Figure 9.
(a) Illustration of the linewidth broadening due to scattering of a plane wave from a moving molecule. The frequency of the incident wave changes upon scattering from the moving molecule (water), in analogy to the Doppler effect (exaggerated by the distance between the wavefronts). The wavelength distribution of the scattered waves is broadened (Δω) with respect to the nearly monochromatic incident wave with ω0. (b) Movement on a sample surface can be probed by scattering two wavepackets, spread by a time delay tSE. Upon recombination of the two scattered wavepackets a loss in correlation is measured due to a small Doppler broadening when scattering from moving adsorbates. The measured ISF shows an exponential decay in spin polarisation with tSE, from which the decay constant (scattering linewidth) is obtained.