Isotope effect of hydrogen atoms scattering from Au(111)

In this section, the isotope effect of hydrogen atom scattering on Au(111) is discussed. As shown in section 4.1, we confirmed that H atom scattering on Au(111) is mainly non-adiabatic, and an electronic friction model based on LDFA could describe the non-adiabatic excitations. Non-adiabatic excitation is believed to exhibit strong isotope effect. Due to the proportionality of the electronic friction force to the velocity of the atom, H atom shows stronger non-adiabatic excitations than D atom does for the same kinetic energy. A strong isotope effect has been observed in the chemi-current measurements. Chemicurrents induced by H atom adsorption are typically 2~5 times larger than those induced by D atom adsorption (18, 21). This isotope effect is confirmed by another electronic friction based calculation (23).

However, things could be different in atom-surface scattering experiments. For the same incidence energy, the H atom exhibits stronger non-adiabatic excitations, but the D atom shows stronger adiabatic excitations. In order to get a quantitative answer for this problem, we studied the isotope effect in scattering experiments by exchanging H atom with D atom. The isotope

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effect is studied in two ways, same incidence energy and same incidence speed, considering the speed dependence of the electronic friction.

4.2.1 Comparison of H and D atoms scattering from Au(111)

Figure 4.4: H and D atom scattering from Au(111). Black dots represent data from H atom scattering, and red dots represent data from D atom scattering. Panel (a) and (b) show the speed and energy loss distribution for H (3.31 eV) and D (3.25 eV) atoms scattering at similar incidence energies. Panel (c) and (d) show the speed and energy loss distribution for H and D atoms scattering at the same incidence speed (17.7 km/s). All distributions are normalized to the integral. The incidence and scattering angles are both 45^o. The incidence and scattering beams are in the plane defined by the surface normal and [101̅] direction.

Panel (a) and (b) of Figure 4.4 show the speed and translational energy loss distributions for H (3.31 eV) and D (3.25 eV) atom scattering from Au(111) at similar incidence energies. The incidence and scattering angles are both 45^o. The incidence and scattering beams are in the plane defined by the surface normal and [101̅] direction. In terms of translational energy loss, only a small isotope effect is observed. The average energy losses are 1080 meV for H and 960 meV for

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D, indicating an isotope effect of <E_lossH>/<E_lossD>=1.12. The shapes of both translational energy loss distributions are similar. Only minor deviations are observed at small translational energy losses. Panel (c) and (d) show the speed and translational energy loss distribution for H and D atom scattering from Au(111) at the same incidence speed of 17.7 km/s. The incidence energies are 3.25 eV for D and 1.64 eV for H. On average, D atoms lose 960 meV and H atoms lose 540 meV. However, the isotope effect on the speed distributions for H and D is minor. Deviations can only be seen at high speeds.

Figure 4.5: Comparisons between experimentally obtained and theoretically calculated energy loss distributions of H and D atoms scattering from Au(111). The theoretical results are from MD simulations including electronic frictions (described in section 4.1). Panel (a) and (b) show the comparisons for H and D, respectively. Black and red lines show the experimental data, and green and blue lines represent the theoretical results. The inset in panel (a) shows the comparison of theoretical results for H (green) and D (blue). The incidence and scattering conditions are the same as those shown in panel (a) and (b) of Figure 4.4.

The small isotope effect in the scattering experiments is stunning compared to chemi-current measurements. In order to understand the difference in the two experiments, MD calculations

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described in Section 4.1 were carried out to simulate the H and D atoms scatterings on Au(111) by Svenja Janke et al. Figure 4.5 shows the comparison between the experimental and calculated results. The incidence and scattering conditions are the same as those shown in Panel (a) and (b) of Figure 4.4. Experiment and theory agree well with each other for both H and D atoms scattering. For theoretical simulations, the calculated average translational energy loss is 1220 meV for H and 1080 meV for D, slightly overestimating the experimental values. The calculated isotope effect <ElossH>/<ElossD>=1.13 is very close to the experimental value. The inset in Panel (a) shows a direct comparison of theoretical results for H and D atoms scattering. The deviation in the distributions at small energy losses is weaker than the experimental measurements but still presents in the model calculations.

4.2.2 Discussion on the isotope effect of H/D atoms scattering from Au(111) Detailed analysis on calculated results gives insights on the scattering process. The small isotope effect in scattering experiment arises from the compensation of adiabatic phonon excitations and non-adiabatic ehps excitations. Owing to the higher mass, the efficiency of phonon excitations is higher for D, but the efficiency of electronic excitations is decreased than H due to a lower speed at the same incidence energy. By separating the contributions of phonon and electronic excitations, the simulation predicts that 90% of the translational energy loss transfers to electronic system for H, while it is 79% for D.

The same incidence velocity scattering experiment provides us insights to understand the compensation effect. Under the framework of electronic friction theory, electronic excitations scales with speed only and should be the same for H and D atoms at the same velocity.

Assuming phonon excitations scale linearly with atomic mass and atom translational energy. At the same velocity, D atoms lose 4 times more energy to phonon excitations than H atoms (∆𝐸_𝑝,𝐷 = 4 ∙ ∆𝐸_𝑝,𝐻), but electronic excitations should be the same for H and D (∆𝐸_𝑒,𝐷 =

∆𝐸_𝑒,𝐻). This is a good approximation for single bounce events (due to the small change in velocity during scattering). The difference between the observed energy losses is then only caused by the difference in phonon coupling caused by different masses. According to the unpublished theoretical data from Alexzander Kandratzenka et al, the average energy loss for single bounce is close to the most probable energy loss. In experiments, the most probable energy losses are 380 meV for H and 500 meV for D, which results in electronic excitations

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∆𝐸_𝑒,𝐷 = ∆𝐸_𝑒,𝐻= 340 meV and phonon excitations ∆𝐸_𝑝,𝐷 = 4 ∙ ∆𝐸_𝑝,𝐻 =160 meV. This means that 89% of the energy loss transfers to the electronic system for H, and that 68% of the energy loss transfers to the electronic system for D. These experimentally deduced proportions are close to the theoretically predicted values (90% for H and 79% for D).