Knowledge Bank

The structure of the $H2O+$, $D2O+$, and $HDO+$ ions in their three lowest electronic states $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}B1$, $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}A1$, and $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}B2$, has been investigated by means of photoelectron spectroscopy. The resolution of these spectra is sufficient to partially resolve rotational structure in the ground ionic state, $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}B1$, and K-type rotational structure in the first excited ionic state, $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}A1$. It is shown that these two state coalesce into one doubly degenerate state of $\Pi$ symmetry in the linear ion. Similarities in the structure of the ion in its ground state and the molecule in its first Rydberg state are pointed out. The transition to the first excited ionic state exhibits an unusual type of vibronic structure. Successive levels of the bending vibration consist alternately of $\Sigma$, $\Delta$, $\Gamma$, ... and $\Pi$, $\Phi$, ... vibronic sub-levels with large vibronic splittings. The origins of the vibronic sub-bands may be represented by the formula $\nu 0K=\nu 0-GK2$, where G is $-25$ $cm-1$ for $H2O+$ and $\sim$ $18cm-1$ for $D2O+$. The configuration of the ion in this $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}A1$ state is found to be non-linear with a very low potential barrier at the linear configuration. Only the first vibrational level of $H2O+$ lies below this barrier. The vibrational structure observed in the transition to the second excited ionic state, $\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}B2$, is complex. All three vibrational modes of the ion are excited in this transition.