Mode-specific low barrier tunneling dynamics in the à state of formaldehyde: The ν1 fundamental and ν4 + ν5 combination levels.

We present a joint experimental and theoretical study of mode-specific tunneling splittings in the à state of formaldehyde. We report the first observation of the symmetric CH stretch fundamental level (ν1'), and of the out-of-plane bend with the antisymmetric CH stretch combination level (ν4'+ν5') including rotational analysis, using infrared-ultraviolet double-resonance laser-induced fluorescence spectroscopy. The experimental results agree very well with full-dimensional ab initio quasi-variational discrete variable representation calculations on a frozen core equation of motion coupled cluster (EOM-CCSDT/ANO1) surface. We compare our findings with various models, including effective potentials, a semiclassical approach, and a quasidiabatic vibronic coupling treatment, to derive insights into the dynamics and coupling between different degrees of freedom in the vicinity of the tunneling barrier.