We present momentum resolved measurements of quasielastic helium atom scattering made using a new 3He spin-echo spectrometer. Our data for the dynamics of CO on Cu(001) indicates an activated jump mechanism which we analyze in detail using molecular dynamics simulations. A nearly isotropic potential energy surface is found with an average barrier height of approximately 125 meV, yielding comparable hopping rates along both the <110> and <100> directions. The measurements provide the first rigorous experimental test of state-of-the-art first-principles calculations previously made on this system.