. with a nanosecond laser and then photodissociated with a femtoseco nd laser,, to produce fragment ion distributions. This is the first time th at the angular distributions from a tetrahedral molecule have been presente d using femtosecond laser pulses only and in the case of CH3I, for fragment s other than CH3+ and I+. The fragment energetics from the single CH3I mole cule have been compared with those from recent work dealing with the Coulom b explosion of CH3I clusters.