UV-induced dynamics of isoxazole and oxazole investigated using time-resolved Coulomb Explosion Imaging

Ring-puckering and ring-opening mechanisms play pivotal roles in the photophysics and photochemistry of heterocyclic compounds. In this work, we use a 200-nm ultraviolet pulse to excite the pentacyclic ring molecules isoxazole and oxazole into their first and/or second excited states, initiating ultrafast nonadiabatic transitions leading to a variety of molecular dynamics, including ring opening, ring puckering, and dissociation. Subsequently, an intense 800-nm near-infrared pulse is employed to ionize the molecules to highly charged states, leading to Coulomb explosion into multiple ionic products. These products are detected in coincidence, and their asymptotic momentum is obtained using a COLTRIMS apparatus. The transient molecular geometries probed by Coulomb explosion imaging can be visualized by a timed sequence of Newton plots, illustrating structural changes as a function of pump-probe delay. By scrutinizing the two isomers, we show that the position of nitrogen and oxygen atoms in the pentacyclic ring has an effect on the excited state relaxation dynamics. This result is complemented by preliminary data on ultrafast electron diffraction probe of the same dynamics. Both experimental datasets are compared with simulations using trajectories obtained from ab initio calculations to elucidate the dynamics.