Ro-vibrational spectroscopy of diatomic molecules using Laguerre-Gaussian beams

We consider a diatomic molecule, like HCl or CS, interacting with the electric field of a Laguerre-Gaussian beam. We derive the Hamiltonian of the system within the harmonic approximation for the vibrational modes of the molecule. We study the angular momentum exchange between the vortex beam, rotation of the molecule and its center-of-mass motion. We demonstrate analytically that the selection rules for the magnetic quantum number of the rotational state are determined by the orbital angular momentum of light. In particular, for Laguerre-Gaussian beams we predict novel transitions that are enabled by the non-zero vorticity of the electric field. These transitions can be resolved in state-of-the-art spectroscopy experiments by comparing the absorption spectra produced by a plane wave and a vortex beam in the presence of a Stark field. Our findings are relevant for a broad range of molecular spectroscopy measurements and emphasize the prospective benefits of using vortex beams.