Trapped Fragments Reshape Ultrafast Photoinduced Surface Reaction Dynamics and Pathways on TiO2(110)

Understanding how intermediate species form, evolve, and interact with semiconductor surfaces is essential for controlling heterogeneous photoinduced reactions. Here, we employ a combined time-of-flight mass spectrometry and femtosecond laser spectroscopy approach, coupled with density functional theory, to unravel the photoinduced reaction mechanism of CH3I, used as a model system, on TiO 2 (110). Direct detection of intermediates and final products reveals that on a freshly prepared surface, a fraction of the photogenerated fragments becomes trapped, passivating reactive sites. On this fragment-decorated surface, CH 3 radicals react with undissociated CH 3 I to form surface-bound CH 3 ICH 3 species, an intermediate stabilized exclusively by the modified TiO 2 (110) interface. The presence of CH 3 ICH 3 profoundly alters the ultrafast surface dynamics, giving rise to coherent oscillations of the CH 3 + transient signal with a ~100 fs periodicity, attributed to the dissociation of CH 3 ICH 3 through an intermediate vibrational exited bound electronic state. The concurrent reformation dynamics of the CH 3 I + signal indicates two parallel reaction channels: recombination of CH 3 and I fragments, and dissociation of CH 3 ICH 3 . Density functional theory calculations support these observations by describing the formation and vibrational modes of CH 3 ICH 3 at the TiO 2 (110) surface. This study demonstrates that fragment-induced surface modification can dynamically reshape potential energy landscapes, opening new reaction pathways and altering ultrafast surface reactivity, a principle broadly relevant to photocatalysis, surface photochemistry, and semiconductor interfaces.