Post ionization alignment of the fragmentation of molecules in an ultrashort intense laser field

We studied the angular distributions of the fragmented ions of diatomic molecules in an intense linearly polarized short laser pulse. In addition to the well-known dynamic alignment of the neutral molecules before ionization, we identified a more important post ionization alignment effect of the molecular ions. The latter is modelled quantum mechanically as resulting from the breakup of a rotating linear rotor. We showed that only for very short pulses are the two alignment mechanisms not important. In this case the angular distributions of the fragmented ions mimic the shape of the electronic density of the outermost molecular orbital. (Some figures in this article are in colour only in the electronic version) When neutral molecules are exposed to an intense laser pulse, they can be aligned as well as ionized by the electric field of the laser (Stapelfeldt and Seideman 2003). The coexistence of both effects makes it difficult experimentally to extract the dependence of ionization rates on the alignment of molecules with respect to the direction of laser polarization. Recently, Litvinyuk et al (2003) exploited the pump–probe technique on N2 molecules where a weak laser pulse was used to align the rotational wave packet and a strong probe pulse was used to determine the ionization rates of these aligned molecules. Their results are in general agreement with the prediction of the molecular tunnelling ionization theory (Tong et al 2002), as well as with the interference model (Muth-Bohm et al 2000, Kjeldsen and Madsen 2004) in the strong field approximation, that the ionization rates peak at θ = 0 (θ is the angle between the molecular axis and laser field polarization direction) for N2 molecules. A similar measurement on O2 was inconclusive, however. More recently Légáre (2004) has shown that the harmonic generation from O2 peaks when the molecule is not aligned along the polarization axis, again in agreement with the molecular tunnelling ionization theory. According to the molecular tunnelling ionization theory (MO-ADK) (Tong et al 2002), the alignment dependence of the ionization rates of molecules by an intense laser pulse depends critically on the symmetry of the outermost orbital of the molecule. For N2, it 0953-4075/05/040333+09$30.00 © 2005 IOP Publishing Ltd Printed in the UK 333