Empirical potential energy surface and bending angle probability densities for the electronic ground state of HCO+

Abstract

In molecular spectroscopy parlance, a linear molecule is defined as one whose potential energy minimum is at a linear geometry. The nature of the average structure of a linear triatomic molecule – linear or bent? – has given rise to much discussion recently. The expectation value of the bond angle (calculated, for example, with the ground vibrational state wavefunction) is less than 180° for a linear molecule, and so we contend that the average structure of a linear triatomic molecule is bent. The purpose of the present work is to underpin this argument with results from Coulomb Explosion Imaging experiments for the linear DCO+ ion; these experiments allow a direct experimental measurement of the bending angle probability density. We aim at simulating theoretically the CEI experiments for DCO+. The possibility of obtaining a detailed simulation is hampered by the fact that the DCO+ ions investigated in the CEI experiment populate very highly excited vibrational states. This creates numerical problems for the theoretical calculations. We can conclude, however, that the results of the CEI experiment are in support of the average structure of a linear molecule being bent.