Laser-Controlled Ion–Molecule Reaction Kinetics: Be ⁺ + H ₂ O at Variable Reaction Temperatures

We investigate the Be⁺ + H₂O → BeOH⁺ + H reaction using a laser-cooled ion trap coupled with a high-resolution time-of-flight mass spectrometer (TOF-MS) over a range of reaction temperatures (E_col/k_B ≈ 127-416 K). By selectively preparing Be⁺ in either the Be⁺(²P_3/2) or Be⁺(²S_1/2) electronic states and controlling ion micromotion to tune the reaction temperature, we determine state-specific reaction rate coefficients. The measured rates for ground-state Be⁺(²S_1/2) exhibit a pronounced decrease with increasing reaction temperature, in good agreement with quasi-classical trajectory (QCT) calculations. In contrast, the rates for excited-state Be⁺(²P_3/2) decrease even more rapidly with increasing reaction temperature, deviating significantly from predictions by the classical capture model. This temperature-dependent behavior underscores the key role of a submerged reaction barrier in shaping the kinetics. Our measurements demonstrate the importance of experimental access to temperature-controlled ion-molecule reactions, revealing limitations of widely used capture theories.