Capturing the hydrogen radical is of central importance in various systems ranging from catalysis to biology to astronomy, but it has been proven to be challenging experimentally because of its high reactivity and short lifetime. Here, neutral MO 3 H 4 (M = Sc, Y, La) complexes were characterized by size-specific infrared-vacuum ultraviolet spectroscopy. All these products were determined to be the hydrogen radical adducts in the form of H•M(OH) 3 . The results indicate that the addition of the hydrogen radical to the M(OH) 3 complex is both thermodynamically exothermic and kinetically facile in the gas phase. Moreover, the soft collisions in the cluster growth channel with the helium expansion were found to be demanded for the formation of H•M(OH) 3 . This work highlights the pivotal roles played by the soft collisions in the formation of hydrogen radical adducts and also opens new avenues toward the design and chemical control of compounds.

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Experimental characterization of microscopic events and behaviors of SO2–H2O interactions is crucial to understanding SO2 atmospheric chemistry but has been proven to be very challenging due to the difficulty in size selection. Here, size-dependent development of SO2 hydrate structure and cluster growth in the SO2(H2O) n (n = 1–16) complexes was probed by infrared spectroscopy based on threshold photoionization using a tunable vacuum ultraviolet free electron laser. Spectral changes with cluster size demonstrate that the sandwich structure initially formed at n = 1 develops into cycle structures with the sulfur and oxygen atoms in a two-dimensional plane (n = 2 and 3) and then into three-dimensional cage structures (n ≥ 4). SO2 is favorably bound to the surface of larger water clusters. These stepwise features of SO2 hydration on various sized water clusters contribute to understanding the reactive sites and electrophilicity of SO2 on cloud droplets, which may have important atmospheric implications for studying the SO2-containing aerosol systems.