Optical nano cavities: a control knob for tuning photo-chemical reactions on a quantum level

Researchers at Fysikum have theoretically investigated the light activated hydrogen abstraction reaction of pyrrole in an optical nano cavity. The electronic excitation of pyrrole with UV light triggers the hydrogen detachment reaction along the NH bond. Their latest paper investigates how strong light-matter coupling of a tightly confined electromagnetic mode can be used to steer this reaction. The results suggest that the cavity can hugely influence the reaction efficiency and open up alternative reaction pathways.

The principle of strong coupling with confined light fields has been proposed five decades ago by Jaynes and Cummings. It has been successfully applied to atomic systems ever-since and resulted in a Nobel prize for Serge Haroche in 2012. Approximately around
the same time a team from France Thomas Ebbesen demonstrated that optical nano-cavitiescan modify the photo-chemistry of molecules opening up a new research area. The strong light-matter coupling greatly mixes the electronic degrees of freedom with the vibrational
modes and the photon mode to form hybrid field-matter states called polaritons. The manipulation of molecular potential energy surfaces by means of optical cavities is intriguing and opens up new ways of thinking about manipulating chemical reactions on a atomic level.

The top part of the figure illustrates the system where pyrrole embedded inside an optical cavity. The bottom part represents the ground and excited state potential energy surfaces of pyrrole along NH dissociation.

The investigated reaction in pyrrole is a good model system to understand how nano-cavities may be used to control more complex photo-chemical reactions. In the natural photo dissociation process, the molecule experiences a so-called conical intersection, which acts like a
funnel for the excitation. Pyrrole on the excited state dissociates completely within 200 fs. The main question that M. Gudem and M. Kowalewski have investigated is, if this photo reaction can be altered using the virtual photons from the light field. The strong light-matter couplings generated by the optical cavity create additional conical intersections along with the intrinsic CI in pyrrole. By varying the cavity frequency and the field strength, the dissociation barrier and the energy gap at the newly formed conical intersection can be modified. Consequently, the photolysis reaction dynamics of pyrrole can be manipulated. Depending on the cavity parameters, the hydrogen elimination reaction can either be accelerated to the lifetime of 10 fs
or suppressed to the lifetime of 1.5 ps.

For more details, read here: https://pubs.acs.org/doi/abs/10.1021/acs.jpca.0c09252

Scientific Contact:
Markus Kowalewski, markus.kowalewski@fysik.su.se
Mahesh Gudem, mahesh.gudem@fysik.su.se

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