Progress report on the development of a Coulomb imaging experiment

The enhanced ionisation thresholds for the triatomic OCS molecules have been determined using a classical model. By using these thresholds to determine the dissociative motion of an OCS molecule in a 55fs laser pulse of intensity 2x10'5 W/cm2, the accuracy a Coulomb imaging experiment has been quantified.

The enhanced ionisation thresholds for the triatomic OCS molecules have been determined using a classical model. By using these thresholds to determine the dissociative motion of an OCS molecule in a 55fs laser pulse of intensity 2×l0¹⁵ W/cm², the accuracy a Coulomb imaging experiment has been quantified.

The results of a Coulomb imaging experiment employing a momentum mapping technique [1] have been analysed using an adaptation of the classical enhanced ionisation approach [2]. In the momentum mapping method molecular geometry is derived by firstly recording the momenta of each fragment ion as a function of trajectory and then simulating the Coulomb explosion process with model bed angle, bond length and alignment distributions. For speed of calculation the simulation assumes an initially stationary molecule. We are now able to model the motion of the molecule before the final stage of ionisation and can therefore quantify how accurate our structure calculations are.

The threshold for the ionization of OCS was set at 10¹³ Wcm². Even at this low threshold, the trajectory still crosses the (3,3,4) curve on the rising edge of the laser pulse for a peak intensity of 10¹⁵ Wcm². Final fragment energies have been found to be strongly dependent only on the final ionization threshold curve crossed, and therefore are largely independent of initial conditions, as it must be possible to reach the final observed ionization channel, in this case (3,3,4), from a range of initial geometries. This is how the mechanism of enhanced ionization itself chooses the temporal and spatial boundaries which give rise to a particular charge state. The initial geometry for enhanced ionization calculations was chosen from within the range specified by the zero point motion shown in figure 3, whereas the initial charge distribution is chosen to model the ‘pre-Coulombic’ explosion dynamics of OCS⁺ in a similar manner to that adopted by Hseih and Eland [3]. The charge on the molecular ion was distributed unevenly, as if to form a channel (Q₁,Q₂,Q₃) where Q_i are fractions representing the degree of charge localization on each atom such that ΣQ_i = 1. The charged fragments are then allowed to repel each other Coulombically. The geometry evolution was calculated within the changing laser pulse envelope and the charge state is subject to the ionization thresholds derived in figure 1. Evolution of the molecular geometry occurs under the fractional charge state until the bond trajectories intersect the (1,1,1) channel ionization threshold curve, after which the molecular ion evolves with integer charges up to the (3,3,4) channel in this case. For ease of calculation we have omitted the bond angle dependence on ionization as it has been shown to be of secondary importance. Figure 1 shows the bond trajectories R_oc and R_cs for a molecule with an initial bond angle of 1758. In effect the bond trajectories are the 55fs laser pulse intensity profiles with the time ordinate transposed to internuclear separation, where the time transformation is derived from the Coulomb repulsion dynamics.