Knowledge Bank

An electric-resonance optothermal spectrometer with frequency synthesizers and phase-locked backward-wave oscillators, a slit-jet molecular-beam with a $CO2$ laser difference-frequency microwave-sideband spectrometer, and a pulsed-nozzle Fourier-transform microwave spectrometer are used to investigate the nine mixed deuterated-protonated isotopomers of water dimer. The use of He and mixed He-Ne carrier gases allows the observation of the metastable Q-H bonded isotopomers, not observed in previous studies in Ar beams. Spectra are observed for the $K = 0 \leftarrow 0, $K and $K=1\leftarrow 0$ subbands. The spectra are complicated by the tunneling interchange of equivalent proton or deuterons and, for the two $(HOD)2$ isotopomers, by the tunneling interchange of the hydrogen-bond donor and acceptor roles of the two HOD moietics. To guide the assignments microwave or submillimeter - radiofrequency double resonance is used, together with the observation that the A, B, and C rotational constants and methyl-amine type tunneling splittings are nearly insensitive to isotopic substitution of the deuteron/proton in the hydrogen bond. The tunneling splittings are tunneling state selection rules furnish a critical test of proposed tunneling pathway in water dimer.