Using a simple classical model, termed the tilted projection picture 7 for designing and interpreting the measurements, ultra-high resolution measurements of surface phonons have been used to measure the low energy part of dispersion curves, anharmonicity, and phonon lifetimes of a metal surface. 5 The main application of the HSE technique is for quasi-elastic scattering studies used to detect and characterise surface diffusion, 6 however, it can also be used to study inelastic scattering in general, and surface phonons in particular. Unlike TOF measurements, within a first order approximation, the velocity spread of the incident beam does not broaden the spectrum, which leads to a dramatic improvement in energy resolution, especially when using beams with a relatively wide energy spread, allowing reconstruction of phonon spectra in the μeV range. HSE uses 3He beams which have a nuclear spin I = ½, and similarly to neutron spin-echo, the precession of the spin in magnetic fields before and after scattering offers an alternative way for measuring energy changes due to the scattering event. 3,4Ī way to circumvent the incident energy broadening effect is offered by the helium spin-echo (HSE) technique. Other contributions broadening the spectrum further are related to the chopper width, detector time response and other experimental restrictions, all of which have been minimised over the years by optimising TOF instruments. 1 Nevertheless, in various circumstances, for example when trying to separate two phonon modes which are close in energy, when trying to separate a very low energy mode from the elastic peak, or when we are attempting to measure the true linewidth of a phonon, the fact that the resolution of TOF experiments is on the order of a fraction of a meV can become a significant problem. This restriction has been addressed by improving supersonic expansions, where helium beams with very narrow energy distributions have been successfully produced and relatively high resolution spectra were obtained from numerous surfaces. One of the fundamental restrictions on the achievable resolution in TOF experiments is the velocity spread of the incident beam, since the measured spectra are a convolution of the real excitation spectra with the energy distribution of the beam. While a great deal of our understanding of low energy phonon modes is the result of HAS TOF experiments, this technique has its limitations. Helium atoms are an excellent choice for studying surface phonons, and the majority of helium atom scattering (HAS) experiments which characterised phonons and their dispersion curves were performed using the time-of-flight (TOF) technique. 2 Surface phonons often provide an important channel for this energy exchange, making them particularly important to investigate. 1 Furthermore, essentially any surface chemistry process is affected, and in many cases dominated, by the ability of molecules to exchange energy with the surface. Introduction Inelastic scattering measurements using atomic and molecular beams provide unique insight into the atomic scale dynamics of surfaces. A classical approach, analogous to that used for helium spin-echo, explains the most dominant excitation peaks measured, whereas a semi-classical approach allows us to identify smaller peaks which are related to the complexity of the multiple spin-rotation states which exist for molecules. Two analysis methods for these molecular spin echo experiments are presented. Similarly to helium spin-echo, experiments can be performed along optimal tilted projections making it possible to resolve energy peaks with a high energy resolution which is not restricted by the spread of energies of the incident beam. Measurements are presented for inelastic scattering of D 2 from a Cu(111) surface. The method extends the principles of 3He spin-echo spectroscopy, to the more complex case of a molecular beam exchanging energy with the surface. A new method to measure surface phonons with a molecular beam is presented.

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