  Spin and orbital angular momentum of an optical beam are two independent
parameters that exhibit distinct effects on mechanical objects. However, when
laser beams with angular momentum interact with plasmas, one can observe the
interplay between the spin and the orbital angular momentum. Here, by measuring
the helical phase of the second harmonic 2{\omega} radiation generated in an
underdense plasma using a known spin and orbital angular momentum pump beam, we
verify that the total angular momentum of photons is conserved and observe the
conversion of spin to orbital angular momentum. We further determine the source
of the 2{\omega} photons by analyzing near field intensity distributions of the
2{\omega} light. The 2{\omega} images are consistent with these photons being
generated near the largest intensity gradients of the pump beam in the plasma
as predicted by the combined effect of spin and orbital angular momentum when
Laguerre-Gaussian beams are used.
