Spectroscopy of the H⋯H + and D⋯D + charge/induced-dipole complexes

The first excited electronic state of the hydrogen molecular ion is essentially repulsive, but exhibits a shallow potential well with a minimum at 6.64 Å because of the long-range H⋯H+ charge/induced-dipole interaction. This well supports a small number of stable vibration–rotation levels, and we have observed an infrared electronic spectrum of the D+2 ion arising from transitions to the D⋯D+ levels from high-lying vibration–rotation levels of the ground state. We have also detected two microwave electronic transitions in D+2 and one in H+2, thereby confirming the existence of these long-range states. This paper describes the principles, experimental techniques and interpretation of these studies.