Light-induced Torque on Moving Atoms

We show that a two-level atom moving in a Laguerre-Gaussian beam is subject to a light-induced torque, $T$, about the beam axis which is directly proportional to $l$, the orbital angular momentum quantum number of the mode. The torque, which has a Lorentzian frequency response which includes an azimuthal $l$-dependent Doppler shift in the detuning term, reduces in the saturation limit to the simple form $T\approx \hslash l\Gamma$, where $2\Gamma$ is the decay rate of the excited state.