TRACKING COMPLEX PHOTOINDUCED DYNAMICS FROM ATMOSPHERIC MOLECULES AND AEROSOLS BY TIME-RESOLVED SPECTROSCOPY

Chemical reactions associated with trace gases and aerosols contribute significantly to biogeochemical cycles, climate, and pollution in the atmosphere. During the daytime, sunlight's illumination imposes energy into the atmosphere and subsequently initiates the associated photochemistry. In the gas phase and embedding in the aerosol phase, the photoexcited molecules involve various ultrafast intra-molecular and inter-molecular decays for transferring the excess energy. The light-induced ultrafast nonradiative decays, leading to different transient states, reactive products, or different isomers, may influence the subsequent chemical reactions. Therefore, studying the complex nonradiative processes in isolated and solvated circumstances paves a route to understanding photochemistry in the atmosphere. Notably, the aerosol surface serves as an interface connecting the gas and the solvation environments and comprises distinctive physicochemical properties and compositions.
These experiments aim to disentangle the complex, interwoven nonradiative processes from atmospherically relevant molecules utilizing time-resolved spectroscopy. The target mechanisms to be studied involve (1) the roaming (and/or isomerization) pathway and (2) dynamic chirality. The roaming mechanism bypasses the conventional transition state and alters the ratio of the radical to molecular products. Tracking the roaming mechanism in the time domain will directly probe this dynamic behavior, complement previous frequency-domain and theoretical studies, and help decode competing pathways (e.g., isomerization). Besides, exploring the photoinduced isomerization in the time-domain helps realize the nonadiabatic coupling between the molecule's multi-electronic states. Furthermore, the dynamic chirality of molecules (chiral or prochiral) during the nonradiative process in the chiral medium (e.g., circularly polarized light or chiral solvent) may be relevant to their homochirality in nature. Probing the dynamic chirality in the time domain will understand how the asymmetry interactions affect the molecule's transient stereodynamics.