Ultrafast probing of coupled rotational and electronic dynamics within a single molecule

The coupled electronic and nuclear motions within a single molecule are ubiquitous in physics and chemistry, which determine the dominant pathways of energy transfer and charge transfer processes in nearly all chemical reactions. Thus, probing coupled electron-nuclear dynamics on ultrafast time scales is essential not only for our understanding of numerous fundamental processes in physics and chemistry, but also for controlling chemical reactions at the molecular level. However, previous studies about probing electron-nuclear coupling interactions were limited to the coupling of electronic and vibrational degrees of freedom. Compared to vibrational motion, rotational motion occurs on a slower time scale, which is typically five or six orders of magnitude larger than the natural time scale of electron motion. Thus the rotational dynamics can hardly be intertwined with the electronic dynamics. Up to now, observing the direct interactions between the electronic and rotational degrees of freedom has remained unreached.

Recently, the collaborative research team from Huazhong University of Science and Technology and the Institute of Applied Physics and Computational Mathematics reported on a joint experimental and theoretical study of the coupled rotational and electronic dynamics during molecular fragmentation. Using a time-resolved Coulomb explosion imaging method, they traced the evolution of the transient electronic structure of a molecule for the internuclear distance up to 40 a.u.  For such large internuclear distance, they found that Coriolis coupling, i.e., the coupling of molecular rotational angular momentum and electronic axial angular momentum, plays a dominant role, and this process lasts a long time of several hundreds femtoseconds. The work has been published in Ultrafast Science.

To observe the coupled rotational and electronic interactions, a novel time-resolved Coulomb explosion imaging method by controlling the polarizations of the pump and probe laser pulses was used. In contrast to previous methods, this method can be used for a large internuclear distance of the molecule, which is the basis for revealing the coupling interaction between the electronic and rotational degrees of freedom. Moreover, this method can simultaneously obtain the information about the electronic structure and the internuclear distance. Using this method, the team traced the evolution of the electronic structure of a dissociating molecule with the internuclear distance up to 40 a.u., which offers insightful information about how the Coriolis coupling correlates the electronic structures during the transition from a molecule to atoms, as shown in Fig. 1.

To understand the experimental observation, the team further performed quantum time-dependent wave packet simulations for the dissociation process. A good agreement between the measurement and the simulation was achieved only when the Coriolis coupling interaction is included in the simulation. This demonstrated that the nonadiabatic Coriolis coupling plays a significant role during the cleavage of chemical bond, which leads to electronic transitions between adjacent electronic states. The team found that the Coriolis coupling strength depends sensitively on the nuclear rotational angular momentum N, i.e., the Coriolis coupling strength is small for small N. As a result, the electronic transition rate is small for small N. Thus the Coriolis coupling process lasts a long time of several hundred femtoseconds.

The joint experimental and theoretical study demonstrated that one of the most fundamental processes in chemical reaction—the cleavage of a chemical bond doesn't occur at a certain moment. Instead, it lasts for a long time of several hundred femtoseconds due to the Coriolis coupling interaction (Similar to “lotus root with broken threads”). This provides the foundation for our understanding of many fundamental physical and chemical processes, which also paves a way towards controlling chemical reactions at the molecular level using strong laser pulses.