Helium spin-echo spectroscopy: studying surface dynamics with ultra-high-energy resolution

Spin-echo measurements offer a major step forward in the technique of helium atom surface scattering. The experimental energy transfer resolution has been improved by several orders of magnitude, and has resulted in an expansion in the range of surface systems which can be studied. We describe the measurements within a double Fourier transform formalism, which by using appropriately selected 'tilted projections' for measurement allows elastic, inelastic and quasielastic measurements to be treated on an equal footing. We illustrate the wide scope of application for helium spin-echo with measurements from each of these areas.