Determining reaction pathways at low temperatures by isotopic substitution: the case of BeD + + H2O

Abstract

Trapped Be ^+ ions are a leading platform for quantum information science (Gaebler et al 2016 Phys. Rev. Lett. 117 060505 117 117 060505 ), but reactions with background gas species, such as H _2 and H _2 O, result in qubit loss. Our experiment reveals that the BeOH ^+ ion is the final trapped ion species when both H _2 and H _2 O exist in a vacuum system with cold, trapped Be ^+ . The BeH ^+ product in the Be ^+ + H _2 reaction further reacts with H _2 O to form BeOH ^+ . To understand the loss mechanism, low-temperature reactions between sympathetically cooled BeD ^+ ions and H _2 O molecules have been investigated using an integrated, laser-cooled Be ^+ ion trap and high-resolution time-of-flight mass spectrometer (Schneider et al 2014 Phys. Rev. Appl. 2 034013 2 2 034013 ). Among all the possible products, BeH _2 O ^+ , H _2 DO ^+ , BeOD ^+ , and BeOH ^+ , only the BeOH ^+ molecular ion was observed experimentally, with the assumed co-product of HD. Theoretical analyses based on explicitly correlated restricted coupled cluster singles, doubles, and perturbative triples (RCCSD(T)-F12) method with the augmented correlation-consistent polarized triple zeta (AVTZ) basis set reveal that two intuitive direct abstraction product channels, Be + H _2 DO ^+ and D + BeH _2 O ^+ , are not energetically accessible at the present reaction temperature (∼150 K). Instead, a double displacement BeOH ^+ + HD product channel is accessible due to a large exothermicity of 1.885 eV through a submerged barrier in the reaction pathway. While the BeOD ^+ + H _2 product channel has a similar exothermicity, the reaction pathway is dynamically unfavourable, as suggested by a sudden vector projection analysis. This work sheds light on the origin of the loss and contaminations of the laser-cooled Be ^+ ions in quantum-information experiments.

Keywords

• trapped ions
• quantum information
• cold reactions