Proton Tunneling: A Decay Channel of the O-H Stretch Mode in ${\mathrm{KTaO}}_3$

The vibrational lifetimes of the O-H and O-D stretch modes in the perovskite oxide ${\mathrm{KTaO}}_3$ are measured 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 7 orders of magnitude larger than from the ground state in the proton conducting oxide, ${\mathrm{BaCeO}}_3$ [Phys. Rev. B 60, R3713 (1999)].

Figure 1

The ionic configuration around the proton in a perovskite oxide. (a) The proton moves throughout the oxygen octahedral network. (b) The potential energy diagram of the hydrogen atom, tunneling from one well into another. The vibrational ground and first excited state of the O-H stretch mode are shown. The red or middle arrows in the inset correspond to this stretching motion; the blue or outer arrows indicate the O-O assist vibration.

Figure 2

Transient bleaching signal $S_b$ from the O-H and O-D stretch mode as a function of time.

Figure 3

Temperature dependence of the O-H and O-D stretch mode lifetimes.

Figure 4

Experimental proton-tunneling rate, $k_{\mathrm{PT}}$, as a function of inverse temperature is shown along with a fit Eq. (1).