We explore by theory and experiment the effects of spin-orbit coupling (SOC) on the pair creation of electrons by spin-polarized primary electrons incident on a pseudomorphic monolayer of Au on Ir(111). An ab-initio calculation of the electronic structure reveals a Rashba-type sp-like surface state in the Au layer, which turns out to lead to a large electron pair creation intensity. The distribution of this intensity over the momenta of the emitted electrons depends strongly on the primary electron spin due to spin-orbit coupling mainly in the incident and the outgoing electron states. For normal incidence, the six-fold rotation symmetry, which would hold without spin-orbit coupling, is broken in a manner depending on the orientation of the primary electron spin.