Anomalous Resonant Ultrafast Recombination Driven by Physisorption of O₂ on MoS₂

Photoexcited carrier lifetimes are central to two-dimensional optoelectronics and gas sensors, yet the dynamical impact of ambient adsorbates remains unclear beyond static electronic signatures. Using decoherence-corrected ab initio nonadiabatic molecular dynamics, we investigate the unexpected process whereby the adsorption of a single O_2 molecule can counterintuitively regulate electron-hole recombination in monolayer MoS_2, with weakly physisorbed triplet O_2 accelerating recombination more strongly than chemisorption. All adsorption motifs shorten the lifetime, but via distinct pathways. Despite negligible lattice distortion and charge transfer, physisorbed triplet O_2 collapses the lifetime from 8.9 ns (pristine) to 359 ps, much exceeding the effect of chemisorbed O_2 at the sulfur vacancy (2.8 ns). This acceleration originates from high-frequency vibronic modes activated by physisorbed O_2, which enhance nonadiabatic coupling between band-edge states and open an efficient resonance-mediated energy-dissipation channel. In contrast, chemisorbed O_2 is structurally locked and follows a slower, trap-assisted Shockley-Read-Hall pathway mediated by shallow states near the valence-band edge. These results indicate that high-frequency vibronic coupling can render physisorption dynamically consequential, offering microscopic insight into light-enhanced sensing in MoS_2-based gas detectors and electronic devices operating in gas environments.