  Trapped Be+ ions are a leading platform for quantum information science [1],
but reactions with background gas species, such as H2 and H2O, result in qubit
loss. Our experiment reveals that the BeOH+ ion is the final trapped ion
species when both H2 and H2O exist in a vacuum system with cold, trapped Be+.
To understand the loss mechanism, low-temperature reactions between
sympathetically cooled BeD+ ions and H2O molecules have been investigated using
an integrated, laser-cooled Be+ ion trap and high-resolution Time-of-Flight
(TOF) mass spectrometer (MS) [2]. Among all the possible products,BeH2O+,
H2DO+, 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 + H2DO+ and D + BeH2O+, 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+ + H2 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.
