Theory of angular momentum transfer from light to molecules

We present a theory describing interaction of structured light, such as light carrying orbital angular momentum, with molecules. The light-matter interaction Hamiltonian we derive is expressed through couplings between spherical gradients of the electric field and the (transition) multipole moments of a particle of any non-trivial rotation point group. Our model can therefore accommodate for an arbitrary complexity of the molecular and electric field structure, and can be straightforwardly extended to atoms or nanostructures. Applying this framework to ro-vibrational spectroscopy of molecules, we uncover the general mechanism of angular momentum exchange between the spin and orbital angular momenta of light, molecular rotation and its center-of-mass motion. We show that the non-zero vorticity of Laguerre-Gaussian beams can strongly enhance certain ro-vibrational transitions that are considered forbidden in the case of non-helical light. We discuss the experimental requirements for the observation of these forbidden transitions in state-of-the-art spatially-resolved spectroscopy measurements.