Work Function Dependence of Vibrational Relaxation Probabilities

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Work Function Dependence of Vibrational Relaxation Probabilities: NO(v = 2) Scattering from Ultra-Thin Metallic Films of Ag/Au(111) Christoph Steinsiek, Pranav R Shirhatti, Jan Geweke, Christof Bartels, and Alec M. Wodtke J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01950 • Publication Date (Web): 19 Apr 2018 Downloaded from http://pubs.acs.org on April 19, 2018

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The Journal of Physical Chemistry

Work Function Dependence of Vibrational Relaxation Probabilities: NO(𝑣 = 2) Scattering from Ultra-Thin Metallic Films of Ag/Au(111) Christoph Steinsiek *1, Pranav R. Shirhatti

1,2(a)

, Jan Geweke

1,2,3

, Christof Bartels

1,5

, Alec M.

Wodtke 1,2,3,4. 1

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

37077 Göttingen, Germany. 2

Department of Dynamics at Surfaces, Max Planck Institute for Biophysical Chemistry, Am

Faßberg 11, 37077 Göttingen, Germany. 3

Max-Planck-EPFL Center for Molecular Nanoscience and Technology, Institute of Chemical

Science and Engineering (ISIC), Station 6, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland. 4

International Center for Advanced Studies of Energy Conversion, Georg-August University of

Göttingen, Tammannstraße 6, 37077 Göttingen, Germany, 5

Physikalisches Institut, Universität Freiburg, Hermann-Herder-Straße 3, 79104 Freiburg,

Germany

ACS Paragon Plus Environment

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(a)

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Present address: Tata Institute of Fundamental Research, 36/P, Gopanapally Village,

Serilingampally Mandal, Ranga Reddy District, Hyderabad 500107, India ABSTRACT: We present measurements on the vibrational relaxation of NO(𝑣 = 2) scattered from atomically defined thin films of Ag on Au(111). The vibrational relaxation probability is strongly dependent on film thickness, increasing with each of the first three Ag monolayers. We interpret this as the influence of work function which changes with layer thickness. This supports the postulated mechanism of NO vibrational relaxation involving a transient NO- anion.

Introduction The loss of vibrational energy from a molecule when it collides with a metal surface often proceeds by exciting the metal’s electron-hole pairs (EHPs).1-2 An illustrative experiment compared vibrational relaxation of highly excited NO when scattered from Au(111) and LiF surfaces.3 While there is almost no loss of vibrational energy on the insulating LiF surface, multiquantum relaxation is seen for the metallic Au(111) surface. Such processes belong to the group of nonadiabatic effects present in a number of gas-surface interactions.4-7 Measurements of vibrational energy transfer offer a way to quantify the breakdown8 of the Born-Oppenheimer (electronically adiabatic) approximation (BOA),9 allowing emerging theories to be tested. Two of the most notable models are: 1) independent electron surface hopping (IESH),10 which postulates that electron transfer forming a transient NO anion is essential to the production of a single excited EHP and 2) electronic friction theory,11-12 which dissipates vibrational energy into many low-energy EHPs. Additional experiments on NO scattering from noble metals examined the dependence of vibrational energy transfer on NO initial vibrational and translational energy.13-16 The effect of NO orientation on vibrational relaxation has also been observed16 and is believed to reflect the


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orientation dependence of electron transfer to the molecule.17 Unfortunately, neither IESH nor electronic friction theory were able to reproduce the dependence of vibrational relaxation probability on incidence energy,14 exhibiting at best qualitative agreement with experimental data.10 Hence, more direct experimental probes of the fundamental mechanism are still required. Finding a strong relaxation probability dependence on surface work function 𝜙 would help confirm the importance of electron transfer in the electronically nonadiabatic dynamics. The hypothesis is supported by the observation that vibrational relaxation of NO(𝑣 = 11) on Ag(111) (𝜙=4.7 eV) was found to be much more efficient than on Au(111) (𝜙=5.3 eV).15 Scattering experiments of NO from Cs-films on Au(111)18 have also been performed (𝜙=1.6 eV19); however, those studies were designed to measure electron exo-emission, an indirect probe of electronically nonadiabatic vibrational relaxation probabilities. In this work, we systematically vary the surface composition and structure and observe its influence on electronically nonadiabatic vibrational energy transfer. Due to the demonstrated layer-by-layer growth of Ag on Au,20-22 samples can be prepared with a defined number of Ag atomic layers, allowing control of surface-localized electronic states23-24 and work function.22, 25 Our results reveal a strong dependence of vibrational relaxation probabilities on work function, confirming the importance of electron transfer in electronically nonadiabatic surface dynamics.

Experimental Methods The experiments were performed with a molecular-beam surface scattering instrument26 augmented by an evaporator and an ultra-high vacuum (UHV) sample transfer system. In the source chamber, a gas mixture of 7% NO/H2 was expanded in a supersonic jet (mean NO translational energy Ei = 0.59 eV) from a pulsed piezoelectric nozzle (3 bar stagnation pressure, 298 K; pulse width of 50 μs FWHM). The molecular beam passes a skimmer (1.5 mm diameter)


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and then two differential pumping stages (separated from neighboring chambers by apertures with diameters of 3 and 2 mm) before entering the UHV scattering chamber, whose base pressure was 5x10-10 mbar increasing to 3.0x10-9 mbar with the molecular beam running at 10 Hz. The scattering target was a Au(111) single crystal (surface-orientation better than 0.1°, purity 99.9999%, MaTeck GmbH) employed as a substrate and coated with silver in a controlled manner to produce the desired thin film samples. Before preparation of a thin film sample, the Au(111) substrate was cleaned by argon ion bombardment (3 kV, 30 min) and subsequently annealed at 950 K. Auger Electron Spectroscopy (AES) was used to confirm the cleanliness of the sample. Thin film growth was performed within the scattering chamber (see Supporting Information Fig. SI-1) using a home-built evaporator and a quartz crystal microbalance (QCM). With the substrate at room temperature, silver was deposited at a rate of 0.9 ML/min on the clean and annealed Au(111) surface. During that process, a pressure increase to − 𝜙 as a simple parameter capable of describing the degree of Born-Oppenheimer failure in electronically nonadiabatic vibrational energy transfer.


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Figure 6: Relaxation probability plotted against the difference between the vertical electron binding energy at the outer turning point (𝑉𝐸𝐵𝐸𝑟> ) and the surface work function for a number of different molecules-surface systems (Ei = 0.5-0.6 eV). The data of this work is represented by black points. Other references: see text. Inset: Visualization of the 𝑉𝐸𝐵𝐸𝑟> . Potential energy curves of NO and NO- are sketched. The VEBE at the outer turning point is depicted (red) for vibrationally excited NO(𝑣 = 15). Figure 6 shows how Prelax≡ 1 − 𝑃𝑠𝑢𝑟𝑣𝑖𝑣𝑎𝑙 depends on Δ including results for several different experiments on both NO and CO scattering from noble metals.3, 14-15, 38 The plot can be divided into three regions. On the left, one extreme is the scattering data of CO(𝑣 = 2) from Au (orange diamond) and thick films of Ag/Au(111) (green diamond) which exhibit relatively inefficient vibrational relaxation. At the other extreme, results from highly vibrationally excited NO scattered from Au (blue and pink circles) and Ag (orange circle) are found. The data set of this work (black circles) is found at the center of the plot and forms a transition region. We point out that the relaxation probabilities for CO(𝑣 = 17) (purple diamond)38 and NO(𝑣 = 3) (red circle)14 are of a similar magnitude. This strongly suggests that transient negative ion formation is important in both CO and NO scattering from noble metals.38


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It is remarkable how much these experimental findings qualitatively coincide with the simple model developed for vibrational excitation in NO/Ag(111) scattering by Newns.35 This model depends on several parameters, but work function and electron affinity (≈ 𝑉𝐸𝐵𝐸𝑟> for v = 0) only enter through their difference. Using the parameters of the paper35 for the case of NO/Ag(111), increasing relaxation probability with Δ is predicted (as elaborated in the supporting information). Several alternative explanations can be discarded. First, a lower dissociation barrier for NO on Ag(111) compared to Au(111) might enhance vibrational relaxation. Experiments on HCl scattering from Au(111) have been interpreted in this way;39 there, enhanced electronically nonadiabatic vibrational relaxation was associated with nonreactive scattering that sampled geometries near the dissociation transition state. However, in the current study with relatively low initial vibrational energy - NO(𝑣 = 2, Evib = 0.46 eV) - dynamics associated with the transition state for dissociation are unimportant. Second, electronic surface states found close to the Fermi level are also strongly dependent on film thickness. However, focusing on the growth of the first monolayer of Ag/Au(111) with the strongest influence on vibrational relaxation, it was reported that the change of the work function (0.48 eV) is much larger in comparison to the energetic shift of the surface state (0.17 eV).24 As a result, a special influence of the surface states on the collision of vibrational excited molecules seems unlikely. Third, photoelectron spectroscopy on Ag/Au(111) revealed additional quantum well electronic states (QWS) between thicknesses of 7 and 40 ML.23-24 The energetic position and density of such states are strongly affected by the film thickness. We also note that an effect of QWS on the magnitude of chemicurrents was reported in a prior study.40 In our work, we observe no change in vibrational relaxation probability in this thickness range.


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Conclusion We have experimentally studied the vibrational relaxation of NO(𝑣 = 2) as a function of Aglayer thickness for Ag deposited on Au(111). Vibrational relaxation strongly increases between 0 and 3 ML after which it becomes constant. With increasing Ag film thickness, the NO(𝑣 = 2 → 2) survival probability decreases while the NO(𝑣 = 2 → 1) remains approximately constant. We conclude, that the missing fraction is in the NO(𝑣 = 2 → 0) relaxation channel, i.e. the NO(𝑣 = 2 → 0) relaxation increases with Ag layer thickness. We argue that the work function change with layer thickness is the key physical variable controlling the strength of nonadiabatic coupling during the scattering process. We are also able to explain a wide variety of results for both NO and CO scattering from noble metals based on a the difference between the VEBE at the outer turning point and the surface work function 𝜙. This study provides some of the strongest evidence yet that transient anion formation is important in the electronically nonadiabatic vibrational energy transfer of molecules at metal surfaces.

Associated Content The supporting material contains the following: (1) Details about the QCM calibration and film characterization (2) Details about the calculation of absolute scattering probabilities (3) Measurements of the angular distributions (4) Molecular vibrational relaxation within the Newns model AUTHOR INFORMATION Corresponding Author


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*Dr. Christoph Steinsiek, E-mail: [email protected] Present Addresses 1

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

37077 Göttingen, Germany. 2

Department of Dynamics at Surfaces, Max Planck Institute for Biophysical Chemistry, Am

Faßberg 11, 37077 Göttingen, Germany. 3

Max-Planck-EPFL Center for Molecular Nanoscience and Technology, Institute of Chemical

Science and Engineering (ISIC), Station 6, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland. 4

International Center for Advanced Studies of Energy Conversion, Georg-August University of

Göttingen, Tammannstraße 6, 37077 Göttingen, Germany, 5

Physikalisches Institut, Universität Freiburg, Hermann-Herder-Straße 3, 79104 Freiburg,

Germany (a)

Present address: Tata Institute of Fundamental Research, 36/P, Gopanapally Village,

Serilingampally Mandal, Ranga Reddy District, Hyderabad 500107, India Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS AMW and CB acknowledge support from the Alexander von Humboldt foundation. AMW and CS acknowledge support from the Sonderforschungsbereich 1073 under project A04. JG


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acknowledges support from the Max-Planck—EPFL Center for Molecular Nanoscience and Technology. The authors would also like to thank Bastian Krüger, Jan Altschäffel and Sascha Kandratsenka for helpful discussions throughout the course of the work.

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36. Gadzuk, J. W., Vibrational Excitation in Molecule–Surface Collisions Due to Temporary Negative Molecular Ion Formation. J. Chem. Phys. 1983, 79, 6341-8. 37. Magoulas, I.; Papakondylis, A.; Mavridis, A., Structural Parameters of the Ground States of the Quasi-Stable Diatomic Anions CO−, BF−, and BCl− as Obtained by Conventional Ab Initio Methods. Int. J. .Quantum Chem. 2015, 115, 771-8. 38. Wagner, R. J. V.; Henning, N.; Krüger, B. C.; Park, G. B.; Altschäffel, J.; Kandratsenka, A.; Wodtke, A. M.; Schäfer, T., Vibrational Relaxation of Highly Vibrationally Excited CO Scattered from Au(111): Evidence for CO- Formation. J. Phys. Chem. Lett. 2017, 8, 4887-92. 39. Geweke, J.; Shirhatti, P. R.; Rahinov, I.; Bartels, C.; Wodtke, A. M., Vibrational Energy Transfer near a Dissociative Adsorption Transition State: State-to-State Study of HCl Collisions at Au(111). J. Chem. Phys. 2016, 145, 054709. 40. Hagemann, U.; Hermann, N., Quantum Size Effects in Chemicurrent Measurements During Low-Temperature Oxidation of Mg(0001) Epilayers. New J. Phys. 2014, 16, 113035.


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Figure 1: Schematic diagram (not to scale) of the wedge sample. Thickness is increasing when moving along the surface in the direction of the blue arrow. Scattering experiments have been performed along the center of the crystal (dashed line). 82x46mm (300 x 300 DPI)


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Ei = 0.5-0.6 eV

NO(v=2) / thin films Ag/Au (current work) NO(v=3) / Au NO(v=11) / Au NO(v=16) / Au NO(v=11) / Ag CO(v=17) / Au CO(v=2) / Au CO(v=2) / 5 ML Ag/Au guide to the eye

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Work Function Dependence of Vibrational Relaxation Probabilities

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