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Register a new userStudy of energy transfer in helium atom scattering from surfaces
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Recently developed quantum mechanical theory of inelastic He atom scattering (HAS) from solid surfaces is employed to analyze the energy transfer between projectile particles (thermal energy He-atoms) and vibrational degrees of freedom (phonons) characteristic of a variety of experimentally studied surfaces. We have first calculated the angular resolved energy transfer which can be directly compared with the values deducible from the HAS time-of-flight spectra and a good agreement with experimental data has been found. This enabled us to calculate the total or angular integrated energy transfer, which is of paramount importance in the studies of gas-surface scattering, but is neither accessible in HAS (which yields only the angular resolved quantities), nor in the wind tunnel measurements for surfaces whose atomic composition and cleanliness must be maintained during the experiment. Here we present the results for prototype collision systems of this kind, viz. He => Cu(001), He => Xe/Cu(111) and He => Xe(111) which are representative of the very different types of surface vibrational dynamics and thereby provide an insight into some common properties of energy transfer in gas-surface scattering.
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Nonperturbative expressions are derived for the angular resolved energy transfer spectra in the quantum regime of multiphonon scattering of inert gas atoms from surfaces. Application to He atom scattering from a prototype heatbath Xe/Cu(111) shows good agreement with experiments. This enables a full quantum calculation of the total energy transfer $mu$ and therefrom the much debated recovery or equilibrium temperature $T_{r}$ characteristic of zero energy transfer in gas-surface collisions in the free molecular flow regime. Classical universal character of $mu$ and $T_{r}$ is refuted.
He atom scattering has been demonstrated to be a sensitive probe of the electron-phonon interaction parameter $lambda$ at metal and metal-overlayer surfaces. Here it is shown that the theory linking $lambda$ to the thermal attenuation of atom scattering spectra (the Debye-Waller factor), can be applied to topological semimetal surfaces, like the quasi-one dimensional charge-density-wave system Bi(114) and the layered pnictogen chalcogenides.
The Sudden Approximation is applied to invert structural data on randomly corrugated surfaces from inert atom scattering intensities. Several expressions relating experimental observables to surface statistical features are derived. The results suggest that atom (and in particular He) scattering can be used profitably to study hitherto unexplored forms of complex surface disorder.
A new quantum-theoretical derivation of the elastic and inelastic scattering probability of He atoms from a metal surface, where the energy and momentum exchange with the phonon gas can only occur through the mediation of the surface free-electron density, shows that the Debye-Waller exponent is directly proportional to the electron-phonon mass coupling constant $lambda$. The comparison between the values of $lambda$ extracted from existing data on the Debye-Waller factor for various metal surfaces and the $lambda$ values known from literature indicates a substantial agreement, which opens the possibility of directly extracting the electron-phonon coupling strength in quasi-2D conducting systems from the temperature or incident energy dependence of the elastic Helium atom scattering intensities.
Deep inelastic neutron scattering experiments using indirect time-of-flight spectrometers have reported a smaller cross section for the hydrogen atom than expected from conventional scattering theory. Typically, at large momentum transfers, a deficit of 20-40% in the neutron scattering intensity has been measured and several theories have been developed to explain these results. We present a different approach to this problem by investigating the hydrogen cross section in polyethylene using the direct geometry time-of-flight spectrometer MARI with the incident energy fixed at a series of values ranging from Ei=0.5 eV to 100 eV. These measurements span a much broader range in momentum than previous studies and with varying energy resolutions. We observe no momentum dependence to the cross section with an error of 4% and through a comparison with the scattering from metal foil standards measure the absolute bound cross section of the hydrogen atom to be sigma(H)= 80 +/- 4 barns. These results are in agreement with conventional scattering theory but contrast with theories invoking quantum entanglement and neutron experiments supporting them. Our results also illustrate a unique use of direct geometry chopper instruments at high incident energies and demonstrate their capability for conducting high-energy spectroscopy.
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