Proton Tunneling: A Decay Channel of the O-H Stretch Mode in $\mathrm{KTaO}_{3}$

Proton vibrational dynamics play a key role in the important processes of hydrogen diffusion and transport. In particular, perovskite structured proton conductors are an important class of hydrogen transport materials with a wide range of potential applications. We have measured for the first time the vibrational lifetimes of the O-H and O-D stretch modes in the perovskite oxide, $\mathrm{KTaO}_{3}$, by pump-probe infrared spectroscopy. Both stretch modes are exceptionally long lived and exhibit a large ``reverse'' isotope effect, due to a phonon-assisted proton tunneling process, which involves the O-Ta-O bending motion. The excited-state tunneling rate is found to be seven orders of magnitude larger than from the ground state in the proton conducting oxide, $\mathrm{BaCeO}_{3}$ [1]. [1] I. Kuskovsky et al., Phys. Rev. B \textbf{60}, R3713 (1999).