Directly measure the ground-state acceptor-switching splitting in (D2O)2

Determine the direct experimental value of the acceptor-switching tunneling splitting between the "1s" and "2s" rotation–tunneling manifolds in the ground vibrational state (Ka = 0) of the deuterated water dimer (D2O)2, which is currently only estimated to be approximately 53 GHz (1.77 cm−1) without a direct measurement.

Background

In the water dimer, acceptor switching is the largest tunneling motion and splits each rotational level into two sublevels labeled "1s" and "2s." Accurate knowledge of this splitting in the ground vibrational state is fundamental for modeling the rotation–tunneling structure and for benchmarking theoretical treatments of tunneling dynamics.

The authors assume a value of exactly 53.0 GHz for convenience but note that the splitting has not been directly measured, indicating a clear experimental gap.