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A two dimensional approach for measuring surface phonons using helium spin echo

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Helium spin echo is a powerful technique for measuring surface dynamics on atomic length-scales with high temporal resolution. Besides adsorbate diffusion, the technique can also be employed for studying inelastic processes, such as surface phonons and adatom vibrational modes. A unique property of Spin Echo is its ability to measure scattered intensity as a function of both incoming and outgoing wavelengths, known as the two-dimensional wavelength intensity matrix, containing all information about elastic and inelastic scattering processes. The approach is independent of incident beam energy and therefore attains ultra high energy resolution.

Here we present the first measurement of a two-dimensional wavelength intensity matrix, performed on the Rayleigh mode of a Cu(111) crystal of which the dispersion relation is well known. The conditions were chosen such that the phonon peak and elastically scattered intensity are very narrowly spaced, making it extremely difficult for other techniques to separate and identify the two peaks. In the matrix measured with the Spin Echo technique, two separate features are clearly visible, corresponding to the Rayleigh mode and elastically scattered intensity.

The main advantage of the 2D intensity matrix over traditional phonon measurements is that line shapes and peak positions can be obtained directly as a function of incident energy, without the need for deconvolving experimental influences. We discuss the benefits and difficulties of the technique and compare them with results of 1D phonon measurements.

This talk is part of the Surfaces, Microstructure and Fracture Group series.

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