INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS B: ATOMIC, MOLECULAR AND OPTICAL PHYSICS J. Phys. B: At. Mol. Opt. Phys. 34 (2001) 4015–4026 PII: S0953-4075(01)27030-8 On the fragment ion angular distributions arising from the tetrahedral molecule CH 3 I P Graham 1 , K W D Ledingham 1,4, 5 , R P Singhai 1 , S M Hankin 1 , T McCanny 1 , X Fang 1 , C Kosmidis 2 , P Tzallas 2 , P F Taday 3 and A J Langley 3 1 Department of Physics and Astronomy, University of Glasgow, Glasgow, G12 8QQ, UK 2 Department of Physics, University of Ioannina, GR-45110, Ioannina, Greece 3 Central Laser Facility, Rutherford Appleton Laboratory, Didcot, Oxfordshire OX11 0QX, UK 4 AWE Plc, Aldermaston, Berkshire RG7 4PR, UK E-mail: k.ledingham@physics.gla.ac.uk Received 17 July 2001 Published 5 October 2001 Online at stacks.iop.org/JPhysB/34/4015 Abstract The mass spectra for both horizontal and vertical polarizations and the angular distributions of fragment ions arising from Coulomb explosion of tetrahedral methyl iodide (CH 3 I) 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 I n+ ion (n  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 CH + m (m  3), C p+ (p  4) and H + ion distributions show that these are also maximal along the polarization direction. Furthermore, there is also a CH 2+ 2 ion peak present in the CH m 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 CH 3 I 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 CH 3 I 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 CH 3 I, for fragments other than CH + 3 and I + . The fragment energetics from the single CH 3 I molecule have been compared with those from recent work dealing with the Coulomb explosion of CH 3 I clusters. 5 Author to whom correspondence should be addressed. 0953-4075/01/204015+12$30.00 © 2001 IOP Publishing Ltd Printed in the UK 4015