Optical Control of Reactions between Water and Laser-Cooled Be Ions.

We investigate reactions between laser-cooled Be ions and room-temperature water molecules using an integrated ion trap and high-resolution time-of-flight mass spectrometer. This system allows simultaneous measurement of individual reaction rates that are resolved by reaction product. The rate coefficient of the Be(S) + HO → BeOH + H reaction is measured for the first time and is found to be approximately two times smaller than predicted by an ion-dipole capture model. Zero-point-corrected quasi-classical trajectory calculations on a highly accurate potential energy surface for the ground electronic state reveal that the reaction is capture-dominated, but a submerged barrier in the product channel lowers the reactivity. Furthermore, laser excitation of the ions from the S ground state to the P state opens new reaction channels, and we report the rate and branching ratio of the Be(P) + HO → BeOH + H and HO + Be reactions. The excited-state reactions are nonadiabatic in nature.