An investigation of the angular distributions of fragment ions arising from the linear CS2 and CO2 molecules

Abstract

The nonlinear interaction of the triatomic molecules CS2 and CO2 with the intense field of a linearly polarized laser beam of femtosecond (fs) pulse duration, was used to study the ionization and dissociation of the parent molecule. The fragment ion angular distributions arising from the Coulomb explosion of the parent ions were also measured. For CS2, the angular distributions of CS2+, CS22+, CS23+, CS+, CS2+, Sn+ (n6) and Cm+ (m4) ions are presented for a laser intensity of 1 × 1016 W cm-2 at a wavelength of 790 nm and pulse duration of 50 fs. The angular distributions of the parent molecular ions are all isotropic. The Sn+ fragments are peaked along the time-of-flight (TOF) axis, whereas the Cm+ fragments explode perpendicularly to this. Similar results for CO2 are also presented for comparison. The S ion distributions do not narrow as their ionic charge increases, and it is argued that the angular distributions for CS2 are due mainly to the angular dependence of the ionization probability. On the other hand, the distributions from the lighter CO2 molecule are thought to be at least partly due to alignment via dipole moments induced by the laser, as in this case the On+ angular distributions are seen to narrow as their charge increases. The conclusion of these results is that the laser pulse may be too short for the CS2 molecule to align in the pulse. Angular distributions are also presented for varying laser pulse durations, in the range of 50 fs to 300 ps. The dynamics of the ionization/dissociation mechanism are discussed in the context of the TOF mass spectra and angular distributions recorded for CS2.