CO2 molecule as a quantum realization of the 1 : 1 : 2 resonant swing-spring with monodromy

We consider the wide class of systems modeled by an integrable approximation to the 3 degrees of freedom elastic pendulum with 1∶1∶2 resonance, or the swing-spring. This approximation has monodromy which prohibits the existence of global action-angle variables and complicates the dynamics. We study the quantum swing-spring formed by bending and symmetric stretching vibrations of the CO2 molecule. We uncover quantum monodromy of CO2 as a nontrivial codimension 2 defect of the three dimensional energy-momentum lattice of its quantum states.