2) from Hot Molten Au

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

On the Role of Thermal Electron-Hole Pairs in Vibrational Excitation of NO Scattered from Hot Molten Au Amelia Zutz, and David J. Nesbitt J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b03503 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on July 20, 2018

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The Journal of Physical Chemistry

Quantum State Resolved Scattering of NO(2Π1/2) from Hot Molten Au(liq): On the Role of Thermal Electron-Hole Pairs in the Vibrational Excitation Dynamics Amelia Zutz and David J. Nesbitt* JILA, University of Colorado and National Institute of Standards and Technology, and Department of Chemistry and Biochemistry, Department of Physics, University of Colorado, Boulder, Colorado 80309-0440 6/15/2018 Abstract Energy transfer at the gas-molten Au interface is investigated in quantum-state resolved molecular beam experiments of supersonically cooled NO scattered from liquid Au (TS = 1400(40) K) and detected via laser induced fluorescence. Inelastic dynamics at the gas-Au(liq) interface is evidenced through collisional excitation of both i) non-adiabatic electronic/spin-orbit and ii) rovibrational degrees of freedom of NO at near thermal (2.0(7) kcal/mol) and hyperthermal (20(2) kcal/mol) collision energies. The studies represent first molecular beam scattering experiments from molten Au, as well as the first observations of vibrational excitation by scattering at the gas-liquid metal interface. In the vibrationally elastic NO(v = 0  0) channel, spin-orbit and rotational excitation are found to increase significantly with collision energy. However, excitation in these degrees of freedom are largely energy independent for the vibrationally inelastic (v = 1 0) channel. One simple physical interpretation of these results is that excitation of NO(v = 1) arises from thermally populated electron hole pair excitations in the metal itself. Finally, comparisons are made between scattering dynamics from single crystal Au(111) and molten gold, whereby rotational and vibrational excitation of scattered NO appear to follow a smooth trend as a function of Au surface temperature across the crystal-to-liquid phase transition.

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The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

I. Introduction

Electronically nonadiabatic interactions have been investigated extensively both experimentally and theoretically in collisions of molecules with solid metallic surfaces.1-18 In particular, numerous experiments dating back to the mid-1980’s have probed vibrational excitation of molecular NO scattered from solid crystalline surfaces,3,19-24 which report a rapid increase in excitation efficiency with surface temperature characteristic of a thermally activated process. Indeed, such behavior has become well established for scattering from solid, single crystal surfaces, with an Arrhenius slope that matches the harmonic vibrational spacing of the oscillator.3,19,20,24,25 Interestingly, there has been relatively little study of molecular energy transfer between molecules at gas-molten metal interfaces. Indeed, in previous studies of molecules scattering from molten metal surfaces,26-30 vibrational excitation has not been observed. The importance of vibrational excitation efficiency in these scattering measurements arises from the nonadiabatic nature of the process. As clearly articulated by Tully and coworkers,13 the conduction band of a metal consists of a continuum of infinitesimally spaced electronic states, into which an electron-hole pair can be formed when an electron is excited above the Fermi level. Because of the continuum of electronic states present, such electron hole pairs can be resonant with any internal (electronic, vibrational, rotational) excitation of an incident molecule on a metallic surface, thereby presenting a potential avenue for efficient energy transfer from the gas molecule to the metal (or vice versa). As a result, this electron-hole pair mediated rovibronic excitation of incident gas molecules is an intrinsically non-adiabatic electronic process, whereby electronic degrees of freedom in the metal couple to vibrational degrees of freedom in the transiently adsorbed molecular projectile. Thus, vibrational excitation


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(or dexcitation) in the scattered NO can act as a proxy for a wide variety of fundamental collision physics. Wodtke, Tully and coworkers have contributed extensively toward understanding these nonadiabatic, electron-hole pair mediated collision dynamics occurring at NO + Au(111) surfaces.7,12-14,20,21,31,32 Of particular dynamical interest, evidence has been presented that suggests vibrational excitation and deexcitation of NO molecules at a metal surface are mediated by the resonant formation of electron-hole pair excitations in the metal. In one theoretical formulation of this mechanism, an electron from the metallic “Fermi sea” can ‘hop’ onto the singly occupied antibonding π* molecular orbital (SOMO) of the NO molecule, thereby forming a transient anion stabilized by an image charge inside the metal. As the NO molecule then subsequently scatters and leaves the surface, the electron returns abruptly back to the metal and neutralizes the Coulombic image charge. As a result of such rapid, transient electron transfer into/from antibonding molecular orbitals, vibrational modes in the NO molecule can be excited (or relaxed). A number of experiments have studied how nitric oxide transfers/accepts vibrational quanta from a variety of solid crystalline surfaces, which all provide support for this simple physical picture of electron-hole pair mediated vibrational/electronic energy transfer between NO and metal surfaces. By way of example, vibrationally excited NO(v = 15) has been scattered from crystalline Au(111), which results in the loss of as much as ~ 8 vibrational quanta in the scattered flux.12 What makes this remarkable is the result of similarly excited NO from electrically insulating surfaces. Indeed, for NO(v = 12) scattered from a single crystal LiF surface,9 vibrational relaxation was almost entirely absent, which would be consistent with predictions of weakly perturbative, first order coupling and consistent with the absence of



continuum electron-hole pair states perfectly resonant with the incoming NO(v). These results provide strong evidence that the presence of a metal surface is responsible for multiquantum vibrational relaxation. Particularly compelling additional experiments investigated vibrationally mediated electron emission from Au(111) covered in Cs,8,33 which lowered the work function of the surface down to 1.6 eV.11 A molecular beam of NO(v) molecules was then directed at the surface in a tunable series of incident vibrational states (v” = 0 – 18), which yielded electron emission only when the incident NO vibrational energy was in excess of the work function of surface. These results provide remarkable evidence for strong coupling of nuclear vibrational energy to electronic excitations in the metal, which in turn represents a fundamental breakdown of the Born-Oppenheimer approximation. It therefore becomes a particularly interesting question to pose, what happens to the nature of such nonadiabatic dynamical processes when the interface changes from gas-single crystal/solid to a gas-molten metal interface? Does the nature of the electron-hole pair excitations depend on the short and long range translational invariance of the Au(111) surface, or are the dynamics of electron transfer from metallic Au to the NO molecule sensitive to the additional presence of dynamical surface roughness created by thermally excited surface capillary waves in the molten metal? Are the electronic dynamics of a gas-molten metal interface simply a dynamically rougher and more disordered version of a perfect single metallic crystal? The answers to these questions will require significant advances on both experimental as well as theoretical fronts, for which this work hopes to provide additional stimulus and first data. We have built up new experimental capabilities for exploring quantum state resolved rovibronic scattering dynamics of NO at gas-liquid metal interfaces. Given the wealth of data from the Wodtke group on scattering of NO from solid single crystal Au(111), we felt it would


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be particularly interesting to explore such dynamics at the gas-molten Au interface, for which Au (Tmelt = 1337 K) requires much higher temperatures to access than previously explored. Recent modifications and additions to our scattering apparatus have now enabled us to probe interfacial collisional dynamics of NO from molten Au (TS = 1400(40) K), in addition to quantum state resolved energy transfer into rotational, electronic and vibrational degrees of freedom as a function of collision energy (Einc = 2 and 20 kcal/mol). The experimental results presented in this paper not only represent the first quantum state resolved molecular scattering from the gasmolten gold interface, but also the first reports of vibrational excitation in scattered molecules from any molten metal surface. The organization of this paper is as follows. In Sec. II, we describe modifications to the NO scattering apparatus that permits experimental access to such high temperature regimes, with quantum state resolved NO populations presented in Sec. III as a function of both near thermal (2 kcal/mol) and hyperthermal (20 kcal/mol) collision energies. In Sec. IV, we analyze energy transfer at the gas-molten metal interface and discuss dynamical correlations between electronic, vibrational and rotational excitation efficiencies. Finally, we conclude by making a first comparison of energy transfer at the NO-Au interface between solid single crystalline Au(111) and molten Au(liq) phases, which indicates rotational and vibrational excitation to be less dependent on the actual phase of the metal and more dependent simply on surface temperature. These results are qualitatively consistent with energy transfer from thermally excited electronhole pairs in the liquid metal into the NO vibrational degree of freedom and highlight the need for further experimental and theoretical efforts.

II. Experimental



The experimental apparatus for NO gas-liquid scattering at low temperature interfaces has been described in detail previously.29,34 Hence only a brief description is provided, highlighting experimental changes made to the heating system that enable the study of molten metals up to temperatures TS = 1400 K and beyond (see Fig. 1a). A beam of supersonically cooled NO(2П1/2, v = 0, J = 1/2) is scattered from the surface of molten Au, with the distribution of final rovibronic states measured via quantum state resolved laser induced fluorescence (LIF) detection. The molecular beam of NO is produced from an Even-Lavie valve, with all molecules cooled to the ground 2П1/2 spin-orbit state and Trot ~ 1 K. The NO is seeded in a buffer gas diluent of either Ne or H2 in a 1:99 ratio, which yields fast pulses (∼80 µs) with incident collision energies of 2 and 20 kcal/mol (0.09 and 0.87 eV), respectively. The molecular beam passes through a 3 mm skimmer, creating a collimated beam that strikes the liquid surface at θinc = 45º with a 1º half angle divergence. The molten metal and scattering apparatus reside in a 90 L stainless steel vacuum chamber that is pumped by a 1500 L/s turbomolecular pump with base pressure ~ 2 x 10-8 Torr. Although reasonably short in duration, it is worth noting that the gas pulses are far longer than the presumed residence time of NO on Au(liq). Thus the scattering dynamics are necessarily captured in the steady state regime, with no temporal dependence anticipated in the resulting NO quantum state distributions. The liquid Au sample sits in a tungsten crucible with an alumina coating along the edge to prevent wetting (S21-AO-W, R.D. Mathis). This boat is resistively heated by a 4kW power supply (LV400, R.D. Mathis) with copper bars of sufficiently low electrical resistivity and high thermal conductivity to minimize heat loss while maximizing power delivered to the tungsten boat. Specifically, the power supply is connected by vacuum feedthroughs to two 1” solid Cu feedthroughs (R.D. Mathis, RDM-FT-400) that are each attached to two 1” x 1/8” Cu bars


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(oxygen free Copper, alloy 101) that rise 8” and form an electrically conductive sandwich (1” x 1” overlap on either side) to hold the tungsten boat. The crucible has a resistance of ~ 5 mΩ, with which to achieve temperatures in the TS > 1400 K range requires ~ 150 A and 100 W of delivered power. The scattering apparatus and heating system are depicted in Fig. 1a. The tungsten reservoir presents a liquid Au surface of 3.8 cm x 1.3 cm area, i.e., significantly larger than the 0.8 cm x 1.1 cm spot size of the incident beam. The Au sample is purchased in powder form (99.95%, Goodfellow) and melted in situ in vacuum. We note that, although the 2 x 10-8 Torr base pressures do not eliminate collisions of this background gas with the molten Au surface during the data acquisition, the combination of low Au reactivity and high sample temperature serves to minimize residence times of any background gas on the gas-liquid interface. We have a residual gas analyzer in the vacuum chamber to monitor the background gas, which reveals it to be overwhelmingly water vapor and H2 with very little hydrocarbon content. We also sputter the molten Au surface with 2 KeV Ar+ ions at 10 µA for multiple hours in preparing the sample. Immediately prior to each data scan, the Au metal surface is also cleaned by 30 minutes of sputtering the hot liquid surface with an Ar+ gun operating at 10 µA and 2KeV. Most importantly, after the initial sample preparation, we see no differences in the measured NO signals between data runs or with further increase in the Ar+ sputtering time. This could imply that Ar+ sputtering competes weakly with surface adsorption to achieve only “steady state” levels of cleanliness (which we think unlikely), or more likely that the surface remains sufficiently clean (vis a vis dynamics in the observed NO distributions) for the duration of a given data run. One experimental challenge is in the temperature measurement, which is obtained by a Type K thermocouple connected to the bottom of the tungsten boat. As this can be influenced by



heat loss through the connecting wires and thermal contact, this primary temperature measurement has been calibrated against secondary temperature measurements obtained from (i) optical pyrometry (at high temperatures, 800 - 1200 K) or (ii) a thermocouple submerged in liquid Ga in the reservoir (at low temperatures, 300 - 850 K), respectively. The upper limit for the Au sample is constrained by prohibitive vapor pressures above 1450 K, with the lower limit set by the melting temperature of gold (Tmelt = 1337 K). Therefore gold is studied at a single liquid temperature 1400(40) K, i.e. such that the sample is fully melted, but cool enough to maintain sufficiently low vapor pressures to avoid coating the LIF detection optics and ensure collision-free conditions for the scattered NO flux. Scattered NO molecules are detected with quantum state resolution via laser induced fluorescence (LIF) on the γ band, A2Σ+(v = 0) ⟵ X2П(v = 0, 1, 2). A pulsed UV light source

(223-246 nm) is produced from the tripled output of a Nd:YAG pumped dye laser (Continuum Powerlite/Continuum ND6000) operating with LDS698 dye. The laser beam passes ≈ 1.6 cm above the liquid surface and in the plane of specular scattering with the laser polarization parallel to surface normal. Fluorescence from the scattered NO is imaged onto a photomultiplier tube (PMT) from a ≈ 13 mm3 volume along the laser path. This spatial filtering is a result of a 1:1 confocal lens pair and 4 mm aperture in front of the PMT, which translates into detection of scattered molecules in a near specular θs = 45(6)o angular window. A UV bandpass filter sits in front of the PMT to block incident laser light while maximizing fluorescence gathered. Each vibrational band is detected in a separate wavelength range, with relative vibrational state population ratio measurements corrected for A2Σ+(v = 0) fluorescence transmission spectra of the filters. The PMT photon counts are integrated in a 200 ns window to capture the scattered NO signals and normalized to laser power for each pulse.


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III. Results IIIA. Rotational State Distributions Sample LIF spectra for scattered NO(v = 0) and NO(v = 1) are displayed in Fig. 1b, where fluorescence is obtained on the A2Σ+(v’ = 0)  X2П(v” = 0, 1) transitions. The spectra are least squares fit to a model to extract rovibronic populations.29,35 The reliability of this data analysis has been confirmed by measurements of ambient NO (≈ 10-6 Torr) introduced into the vacuum chamber, which recapitulates the room temperature rotational and spin-orbit distributions of 295(4) K and 298(10) K, respectively. The data in Fig. 1b represent scattering of NO (Einc = 20(2) kcal/mol) from a TS = 1400(40) K molten gold surface, with molecules detected specularly scattered at θinc ≈ θs ≈ 45º. Since > 99.99% of the NO molecules in the incident beam stagnation region originate in the ground vibrational state, essentially all NO(v = 1) signals result from excitation through collisions at the gas-metal interface (lower panel, Fig. 1b). Significant rotational as well as spin-orbit excitation of NO are also observed. The substantial energy transfer into spin-orbit and vibrational degrees of freedom signals the presence of nonadiabatic, surface hopping collision dynamics occurring at the NO-Au(liq) interface, as noted also in the highly efficient vibrational relaxation studies of Wodtke and coworkers for single crystal Au(111).12,36 We next consider more quantitatively the rovibronic distributions of NO scattered from Au(liq). In Fig. 2, rotational distributions from NO(v = 0) are displayed for low (Einc = 2.0(7) kcal/mol, blue) and high (Einc = 20(2) kcal/mol, red) collision energies, where ln[Population(J)/(2J+1)] is plotted vs. rotational energy in a standard Boltzmann plot for the excited 2П3/2 spin-orbit state. These distributions clearly exhibit curvature at low rotational



energies characteristic of non-Boltzmann behavior, but with trends that are adequately captured with double exponential fits characterized by Tcold and Thot. At high collision energies (Einc = 20 kcal/mol), the rotational distributions reflect temperatures of Tcold = 360(30) K and Thot = 1420(80) K. It is worth noting that the higher of these two temperatures is within experimental uncertainty of the 1400(40) K surface temperature of the molten Au, which could signal that this component arises from short term trapping desorption events in equilibrium with the hot metal. However, at the lower collision energies of Einc = 2.0(7) kcal/mol, much colder temperatures of Tcold = 170(40) K and Thot = 830(20) K are observed, suggesting that such a simple picture of the collision dynamics is not justified. Importantly, all the rotational temperatures in both spin orbit manifolds are far below the 1400(40) K surface temperature of the molten Au. Since the incident beam is populated exclusively by the lower 2П1/2 spin-orbit state, the presence of a colder component observed in each of these 2П3/2 rotational distributions can clearly not be ascribed to contamination by the incident beam. Indeed, similar data have been obtained for the ground spin-orbit (2П1/2) state manifold, which exhibit the same qualitative trends observed from the excited (2П3/2) spin-orbit state. Specifically, for NO(v=0) at Einc = 2.0(7) kcal/mol, Trot is only slightly cooler for the 2Π1/2 (480(50) K) than the 2Π3/2 states (670(53) K), with both spin orbit manifolds exhibiting temperatures within experimental uncertainty (880(50) K and 950(50) K for 2Π1/2 and 2Π3/2, respectively) under high collision energy conditions (Einc = 20(2) kcal/mol). Data for the vibrationally inelastic NO(v = 1) channel is reduced sufficiently in S/N to eliminate any statistically meaningful differences between the two spin orbit manifolds, with both 2Π1/2 and 2

Π3/2 states yielding the same rotational temperatures (Trot = 970-980 K) within experimental

uncertainty for both low and high collision energies.


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In previous molecular scattering experiments from both insulating and conducting liquid surfaces, a two-temperature fit has often been invoked to capture the effects due to microscopic branching between a colder, thermal desorption (TD) scattering pathway with Trot ≈ TS, and a rotationally hotter, impulsive scattering (IS) channel with Trot typically much greater than TS.37-40 Indeed, this simple dual temperature characterization for low temperature gas-liquid interfaces has proven to be surprisingly robust, with linear Boltzmann plots of populations over seven efoldings in dynamic range (see Fig. 2) corresponding to rotational energies of greater than 5kT. We have noted previously41-43 that the reasons for such empirical success of this simple physical model are far from obvious and, at some level of signal-to-noise and energy resolution, must fail, for which the present scattering data for molten Au may be providing first indications. A comprehensive understanding of such quantum state resolved scattering phenomena for NO from molten metals will clearly require additional theoretical efforts. Although the rotational excitation of NO from molten Au may not necessarily lend itself to a simple TD/IS scattering picture, what is clear is that the distributions do conform to a twotemperature model. One possible reason could be the combination of high surface temperature and a relatively weak NO-Au interaction strength (kTS >> D0), which could result in much shorter residence times on the gas-liquid interface. Such effects have been nicely demonstrated in the much simpler Ar + Pt(111) system, as nicely confirmed by theory.44 If so, the colder rotational temperatures derived from these fits could represent not the thermal equilibration process, but rather a gas-surface interaction too transient to warm the jet cooled NO prior to escape. Conversely, the higher rotational temperatures could reflect the microscopic branching fraction of NO molecules that successfully accommodate with the molten Au and emerge in much hotter rotational degrees of freedom. The net result could be a first clear breakdown of the



TD/IS paradigm for collisional energy transfer between internally cold, translationally hyperthermal molecules at gas-molten metal interfaces, for which analysis in terms of the corresponding breakdown in atom + surface (Ar + Pt(111)) scattering dynamics would be particularly instructive.44

IIIB. Vibrational State Distributions In order to further investigate the energy transfer dynamics and accommodation with the liquid surface, we now consider distributions in the vibrationally excited NO(v = 1) manifold (see Fig. 2b), again for incident collision energies at both Einc = 2 and 20 kcal/mol. There is considerably more scatter in these NO(v = 1) rotational distributions, due to ~30x lower population density than for NO(v = 0). As a result, these distributions can therefore only be fit to a single temperature, yielding hot and very nearly equivalent values of Trot = 980(70) K at high Einc and Trot = 970(70) K at low Einc. Especially striking is the observation of vibrationally excited NO(v = 1) at Einc = 2 kcal/mol, where the collision energy is less than the vibrational energy spacing, i.e. Einc = 700 cm-1 < ΔEvib = 1876 cm-1. This implies that Einc by itself is insufficient on average to excite NO(v = 1), which therefore requires extraction of thermal energy from the hot molten Au. This would be consistent with an excitation process dominated by thermal electron-hole pair populations in the metal insensitive to incident collision energy. While phonon modes in the metal could also vibrationally excite NO, it is less likely as such processes would require highly multi-phonon processes and longer residence interactions times than anticipated to be reasonable at these high surface temperatures. A Boltzmann plot for the vibrational temperature of NO scattered from Au(liq) can be extracted by integrating over all spin-orbit and rotational states within each vibrational state via


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the simple expression [v = 1]/[v = 0] = exp[-ΔEvib/(kTvib)], where the vibrational energy spacing between NO(v = 0) and NO(v = 1) is experimentally known to be ΔEvib = 1876 cm-1. To extract such populations, the LIF data are all appropriately corrected for relevant differences in FranckCondon factors and fluorescence transmission of the UV filter. The upper (red) two-point Boltzmann plot in Fig. 3 illustrates the vibrational temperature calculation for Einc = 20(2) kcal/mol, with a linear fit corresponding to Tvib = 781(40) K. This is significantly cooler than the 1400(40) K liquid gold surface and equivalent to a ≈ 3.1% vibrational excitation probability for NO(v = 1  0). A similar treatment of the low collision energy data (Einc = 2.0(7) kcal/mol yields the lower (green) Boltzmann plot, which corresponds to an only very slightly cooler vibrational temperature of Tvib = 724(40) K and an excitation probability of ≈ 2.3%. Particularly noteworthy is the remarkable insensitivity in the vibrational excitation probability to an order of magnitude increase in incident collision energy, particularly over a range of energies far below and far above the NO(v = 1) energetic threshold. Again, such an insensitivity to energy points toward a vibrationally inelastic process dominated by thermal electron-hole pair excitations in the hot molten Au. In the interest of completeness, LIF detection of NO(v = 2) was also attempted via the A2Σ+(v’ = 0)  X2П(v” = 2) band; however, no peaks were detected. The NO(v = 2) signals have been integrated over the range of expected frequencies to determine a lower limit to the population ratio of [v = 2]/[v = 0] ≤ 1.5 x 10-3, as indicated on Fig. 3 by the gray dashed line.

Extrapolation of the linear Boltzmann fit from the [v = 0] and [v = 1] populations predicts the current [v = 2] populations to lie below detection sensitivity limits of the LIF based experimental methods presently available. To help circumvent these limitations, we are currently



implementing REMPI ion detection methods, with which we hope to gain at least another order of magnitude in detection sensitivity.

IV. Discussion IVA. Rovibronic Energy Transfer at the NO + Au(liq) Interface The data in Fig. 4 summarize the final distributions of NO scattered from Au(liq) (TS = 1400(40) K) at low (Einc = 2 kcal/mol) and high (Einc = 20 kcal/mol) collision energies. In the interest of simplicity, the data are reported as effective temperatures and calculated as follows. The electronic temperature for NO is computed by equating the ratio of ground [2П1/2] and

excited state [2П3/2] populations to a Boltzmann expression [2П3/2]/ [2П1/2] = exp(-Eso/kTelec),

where the NO spin-orbit splitting in the Hund’s case (a) limit is taken to be Eso = 120 cm-1. As the high collision energy results may show evidence for dual channel pathways, rotational temperatures are simply estimated for each vibrational state by calculating the average rotational energy and equating it to the equipartition expression = kTrot. Finally, NO vibrational temperatures are calculated in a similar statistical mechanical way by integrating over all rotational and spin-orbit states populated and equating [v=1]/[v=0] with exp[-ΔEvib/(kTvib)], as

used previously in Fig. 3. Most evident in the NO + Au(liq) scattering data in Fig. 4 is the lack of thermal accommodation for any of the three NO internal molecular degrees of freedom (i.e. rotational, electronic, or vibrational) with a hot molten gold surface at both collision energies studied. All of these effective temperatures are significantly below the temperature of the molten Au, which implies that complete thermal accommodation for a transient scattering of NO molecules represents an unlikely dynamical scenario. This again suggests residence times on the gas-molten


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Au interface too short for trapping desorption (TD) events to occur with high probability. Broadly speaking, however, the scattering data do reveal a general increase in energy transfer efficiency into NO internal rotation and electronic degrees of freedom as a function of incident energy. One simple picture would be that this reflects a competition between i) lower Einc resulting in a higher probability for longer residence times (and more heating from the surface) and ii) higher Einc resulting in greater system energy, resulting in increased kinematic transfer from translational into rotational degrees of freedom. A closer look at the data, however, reveals striking differences between the vibrationally inelastic and elastic scattering pathways. As noted earlier, the presence of NO(v = 1) scattered at the lowest energies (Einc = 2 kcal/mol 400 K) difference in Trot exists between the two collision energies for NO(v = 0), which contrasts with only ∆T ~ 100 K differences in Trot for NO(v = 1). The data indicate that energy transfer from translation to rotational/spin-orbit degrees of freedom is more prominent when accompanied by vibrationally elastic collisions. By way of contrast, however, in collisions where NO is vibrationally excited (i.e., NO[v = 1  0]),



the excitation energy is predominantly provided by the molten metal surface, where the rotational and spin-orbit distributions are relatively insensitive to Einc. These observations are consistent with electron-hole pair formation in the molten Au coupling to vibrational degrees of freedom in the incident NO, resulting in vibrational excitation of scattered NO. In this regard, it is also interesting to note that studies by the Wodtke group have revealed a strong orientational dependence to NO(v) interactions at the Au(111) surface, whereby collisions with the “N-end” vs. “O-end” are shown to be significantly more facile at vibrational relaxation out of the NO(v = 3) manifold.5 Since molecular orientation in collisions with the surface could also strongly impact the degree of rotational excitation, this could in principle provide an elegantly simple physical mechanism for generating the experimentally observed correlations between vibrational and rotational excitation in the NO + Au(liq) scattering dynamics. Validation of such a physical mechanism will obviously require further theoretical treatment, toward which we hope the current data provides sufficient stimulus.

IVB. Direct comparison of NO scattered from solid Au(111) vs. molten Au(liq) Despite a wealth of molecular beam scattering experiments from both single crystal4,6,9,12,20,32,45-50 and liquid molten metal26,29 interfaces, to the best of our knowledge there has been no direct comparison of quantum state resolved scattering dynamics at the gas-liquid vs. gas-solid interface for the same metal. This could be a particularly interesting comparison for NO scattering from single crystal vs. molten Au, where vibrational excitation/relaxation dynamics are thought to be influenced by short range electron transfer, surface hopping and nonadiabatic formation of a transient NO- anion.12 As another potentially relevant difference, the surface of molten gold at 1400 K is characterized by large amplitude capillary waves with out-


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of-surface rms fluctuations on the order of ~2 Å,51 which contrasts with the more near atomically flat surface of crystalline Au(111). The data presented in Fig. 5 therefore offer a first glimpse at a comparison of NO scattering dynamics from solid and liquid phases of the same material. In Fig. 5, we present results from this work for NO scattered from molten Au(liq) at 1400(40) K, alongside temperature dependent results (300 - 1000 K) from Wodtke and coworkers for NO scattered from single crystalline Au(111).52 Both data sets reflect experiments where supersonically cooled NO(v = 0) is scattered from a temperature controlled Au sample, at normal incidence (θinc = θs = 0º) for the Wodtke study of Au(111), and glancing angles (θinc = θs = 45º) for the current experiments with molten Au(liq). In this regard, it is worth noting the recent ab initio molecular dynamics (AIMD) results from the Guo group18 on reactive scattering of H from Cl-Au(111), which revealed an angular sensitivity both to reaction pathway (Eley-Rideal) and excitation of electron-hole pairs (EHP) and phonons in Au(111) for normal (θinc = θs = 0º) vs. more glancing (θinc = θs = 45º) scattering conditions.53 The effective rotational temperatures corresponding to vibrationally excited NO(v = 1) are shown in Fig. 5(a), with the corresponding vibrational temperatures of the scattered NO(v) displayed in Fig. 5(b). Collisional excitation into the both rotational and vibrational degrees of freedom is also considered a function of incident collision energy (low Einc = 0.09(3) eV for Au(liq) and 0.09 eV for Au(111) and high Einc = 0.87(9) eV for Au(liq) and 0.90 eV for Au(111)). It should be noted that both scattering experiments presume an isotropic distribution (i.e., no preferred molecular orientation) of the incident NO striking the surface. The rotational temperatures in Fig. 5a represent an equipartition principle average over all rotational states populated. Even though not all rotational distributions are well described by a single thermal temperature (e.g., see Fig. 4 at high collision energies), this average Trot is



nevertheless useful to quantify the total energy transferred into the rotational degree of freedom. Fig. 5a reveals an increase in rotational excitation as a function of surface temperature. As crystalline Au(111) warms from 300 to 1000 K, the rotational temperature increases gradually at a rate lower than increase in the surface temperature. As Au melts into liquid form, a further increase is observed that is visually consistent with smooth extrapolation of the results for Au(111). Particularly interesting would be Au(111) data at higher temperatures in the gap between 1000-1300 K, the experiments for which are currently underway. Vibrational temperatures of NO scattered from Au(liq) and Au(111) surfaces are plotted in Fig. 5b as a function of surface temperature. Again, an increase in Tvib is observed for rising surface temperatures at both low and high collision energies. The trend observed for vibrational excitation vs. TS for NO scattering from single crystal Au(111) is consistent with the observed vibrational excitation probability from molten Au. While these results beg for data to fill in the gap between 1000-1300K, they also already suggest a remarkable similarity in rotational and vibrational energy transfer of NO scattered from Au(111) single crystal compared to molten Au. Vibrational and rotational excitations appear to be largely dependent on surface temperature and only weakly influenced by the phase of Au. The spatial fluctuations due to the capillary wave excitation at the molten gold surface, which are roughly twice the size of the bond length for NO, do not seem to have a significant effect on energy transfer from the Au surface into the rovibronic states of NO. If the NO-Au binding potential is relatively weak, it is possibly NO molecules are not able to get close enough to the surface to ‘feel’ the surface roughness of molten Au, thus leading to the qualitative similarities observed for scattering from solid and liquid Au. Further theory and experimental work will clearly be necessary, toward which we hope the current experiments provide the requisite stimulation. We emphasize that recent


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successes in using ab initio molecular dynamics (AIMD) with Au single crystal surfaces might therefore represent the ideal compromise to rigorous calculation of quantum state resolved changes in the non-adiabatic energy transfer and scattering dynamics across the single crystal to molten metal phase transition.18,54,55

V. Conclusion Quantum state resolved molecular beam scattering techniques have been utilized to investigate collision dynamics of NO at the gas-molten Au interface. The results represent first detection of vibrationally excited NO from collisions at a liquid metal surface. Moreover, vibrational excitation is observed even at low collision energies (Einc

2) from Hot Molten Au

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