The H2+ + HD reaction at low collision energies: H3+/H2D+ branching ratio and product-kinetic-energy distributions

Abstract

The fully state-selected reactions between H₂⁺ molecules in the X^{+ 2}Σ_g⁺(v⁺ = 0, N⁺ = 0) state and HD molecules in the X ¹Σ_g⁺(v = 0, J = 0) state forming H₃⁺ + D and H₂D⁺ + H have been studied at collision energies E_coll between 0 and k_B·30 K with a resolution of about 75 mK at the lowest energies. H₂ molecules in a supersonic beam were prepared in Rydberg-Stark states with principal quantum number n = 27 and merged with a supersonic beam of ground-state HD molecules using a curved surface-electrode Rydberg-Stark decelerator and deflector. The reaction between H₂⁺ and HD was studied within the orbit of the Rydberg electron to avoid heating of the ions by stray electric fields. The reaction was observed for well-defined and adjustable time intervals, called reaction-observation windows, between two electric-field pulses. The first pulse swept all ions away from the reaction volume and its falling edge defined the beginning of the reaction-observation window. The second pulse extracted the product ions toward a charged-particle detector located at the end of a time-of-flight tube and its rising edge defined the end of the reaction-observation window. Monitoring and analysing the time-of-flight distributions of the H₃⁺ and H₂D⁺ products in dependence of the duration of the reaction-observation window enabled us to obtain information on the kinetic-energy distribution of the product ions and determine branching ratios of the H₃⁺ + D and H₂D⁺ + H reaction channels. The mean product-kinetic-energy release is 0.46(5) eV, representing 27(3)% of the available energy, and the H₃⁺ + D product branching ratio is 0.225(20). The relative reaction rates correspond closely to Langevin capture rates down to the lowest energies probed experimentally (≈k_B·50 mK).