Evidence for ElectronâHole Pair Excitation in the Associative

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Evidence for Electron-Hole Pair Excitation in the Associative Desorption of H and D from Au(111) 2

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Quan Shuai, Sven Kaufmann, Daniel J. Auerbach, Dirk Schwarzer, and Alec M. Wodtke J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b00265 • Publication Date (Web): 24 Mar 2017 Downloaded from http://pubs.acs.org on March 26, 2017

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The Journal of Physical Chemistry Letters

Evidence for Electron-Hole Pair Excitation in the Associative Desorption of H2 and D2 from Au(111) Quan Shuai,*,†,∥ Sven Kaufmann,*,†,‡ Daniel J. Auerbach,†,‡ Dirk Schwarzer,†,‡ and Alec M. Wodtke†,‡,§ †

Department of Dynamics at Surfaces, Max Planck Institute for Biophysical Chemistry, Am Faßberg 11, 37077 Göttingen, Germany

‡

Institute for Physical Chemistry, Georg-August University of Goettingen, Tammannstraße 6, 37077 Göttingen, Germany

§

International Center for Advanced Studies of Energy Conversion, Georg-August University of Goettingen, Tammannstraße 6, 37077 Göttingen, Germany

Corresponding Author *E-mail: [email protected]; [email protected]

∥Present

address: Institute for Molecules and Materials, Radboud University, Heijendaalseweg 135, Nijmegen 6525 AJ, the Netherlands.

ACS Paragon Plus Environment

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Abstract

The dissociative adsorption reaction of hydrogen on noble metals is believed to be well described within the Born-Oppenheimer approximation. In this work, we have experimentally derived translational energy distributions for selected quantum states of H2 and D2 formed in associative desorption reactions at a Au(111) surface. Using the principle of detailed balance, we compare our results to theory carried out at the same level of sophistication as was done for the reaction on copper. The theory predicts much higher translational excitation than is seen in experiment and fails to reproduce the experimentally observed isotope effect. The large deviations between experiment and theory are surprising as, for the same reactions occurring on Cu(111), a similar theoretical strategy agreed with experiment yielding “chemical accuracy”. We argue that electron-hole pair excitation is more important for the reaction on gold, an effect that may be related to the reaction’s later transition state.

TOC GRAPHIC

KEYWORDS surface chemistry, nonadiabatic effect, detailed balance, translational excitation, vibrational population


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The Born-Oppenheimer approximation (BOA) provides the foundation for virtually all computational studies of chemical binding and reactivity,1 yet its validity in describing reactions on metal surfaces is still not established.2-9 We know that molecular vibrational energy can be efficiently transferred to electronic excitation of a metal7 and that H-atom adsorption at a metal cannot occur without excitation of electron-hole pairs.8,9 Yet, the influence of BOA failure on surface reaction rates (or equivalently probabilities) remains unknown. If we could identify one or more simple surface reactions that clearly exhibit BOA failure, they could serve as a platform for the fruitful interplay between experiment and theory, aiding the development of electronically nonadiabatic theories of surface chemistry. Recently, Hasselbrink et al. performed “chemicurrent” experiments using Au−TaOx−Ta, metal−insulator−metal junctions exposed to a flux of atomic hydrogen.10-12 The signals seen in those experiments were attributed to electron-hole pair (EHP) excitation resulting from the Langmuir-Hinshelwood H−H recombination reaction on gold.13 In other recent work, photoexcitation of surface plasmons on Au nanoparticles promoted H2 dissociation.14 This also implies that the H−H associative desorption reaction on gold is accelerated by hot EHPs. This leads us to the question: is the dissociation of hydrogen on Au an example of a surface chemical reaction that is strongly influenced by electronically nonadiabatic interactions? To answer this question, detailed dynamical experiments can be performed and their results compared to the best available theories of surface chemical reactivity. This approach appears particularly promising since two decades of work carried out on H2 and D2 dissociation on Cu(111) have led to extraordinary agreement between theory and experiment where the theory is believed to achieve “chemical accuracy”,15,16 by which the authors of ref 16 meant that dynamics calculations on H2/D2 + Cu(111) using the SRP functional were able to reproduce the experimentally determined


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probability of dissociative adsorption as a function of incidence energy and internal state within an accuracy of ~4.2 kilojoules per mole.


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Figure 1. Experimental (black symbols) and theoretically predicted (red and blue lines) kinetic energy distributions for H2 and D2. The experimental results were converted from the time-offlight (TOF) distributions we measured (Figure S1). After subtracting the signals from ionization of ambient H2 and D2 - much slower than the molecules desorbed from the surface - we found that these kinetic energy distributions could be well described by Gaussians. Table S1-S4 in the Supporting Information shows the Gaussian fit parameters for all for the quantum states of H2 and D2 observed in this work. The theoretical results are derived using the SRP48 (red) and PBE (blue) functionals.17 They have been scaled to best match the peak heights of the experimental results of H2(v=0; J=1) and D2(v=0; J=2). Note that different v,J states of a given isotope are scaled by the same factor, but the scaling factors can be different for different isotopes.

In this paper, we report on an investigation of the associative desorption of H2 and D2 from Au(111) using a hydrogen permeation technique.18 Via the principle of detailed balance19-23 we are able to compare to recent BOA-based quasi-classical trajectory calculations17 performed at a level of sophistication that was successful in explaining similar results for hydrogen dissociation on Cu(111).16 We find that the theory fails to describe key aspects of the reaction on Au(111). We suggest that participation of EHPs is important during product formation and this is the most likely cause of the deviations between experiment and theory. Figure 1 presents experimental kinetic energy distributions for H2 and D2 produced in the associative desorption reaction on Au(111) at the surface temperature of 1061 K. In these experiments, resonant laser excitation is used to ionize selected quantum states of nascent H2 and D2 reaction products in a field free region. The distributions shown in Figure 1 reflect the translational excitation appearing in each quantum-state resolved reaction channel. Application of the principle of detailed balance to such measurements provides a sensitive test of theoretical


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methods designed to describe the dissociative adsorption reaction probabilities, which are expected to depend on vibrational, rotational and translational excitation of the H2 and D2 molecules. According to the principle of detailed balance,24 the kinetic energy distributions, f(Ei,v,J), can be related to v,J-state specific adsorption probability functions, S0(Ei,θi,v,J), where Ei is the incidence translational energy and θi is the incidence angle. The small angular spread (7°, limited by the geometry of our experimental setup) of detected molecules allows us to ignore angular averaging and set θi = 0°, in which case eq 1 applies.

, , ∝ , , − , , .

(1)

Here, N(v,J,TS) is the Boltzmann population in state v,J, and TS is the surface temperature. Recently, theoretical predictions of S0(Ei,v,J) have been reported for H–H and D−D dissociation on Au(111)17 and in Figure 1, we show the kinetic energy distributions for associative desorption derived via detailed balance from those reaction probabilities. The theoretically predicted translational energy release is higher than seen in experiment. Kinetic energy distributions using theoretical predictions derived by use of another functional (PBE) that generates a lower reaction barrier yield improvement in some aspects of the comparison, but the agreement with experiment is still poor. Figure 2 shows additional comparisons between theory and experiment. Here, the state population distributions are displayed in a plot of ln[N/gn(2J+1)] versus the rotational energy, where N is the calibrated flux, and gn(2J+1) is the statistical weight for rotational level J. For D2, gn=2 for even J and 1 for odd J; for H2, gn=1 for even J and 3 for odd J. A Boltzmann distribution of rotational states appears as a straight line in such a plot. The experimentally derived populations (filled symbols) deviate from Boltzmann distributions at the surface


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temperature (solid lines). Particularly for v=0, the population in excited rotational states exceeds that of a Boltzmann distribution, indicating that molecules in excited rotational states are more likely to adsorb.

Figure 2. State distributions of H2 and D2 desorbed from Au(111), plotted together with the results derived from theory. Filled black circles show the distributions recorded for v=0, and red ones for v=1. The theoretical distributions from calculated reaction probabilities are plotted as open squares. The slopes of the lines represent rotational Boltzmann distributions at the surface temperature.


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Table 1. Comparison of the mean kinetic energies for associative desorption reaction products from gold and copper. Au(111) / 1061 K H2

D2 Cu(111) / 923 K H2

D2 a

Total v=0 v=1 Total v=0 v=1 Total v=0 v=1 Total v=0 v=1

Experimentala / eV 0.806 0.924 0.576 0.897 0.961 0.691 Experimentalc / eV 0.594 0.608 0.336 0.599 0.625 0.426

Theory (SRP48)b / eV 1.121 1.178 0.806 1.177 1.254 0.964 Theory (SRP48)d / eV 0.670 0.683 0.413 0.71 -

theory-exp. / eV 0.315 0.254 0.230 0.280 0.293 0.273 theory-exp. / eV 0.076 0.075 0.077 0.11 -

This work.

b

Ref 17. We took the theoretical adsorption functions and did detailed balance analysis to get the mean energy in desorption. c

Using the identical experimental setup and procedures, we made measurements of H2 and D2 associative desorption from Cu(111). d

For H2, we took the theoretical adsorption functions from ref 16 and did detailed balance analysis to get the mean energy in desorption. For D2, we directly took the mean kinetic energy from ref 20, as no state-specific adsorption function is available. By summing over J in Figure 2, we derive experimental v=1:v=0 population ratios: 0.51±0.01(0.26) for H2 and 0.33±0.01(0.36) for D2. Theoretical values are shown in parentheses. While the theory accurately reproduces the v,J population distribution for D2, it systematically underestimates the population ratio of H2(v=1) to H2(v=0). For H2 and D2 formation on copper, no such isotope effect was seen.18,25 The most striking aspect of this work is the fact that a theoretical treatment that worked well for hydrogen reacting on copper fails to describe key aspects of the same reaction on gold. We see this particularly in the isotope effect and in the translational excitation of the associative


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desorption products (Figure 1). No similar isotope effect is seen in the reaction on copper, either in experiment or in theory. Table 1 shows a comparison of the translational excitation for the reaction on copper and gold - the deviation between theory and experiment is about four times larger for the case of reactions on gold. For such similar reactions, this comes as a surprise and raises the question: what are the special properties of the reaction on Au that give rise to it being a challenge to theory? Two key aspects are of major importance in the comparing our results to theory. One is that the theory employs the static surface approximation; while the experiment is performed on a surface at 1060 K. Thermal motion of the surface atoms could assist dissociative adsorption and lead to larger energetic corrugation of reaction barriers, and thus to shifted and broader kinetic energy distributions, closer to that seen in the experiment. However, previous studies using ab initio molecular dynamics (AIMD) on H2 dissociation on Cu20,26-28 showed that the shift in the effective barrier is usually less than or around 0.1 eV. Thus, thermal motion probably does not have a large enough effect to explain the difference between experiment and theory for the H2/Au system. Another important difference between Cu(111) and Au(111) is that Au(111) has herringbone surface reconstruction,29,30 and Cu(111) does not. It was found that the herringbonereconstruction would result in an even higher dissociation barrier of H2.17 According to detailed balance, this would lead to faster H2 molecules desorbed from the surface, rather than slower as we observed. Thus, it seems that surface reconstruction cannot cause the disagreement between experiment and theory. While the reactions on Au and Cu exhibit qualitative similarities, specific experimental observations indicate important differences. Inspection of Table 1 shows that the most probable translational energy release is higher for the reaction on Au compared to Cu. This undoubtedly


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indicates that the reaction barrier is higher on the former than on the latter. Hence, an important difference between the two reactions is simply that there is more chemical energy released in the associative desorption reaction on Au compared to that on Cu.

Figure 3. Adsorption probabilities for D2(v=0,1; J=2) plotted versus kinetic energy. The upper panel (a) shows the adsorption probabilities obtained from the kinetic energy distributions as described in the text. The lower panel (b) shows the result of shifting (v=1,J=2) by 0.3 eV to overlap best with (v=0,J=2). The experimental adsorption probabilities were scaled to approach unity at high kinetic energy. The lines in panel (a) indicate the theoretically calculated adsorption probabilities from ref 17 with the SRP48 functional.


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A second difference is found in the vibrational efficacy to promote dissociative adsorption relative to translation. Figure 3 shows how we obtain experimental efficacies. First, using the principle of detailed balance we calculate the v,J state specific adsorption probabilities, S0(Ei,v,J) by directly inverting kinetic energy distributions employing an algebraically inverted form of eq 1. This is shown in Figure 3a for an example D2(v=0&1; J=2). Then, we find the required shift,

!"# ,

along the translation energy axis needed to get best overlap of the two curves (Figure

3b). This defines the vibrational efficacy, ℰ% =

!"# ⁄%'

− % ,

(2)

where %' and % are - in this example - the vibrational energies of (v=1,J=2) and (v=0,J=2), respectively.

Table 2. Vibrational efficacies for dissociative adsorption of H2/D2 on Au(111). The theoretical values were obtained using the SRP48 functional.17 J

0

1

2

3

4

5

6

7

8

9

avg.

theory

H2

0.85± 0.04 0.89± 0.04 0.90± 0.04 0.82± 0.04 0.91± 0.04 0.83± 0.04 0.84± 0.05 0.81± 0.05

0.86± 0.04

0.81(J=3)

D2

0.81± 0.06 0.86± 0.06 0.81± 0.06 0.81± 0.06 0.76± 0.06 0.82± 0.06 0.82± 0.06 0.88± 0.06 0.77± 0.06 0.78± 0.06

0.81± 0.06

0.83(J=2)

Table 2 shows the vibrational efficacies obtained for a variety of v,J states. The efficacy describes how effectively an increase in H2 or D2 vibrational energy leads to a reduction in the translational energy needed to promote reaction. The vibrational efficacy agrees well with theory, as shown in Table 2. At first sight, the observation of different H2(v=1)/H2(v=0) ratios in desorption in theory and experiment on the one hand, and agreement between theory and experiment on the vibrational efficacy of H2 on the other hand, represent a contradiction. By detailed balance, the decrease in the kinetic energy required for dissociative adsorption for


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H2(v=1) from that required for H2(v=0) means that the ratio H2(v=1)/H2(v=0) will increase. If the form of the dissociative adsorption probability curves for H2(v=1) and H2(v=0) were the same, indeed it would not be possible to get agreement on vibrational efficacy and disagreement on the vibrational population ratio. When we fit the TOF curves to a dissociative adsorption probability function of the form )

= 1 + erf 0 *

12 3

4,

(3)

we find that the value of A is lower for H2(v=0) then for H2(v=1) and this difference causes the change in the vibrational population ratio observed. (See Table S6-S7) The efficacies shown in Table 2 are uniformly larger than those found for the reaction on copper, where Ɛv = 0.5 for both H2 and D2.18,25 We interpret this to indicate that the transition state for dissociative adsorption is “later” - that is, the H−H bond distance at the transition state is longer - for the reaction on Au(111) than it is on Cu(111). The agreement of experimental and theoretical efficacies suggests the theory accurately describes the lateness of the reaction barrier. These observations indicate that associative desorption of hydrogen on Au in comparison to Cu, there is more energy to be released and that the transition state of the reaction more closely resembles two separated atoms. Given the now clear evidence that energetic H atoms interacting with Au can produce excited electron-hole pairs,8,9 it strikes us as likely that such effects are important in the associative desorption of hydrogen on gold. If this is the case, we expect to see the recombination of H/D atoms lead to concerted excitation of electron hole pairs, resulting in less translationally excited H2/D2 products. Indeed this is what is seen in Figure 1 and Table 1. By detailed balance, this would mean that the barrier to dissociation is lowered by electron-hole pair effects, namely by some coupling of thermally excited electron-hole pairs to the motion leading to desorption.


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Two previous experiments implied that the hydrogen recombination reaction on Au is coupled to the electronic degrees of freedom of the metal. In one, photo-excitation of surface plasmons on Au nanoparticles promoted H2 dissociation.14 In another, hydrogen recombination on Au proceeding by way of a Langmuir-Hinshelwood reaction produced chemicurrents.10-12 In this work, we have measured energy distributions of recombining H2 and D2 from a Au(111) surface and compared them to the best available theoretical predictions relying on the BornOppenheimer approximation.17 We find that the translational energy of the reaction products is substantially less than predicted by theory. This is consistent with energy uptake by electron-hole pairs being important during the reaction. We believe that thermal motion of the surface cannot fully explain the discrepancy between theory and experiment for the kinetic energy of desorbing molecules, and see no way that thermal motion can explain the observed isotope effect. Currently we also do not have an understanding of how coupling to electron-hole pairs can lead to the observed isotope effect. The observations of this work provide a useful benchmark for further theoretical development. Additionally, it would be intriguing to extend our investigation to the hydrogen recombination reaction on a surface of another coinage metal, namely Ag, where dynamics calculations based on BOA do not agree well with associative desorption experiments at low collisional energies.31

Experimental Methods The experiments were performed in two ultra-high vacuum chambers separated by a differential pumping wall. The first chamber held the manipulator with the sample holder and was equipped for surface preparation. The second chamber held the detection setup. Hydrogen


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atoms were supplied to the surface via permeation through a thin membrane of the single crystal. The surface was heated to 1059-1063 K to allow for permeation.

Figure 4. Schematic drawing of experimental setup.

We probed the flux of hydrogen molecules leaving the surface state selectively by resonantly enhanced multi photon ionization (REMPI). The molecule’s kinetic energy was determined by recording TOF spectra under conditions where the overall flight time was dominated by the passage through a nominally field-free region (Faraday cage) of ~30 mm length. The Faraday cage was heated during experiment to prevent the adsorption of molecules in the background gas on the mesh. An effusive (Knudsen) source, which could be positioned in the same place as that of the crystal, was used to obtain TOF distributions for calibration. A schematic drawing of the apparatus with the gold crystal in the position for recording TOF distributions is shown in Figure 4.


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The permeation source (MaTeck) consisted of a cylindrical Au(111) single crystal, 8 mm long and with a diameter of 8 mm. This crystal was eroded from the backside to form a hollow channel of 3 mm diameter and a length of ~7.7 mm, leaving a ~0.3 mm thick membrane at the tip. This crystal was welded onto a stainless steel support tube, which allowed mounting to the sample holder and gas supply. The crystal was equipped with a heater assembly made of a boron nitride body holding a coil of tantalum wire (Goodfellow, 0.25 mm diameter). This was resistively heated, transferring the heat to the sample by mechanical contact. The crystal was cleaned by Ar+-ion sputtering for 30 minutes followed by annealing at 500°C for 20 minutes. The absence of contaminants was checked by Auger electron spectroscopy and the (111) surface structure by low energy electron diffraction. Permeation was induced by supplying 1 bar of hydrogen (H2: Westfalen Gas, 99,999 Vol.-%; or D2: Sigma Aldrich, 99.96 atom % D) to the backside of the crystal and heating the sample to temperatures above ~950 K. After several hours of permeation the amount of carbon on the surface rose to detectable amounts and the crystal needed to be cleaned again. The hydrogen molecules were detected by a (2+1) REMPI scheme via the Q-branch transitions to the E,F 1Σ56 state.32 Note that this REMPI scheme is not polarization-sensitive, i.e., differently aligned rotational states of H2/D2 can be detected with almost equal probability.33 The laser was generated by a Nd:YAG-pumped dye laser (Sirah, Precision Scan). Depending on the wavelength range needed the dye was either DCM (Radiant Dyes) or a mixture of Rhodamines B and 101 (Lambda Physik) in ethanol. The radiation of the dye was frequency tripled to yield the needed range of 201-215 nm with a pulse power of ~1 mJ at a repetition rate of 50 Hz. This light was then focused into the UHV chamber by a plano-convex lens with a nominal focal length of 250 mm.


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We can derive theoretical state distributions of molecules desorbed into individual internal states in the following way: 9

7, ∝ 8 , , exp − , , ,

(4)

where Ei and N(v,J,TS) are as defined in eq 1, and S0(Ei,v,J) is v,J-state specific adsorption probability functions from theory.17

ASSOCIATED CONTENT Supporting Information The following files are available free of charge. Experimental and theoretically predicted TOF distributions of H2 and D2 formed in associative desorption on Au(111); Gaussian parameters describing the kinetic energy distributions; error function parameters describing the measured TOF distributions. (PDF) AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected]. Author Contributions Q.S. and S.K. contributed equally to this work. Present Address ∥Q.S.:

Institute for Molecules and Materials, Radboud University, Heijendaalseweg 135,

Nijmegen 6525 AJ, the Netherlands.


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Notes The authors declare no competing financial interests. ACKNOWLEDGEMENT We thank Prof. Eckart Hasselbrink, Prof. John Tully, Dr. Alexander Kandratsenka, Prof. Geert-Jan Kroes and Mark Wijzenbroek for many helpful discussions, Prof. Geert-Jan Kroes for sharing the results of ref 17 prior to publication, Francesco Nattino for providing Python code we used as a basis for our data analysis. We gratefully acknowledge Florian Lange and Reinhard Bürsing for contributions to the design and assembly of the vacuum chamber used in this work. We acknowledge support from Alexander von Humboldt Foundation and the Max Planck Society. REFERENCES (1) (2) (3)

(4)

(5)

(6) (7)

(8)

Tully, J. C. Perspective on "Zur Quantentheorie Der Molekeln" - Born M, Oppenheimer R (1927) Ann Phys 84 : 457. Theor. Chem. Acc. 2000, 103, 173-176. Wodtke, A. M. Electronically Non-Adiabatic Influences in Surface Chemistry and Dynamics. Chem. Soc. Rev. 2016, 45, 3641-3657. Diesing, D.; Hasselbrink, E. Chemical Energy Dissipation at Surfaces under UHV and High Pressure Conditions Studied Using Metal-Insulator-Metal and Similar Devices. Chem. Soc. Rev. 2016, 45, 3747-3755. Golibrzuch, K.; Bartels, N.; Auerbach, D. J.; Wodtke, A. M. The Dynamics of Molecular Interactions and Chemical Reactions at Metal Surfaces: Testing the Foundations of Theory. Annu. Rev. Phys. Chem. 2015, 66, 399-425. Nahler, N. H.; White, J. D.; LaRue, J.; Auerbach, D. J.; Wodtke, A. M. Inverse Velocity Dependence of Vibrationally Promoted Electron Emission from a Metal Surface. Science 2008, 321, 1191-1194. Huang, Y. H.; Rettner, C. T.; Auerbach, D. J.; Wodtke, A. M. Vibrational Promotion of Electron Transfer. Science 2000, 290, 111-114. White, J. D.; Chen, J.; Matsiev, D.; Auerbach, D. J.; Wodtke, A. M. Conversion of LargeAmplitude Vibration to Electron Excitation at a Metal Surface. Nature 2005, 433, 503505. Bünermann, O.; Jiang, H.; Dorenkamp, Y.; Kandratsenka, A.; Janke, S. M.; Auerbach, D. J.; Wodtke, A. M. Electron-Hole Pair Excitation Determines the Mechanism of Hydrogen Atom Adsorption. Science 2015, 350, 1346-1349.


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(9)

(10) (11) (12)

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Janke, S. M.; Auerbach, D. J.; Wodtke, A. M.; Kandratsenka, A. An Accurate FullDimensional Potential Energy Surface for H-Au(111): Importance of Nonadiabatic Electronic Excitation in Energy Transfer and Adsorption. J. Chem. Phys. 2015, 143, 124708. Hasselbrink, E. Non-Adiabaticity in Surface Chemical Reactions. Surf. Sci. 2009, 603, 1564-1570. Schindler, B.; Diesing, D.; Hasselbrink, E. Electronic Excitations Induced by Hydrogen Surface Chemical Reactions on Gold. J. Chem. Phys. 2011, 134, 034705. Schindler, B.; Diesing, D.; Hasselbrink, E. Electronically Nonadiabatic Processes in the Interaction of H with a Au Surface Revealed Using MIM Junctions: The Temperature Dependence. J. Phys. Chem. C 2013, 117, 6337-6345. Schindler, B.; Diesing, D.; Hasselbrink, E. Electronic Excitations in the Course of the Reaction of H with Coinage and Noble Metal Surfaces: A comparison. Z. Phys. Chem. 2013, 227, 1381-1395. Mukherjee, S.; Libisch, F.; Large, N.; Neumann, O.; Brown, L. V.; Cheng, J.; Lassiter, J. B.; Carter, E. A.; Nordlander, P.; Halas, N. J. Hot Electrons Do the Impossible: PlasmonInduced Dissociation of H2 on Au. Nano Lett. 2013, 13, 240-247. Kroes, G. J.; Diaz, C. Quantum and Classical Dynamics of Reactive Scattering of H2 from Metal Surfaces. Chem. Soc. Rev. 2016, 45, 3658-3700. Diaz, C.; Pijper, E.; Olsen, R. A.; Busnengo, H. F.; Auerbach, D. J.; Kroes, G. J. Chemically Accurate Simulation of a Prototypical Surface Reaction: H2 Dissociation on Cu(111). Science 2009, 326, 832-834. Wijzenbroek, M.; Helstone, D.; Meyer, J.; Kroes, G.-J. Dynamics of H2 Dissociation on the Close-Packed (111) Surface of the Noblest Metal: H2 + Au(111). J. Chem. Phys. 2016, 145, 144701. Michelsen, H. A.; Rettner, C. T.; Auerbach, D. J.; Zare, R. N. Effect of Rotation on the Translational and Vibrational Energy Dependence of the Dissociative Adsorption of D2 on Cu(111). J. Chem. Phys. 1993, 98, 8294-8307. Comsa, G.; David, R. Dynamical Parameters of Desorbing Molecules. Surf. Sci. Rep. 1985, 5, 145-198. Nattino, F.; Genova, A.; Guijt, M.; Muzas, A. S.; Diaz, C.; Auerbach, D. J.; Kroes, G. J. Dissociation and Recombination of D2 on Cu(111): Ab Initio Molecular Dynamics Calculations and Improved Analysis of Desorption Experiments. J. Chem. Phys. 2014, 141, 124705. Comsa, G.; David, R.; Schumacher, B.-J. Fast Deuterium Molecules Desorbing from Metals. Surf. Sci. 1980, 95, L210-L216. Cardillo, M. J. J.; Balooch, M.; Stickney, R. E. E. Detailed Balancing and QuasiEquilibrium in the Adsorption of Hydrogen on Copper. Surf. Sci. 1975, 50, 263-278. Hodgson, A. State Resolved Desorption Measurements as a Probe of Surface Reactions. Prog. Surf. Sci. 2000, 63, 1-61. Michelsen, H. A.; Auerbach, D. J. A Critical Examination of Data on the Dissociative Adsorption and Associative Desorption of Hydrogen at Copper Surfaces. J. Chem. Phys. 1991, 94, 7502-7520. Rettner, C. T.; Michelsen, H. A.; Auerbach, D. J. Quantum-State-Specific Dynamics of the Dissociative Adsorption and Associative Desorption of H2 at a Cu(111) Surface. J. Chem. Phys. 1995, 102, 4625-4641.


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TOC Graphic. Hydrogen adsorption and desorption on Au(111). TOC Graphic 50x50mm (300 x 300 DPI)


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Figure 1. Experimental (black symbols) and theoretically predicted (red and blue lines) kinetic energy distributions for H2 and D2. The experimental results were converted from the time-of-flight (TOF) distributions we measured (Figure S1). After subtracting the signals from ionization of ambient H2 and D2 much slower than the molecules desorbed from the surface - we found that these kinetic energy distributions could be well described by Gaussians. Table S1-S4 in the supporting information shows the Gaussian fit parameters for all for the quantum states of H2 and D2 observed in this work. The theoretical results are derived using the SRP48 (red) and PBE (blue) functionals. They have been scaled to best match the peak heights of the experimental results of H2(v=0; J=1) and D2(v=0; J=2). Note that different v,J states of a given isotope are scaled by the same factor. Figure 1 142x203mm (300 x 300 DPI)



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Figure 2. State distributions of H2 and D2 desorbed from Au(111), plotted together with the results derived from theory. Filled black circles show the distributions recorded for v=0, and red ones for v=1. The theoretical distributions from calculated reaction probabilities are plotted as open squares. The slopes of the lines represent rotational Boltzmann distributions at the surface temperature. Figure 2 82x120mm (300 x 300 DPI)


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Figure 3. Adsorption probabilities for D2(v=0,1; J=2) plotted versus kinetic energy. The upper panel (a) shows the adsorption probabilities obtained from the kinetic energy distributions as described in the text. The lower panel (b) shows the result of shifting (v=1,J=2) by 0.3 eV to overlap best with (v=0,J=2). The experimental adsorption probabilities were scaled to approach unity at high kinetic energy. The lines in panel (a) indicate the theoretically calculated adsorption probabilities from ref 18 with the SRP48 functional. Figure 3 82x117mm (300 x 300 DPI)



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Figure 4. Schematic drawing of experimental setup. Figure 4 165x84mm (300 x 300 DPI)


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