Laser and Particle Beams


Unusual fragmentation patterns from the dissociation of some small molecules

P.  GRAHAM a1c1, X.  FANG a1, K.W.D.  LEDINGHAM a1, R.P.  SINGHAL a1, T.  McCANNY a1, D.J.  SMITH a1, C.  KOSMIDIS a2, P.  TZALLAS a2, A.J.  LANGLEY a3 and P.F.  TADAY a3
a1 Department of Physics & Astronomy, University of Glasgow, Glasgow, G12 8QQ, Scotland, UK
a2 Department of Physics, University of Ioannina, GR-45110, Ioannina, Greece
a3 Central Laser Facility, Rutherford Appleton Laboratory, Didcot, Oxon., OX11 0QX., England, UK


The nonlinear interaction of molecular matter with the intense field of a linearly-polarized laser beam, allows the study of not only the ionization dynamics of the parent molecule, but also the angular distribution of the exploding fragments. This is carried out by rotating the polarization vector of the laser with respect to the laboratory reference frame. The angular distributions for the CS2, CO2, N2O, H2S, and CH3I molecular ions, at intensities of about 1016 W cm−2, are presented. The distributions seem to be dependent on the molecule under consideration, but common features are that the peripheral atoms of the molecule are ejected along the ToF-axis, and the central atom perpendicularly to it, whenever the polarization vector and ToF-axis are collinear. It would seem that the distributions for the lighter of the molecules are partly due to alignment via dipole moments induced by the laser, as the distributions narrow as their charge-state increases. This is indicative of a larger torque acting on the higher-charged precursor molecular ion, via the interaction of the field with the laser-induced dipole moment. On the other hand, the angular dependence of the heavier molecules studied, are thought to originate from a dependence of the ionization/dissociation probability of the molecular ion on the initial angle made between the molecular axis and the polarization vector, that is, a preferential ionization/dissociation process. Spatial alignment in the laser pulse, in this case, is not thought to occur since the peaks do not narrow as the ionic charge increases. Finally, the results for H2S and also N2O are particularly interesting, since distributions for up to S7+ are presented, while the N-distributions show both a parallel and perpendicular component of the distribution. Neither of these results has, to the author's knowledge, been previously observed.

(Received June 23 1999)
(Accepted October 31 1999)

c1 Address correspondence and reprint requests to: P. Graham, Department of Physics & Astronomy, Kelvin Building, University of Glasgow, Glasgow G12 8QQ, United Kingdom. E-mail: