Christina Sanz-Sanz: Molecular branching ratio using excitation and control pulses parallel to molecular axis

One of the main goals in chemistry is the control of chemical reactions. Laser pulses are one of the tools that we can use to modify our system, in order to obtain a different outcome [1]. The Stark effect is produced when a static field alters molecular states. When the field applied is time dependent the process is known as dynamic Stark effect (DSE). Of particular interest is the non-resonant dynamic Stark effect (NRDSE), in which the time dependent field is unable to produce photon excitation.

The laser pulse is used to shape the PES2. The control of the photo-dissociation of IBr using NRDSE has been studied experimentally by Stolow and coworkers [2]. Non-adiabatic processes, as intersystem crossings, entail charge rearrangements that occur along a reaction path at the intersections of PES. Chemical branching ratios in non-adiabatic processes are very sensitive to the intersection geometry, and therefore the dynamic modification of these processes is an important application of the NRDSE. The photo-dissociation of IBr is initiated by absorption of a visible photon, exciting the system from the ground electronic state to a state which has several crossings with states that could yield the system to different dissociation channels [3]. The application of an infrared NRDSE field can be used to modify the curve-crossing barrier at a specific time, promoting the yield of one dissociation channel over another.

In this study we show that a field that is parallel to the molecular axis, only yields the system to dissociation when the wavepacket is excited to the spin-orbit components of  states. The branching ratio obtained mimicking the experiments, with a laser field parallel to the molecular axis, shows a similar behavior comparing with the experiments.

[1] G. A. Worth, C. Sanz-Sanz, Phys. Chem. Chem. Phys. 12, 15570-15579 (2010).

[2] B. J. Sussman, D. Townsend, M.Y. Ivanov and A. Stolow, Science 314, 278 (2006).

[3] C. Sanz-Sanz, G. W. Richings and G. A. Worth, Faraday Discuss. 153, 275-291 (2011).