Cold Chemistry Probes Isotope Effects in the Be⁺ + HD Reaction

Cold chemical reactions represent exquisite probes of quantum-mechanical phenomena in reaction dynamics. Here, we explore isotope effects in the full quantum-state-controlled reaction between laser-cooled Be⁺ ions in the ²P_3/2 excited state and rovibrationally ground-state HD molecules at 5.7 K in a cryogenic ion trap. Using secular excitation mass spectrometry, we directly measure the product branching ratio of BeD⁺ to BeH⁺, yielding a value of 4.1 ± 0.2. The strong preference for BeD⁺ formation reveals the dominant role of long-range interactions in governing reaction dynamics on the underlying potential energy surface at cryogenic conditions. These detailed experimental benchmarks are crucial for refining theoretical potential energy surfaces and advancing our quantitative understanding of reaction dynamics in the cold regime. Our approach provides a platform for quantum-state-resolved investigations of ion-neutral reactions, with potential relevance to cold chemistry and astrochemical modeling.