Near-infrared high resolution overtone spectroscopy of the hydronium ion H${}_3$O${}^{+}$: the $2{\nu }_3^{2+}$ and $2{\nu }_3^{2-}$ bands

This work presents the rovibrational spectra of the two strongest first overtone bands of the asymmetric stretching mode ${\nu }_3$ ($2{\nu }_3^{2+}$ and $2{\nu }_3^{2-}$) of the hydronium ion, H${}_3$O${}^{+}$. The measurements were performed in a temperature-variable cryogenic 22 pole ion trap using leak-out spectroscopy (LOS), covering an energy range of $6750-6950$ cm${}^{-1}$. The spectra were fit with a standard oblate symmetric top Hamiltonian and additional off-diagonal matrix elements accounting for $l$-doubling to obtain the spectroscopic constants and the band origins, which were determined to be $6845.610(14)$ cm${}^{-1}$ for $2{\nu }_3^{2+}$ and $6878.393(13)$ cm${}^{-1}$ for $2{\nu }_3^{2-}$.