Conserving Coherence and Storing Energy during Internal Conversion: Photoinduced Dynamics of cis- and trans-Azobenzene Radical Cations.

Light harvesting via energy storage in azobenzene has been a key topic for decades and the process of energy distribution over the molecular degrees of freedom following photoexcitation remains to be understood. Dynamics of a photoexcited system can exhibit high degrees of nonergodicity when it is driven by just a few degrees of freedom. Typically, an internal conversion leads to the loss of such localization of dynamics as the intramolecular energy becomes statistically redistributed over all molecular degrees of freedom. Here, we present a unique case where the excitation energy remains localized even subsequent to internal conversion. Strong-field ionization is used to prepare cis- and trans-azobenzene radical cations on the D surface with little excess energy at the equilibrium neutral geometry. These D ions are preferably formed because in this case D and D switch place in the presence of the strong laser field. The postionization dynamics are dictated by the potential energy landscape. The D surface is steep downhill along the cis/trans isomerization coordinate and toward a common minimum shared by the two isomers in the region of D/D conical intersection. Coherent cis/trans torsional motion along this coordinate is manifested in the ion transients by a cosine modulation. In this scenario, D becomes populated with molecules that are energized mainly along the cis-trans isomerization coordinate, with the kinetic energy above the cis-trans interconversion barrier. These activated azobenzene molecules easily cycle back and forth along the D surface and give rise to several periods of modulated signal before coherence is lost. This persistent localization of the internal energy during internal conversion is provided by the steep downhill potential energy surface, small initial internal energy content, and a strong hole-lone pair interaction that drives the molecule along the cis-trans isomerization coordinate to facilitate the transition between the involved electronic states.