![]() ![]() The influence of the parameters of the control pulse (width, intensity, duration of the pulse and delay of the control pulse) using a three-states model with calculated potential curves, dipole moments and polarizabilities was subsequently published. 20 The dynamic Stark control experiments mentioned above, in which an intermediate strength control pulse is used after the excitation to shift the curves involved in the crossing, appeared one decade later: first with a theoretical study using a three-state model 1 and a year later, the experimental evidence 2 of the curve shifting of the excited electronic states controlling the photodissociation channel. The first studies to control the photodissociation used coherent control 18,19 and optimal control theory. 17 In addition to the use of the laser pulse as a method to promote the system from lower to excited vibro-rotational or electronic states, pulses were used to control the photofragmentation of IBr. The application of laser pulses provided several extra studies in vibration-rotation excitation, 14 wavepacket absorption spectra, 15,16 and wavepacket predissociation. The first photodissociation study 13 used several coupling regimes to explain the predominance of excited state of Br in the photodissociation products. With the first descriptions of the potential energy curves the first spectroscopic theoretical studies also appeared 13 focusing on the adiabatic and diabatic effects of the excited states crossings. 12 These optimised basis sets are the basis sets used in this paper. The description of the dissociation energies using ECP's was deficient, and a later study 12 used an optimised basis-set to described correctly all the states and crossings of the states dissociating to the ground and first excited spin–orbit states of I and Br atoms. The description of the halogen atoms used relativistic effective core potentials (ECP) including spin–orbit interactions. The first theoretical studies, 7 a decade later, started a series of papers dedicated to the electronic description of the potential energy curves 8–11 with special attention to the crossing of the excited states. The initial studies, 4–6 more than 80 years ago, were focused on the interpretation of the absorption spectrum by vibrational analysis. ![]() The IBr system has received much attention in the last three decades mainly because of the crossing excited-states which dissociate to two different dissociation channels. Comparison of this full description and the models used previously will provide the missing link. 1,3 To understand the origin of this discrepancy it is first necessary to build a complete and accurate theoretical description of IBr able to reproduce the experimental data. In particular, the time-scale for the control is much shorter in the simulations, with the simulations showing that the photodissociation reaction is over in around 100 fs while the experiments show a control effect out to 500 fs. Computer simulations using a simple model for the coupled potentials have supported the interpretation of the mechanism, but questions remain. The control pulse separates the states avoiding the dissociation in the lower channel, producing an inversion of the dissociation ratio. The crossing modified by the pulse appears between two excited spin–orbit states that dissociate to two different dissociation channels. 1 Introduction The strong-field control of the photodissociation of IBr by Stolow and co-workers 1,2 is a classic example of affecting the outcome of a chemical reaction by modifying the potential curves near a curve crossing using the non-resonant dynamic Stark effect. However, just like earlier results, this restricted model is not able to reproduce the timescale of the control. Preliminary dynamics calculations with the field placed along the molecular axis show that a Hamiltonian including all 36-states agrees with earlier results and is able to model the basic features of the control. ![]() These surfaces will enable full calculations of the molecule in the pump-control field. Both the strength and direction of field have been taken into account and it is seen how the avoided crossing between the states thought to be key in the control mechanism shift as a function of field strength. Potential surfaces for the complete spin–orbit manifold of IBr states dissociating into the ground and first excited states of the constituent atoms have been calculated at the multi-reference configuration interaction (MRCI) level of theory as a function of applied field. In this paper, we revisit the control of this molecule. In a seminal work the photodissociation of IBr has been controlled using a strong non-resonant IR pulse, changing the branching ratio of products in different final states via the relative timing of pump and control pulses. ![]()
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