Arthémise Altman, Frank M J Cozijn, Alexandr S Bogomolov, Roland Tóbiás, Attila G Császár, Wim Ubachs, Clément Lauzin
A novel spectroscopic approach combining wavelength-modulated Noise-Immune Cavity-Enhanced Optical-Heterodyne Molecular Spectroscopy (wm-NICE-OHMS) and two-photon absorption (TPA) is employed to directly determine the energy of the 4ν3 vibrational band origin in H216O from a single spectroscopic transition. The resulting excitation energy, E/h = 435 823 319 572.0 ± 4.6 kHz, is 4 orders of magnitude more accurate than previous determinations based on conventional spectroscopy (35 MHz uncertainty). This measurement provides a stringent benchmark for first-principles calculations in the third stretching overtone region of water. In addition, TPA enables direct connections between opposite-parity states within a vibrational manifold, thereby strengthening a recently established, high-accuracy spectroscopic network of H216O.