Photoexcitation of atoms near the center of vortex light

We investigate the interaction of atoms with a Laguerre-Gaussian beam near the vortex center where an annular nodal area exists on and around the beam propagation axis. To grasp the essential features related to the orbital angular momentum transfer to a bound electron, we first show that it suffices to include in the interaction Hamiltonian the leading (second) term in the usual expansion of e−ik.r to be associated with the transverse (longitudinal) field component of a beam with orbital angular momentum ℓ=±1. We compare the obtained results for the quadrupole transition 4S1/2 to 3D5/2 in Ca+ with published experimental data. For orbital angular momenta |ℓ|>1, this approach predicts that quadrupole transitions are only possible for the configuration where the orbital angular momentum with |ℓ|=2 is antiparallel to the spin degree of freedom. These results are confirmed by another approach based on the multipolar Hamiltonian which, additionally, provides rich information on the emergence of different selection rules for arbitrary value of ℓ including their spatial dependence in the region of interest. This work also extends the discussion of the excitation of target atoms by twisted light to two-electron atomic systems by deriving selection rules for single-electron transitions.