Direct measurement of bending conformations in triatomic dihydride ions

The distributions of bond angles in the triatomic dihydride ion series CH₂⁺, NH₂⁺, and H₂O⁺ have been studied using the Coulomb explosion imaging method. These distributions were measured as a function of the cooling of the internal degrees of freedom of these ions. The distribution for the coldest sample of CH₂⁺ molecules shows the most probable structure to be bent with substantial tunneling through the linear conformation. The most probable geometry for NH₂⁺ was found to be linear, though the angular distribution is significantly different from a shape of a harmonic-oscillator ground-state prediction. In the case of H₂O⁺, we find a bent structure as expected from theory. Evidence for a linear excited state in H₂O⁺ is seen in the hotter distributions. Comparison to the adiabatic theoretical predictions shows good agreement with the most probable geometries. However, the measured distributions are systematically wider than the squared vibronic wave functions derived from the corresponding potential-energy surfaces.