Real-Time Observation of Molecular Motion on a Surface

Integration of conventional surface science techniques with surface-sensitive azimuthal and polarization dependent femtosecond-resolved sum frequency generation spectroscopy

Experimental insight into the elementary processes underlying charge transfer across interfaces has blossomed with the wide-spread availability of ultra-high vacuum (UHV) setups that allow the preparation and characterization of solid surfaces with well-defined molecular adsorbates over a wide range of temperatures. Within the last 15 years, such insights have extended to charge transfer heterostructures containing solids overlain by one or more atomically thin two dimensional materials. Such systems are of wide potential interest both because they appear to offer a path to separate surface reactivity from bulk chemical properties and because some offer completely novel physics, unrealizable in bulk three dimensional solids. Thick layers of molecular adsorbates or heterostructures of 2D materials generally preclude the use of electrons or atoms as probes. However, with linear photon-in/photon-out techniques, it is often challenging to assign the observed optical response to a particular portion of the interface. We and prior workers have demonstrated that by full characterization of the symmetry of the second order nonlinear optical susceptibility, i.e., the χ(2), in sum frequency generation (SFG) spectroscopy, this problem can be overcome. Here, we describe an UHV system built to allow conventional UHV sample preparation and characterization, femtosecond and polarization resolved SFG spectroscopy, the azimuthal sample rotation necessary to fully describe χ(2) symmetry, and sufficient stability to allow scanning SFG microscopy. We demonstrate these capabilities in proof-of-principle measurements on CO adsorbed on Pt(111) and on the clean Ag(111) surface. Because this setup allows both full characterization of the nonlinear susceptibility and the temperature control and sample preparation/characterization of conventional UHV setups, we expect it to be of great utility in the investigation of both the basic physics and applications of solid, 2D material heterostructures.

Bond Selective Photochemistry at Metal Nanoparticle Surfaces: CO Desorption from Pt and Pd

The use of visible photon fluxes to influence catalytic reactions on metal nanoparticle surfaces has attracted attention based on observations of reaction mechanisms and selectivity not observed under equilibrium heating. These observations suggest that photon fluxes can selectively impact the rates of certain elementary steps, creating nonequilibrium energy distributions among various reaction pathways. However, quantitative studies validating these hypotheses on metal nanoparticle surfaces are lacking. We examine the influence of continuous wave visible photon fluxes on the CO desorption rates from 1 to 2 nm diameter Pt and Pd nanoparticle surfaces supported on γ-Al2O3. Temperature-programmed desorption measurements quantified via diffuse reflectance infrared Fourier transform spectroscopy demonstrate that visible photon fluxes significantly enhanced the rate of CO desorption from Pt nanoparticles in a wavelength-dependent manner. 440 nm photons most efficiently promoted CO desorption from Pt nanoparticle surfaces, aligning with the excitation energy for the interfacial electronic transition within the Pt-CO bond. Conversely, visible photon fluxes had no measurable influence on CO desorption rates from Pd nanoparticle surfaces after accounting for photon-induced heating. Density functional theory calculations demonstrate that the Pt-CO bond exhibits a narrower LUMO resonance, stronger coupling between the photoexcitation and forces induced on the metal-C bond, and vibrational energy dissipation that more effectively couples to desorption as compared to Pd-CO. These results demonstrate the specificity photons provide in facilitating chemical reactions on metal nanoparticle surfaces and substantiate the idea that photon fluxes can steer processes and outcomes of catalytic reactions in ways not achievable by equilibrium heating.

Increased Selectivity in Photolytic Activation of Nanoassemblies Compared to Thermal Activation in On-Surface Ullmann Coupling

On-surface synthesis is a powerful method that has emerged recently to fabricate a large variety of atomically precise nanomaterials on surfaces based on polymerization. It is very successful for thermally activated reactions within the framework of heterogeneous catalysis. As a result, it often lacks selectivity. We propose to use selective activation of specific bonds as a crucial ingredient to synthesize desired molecules with high selectivity. In this approach, thermally nonaccessible products are expected to arise in photolytically activated on-surface reactions with high selectivity. We demonstrate for assembled 2,2'-dibromo biphenyl clusters on Cu(111) that the thermal and photolytic activations yield distinctly different products, combining submolecular resolution of individual product molecules in real-space imaging by scanning tunneling microscopy with chemical identification in X-ray photoelectron spectroscopy and supported by ab initio calculations. The photolytically activated Ullmann coupling of 2,2'-dibromo biphenyl is highly selective, with only one identified product. It starkly contrasts the thermal reaction, which yields various products because alternate pathways are activated at the reaction temperature. Our study extends on-surface synthesis to a directed formation of thermally inaccessible products by direct bond activation. It promises tailored reactions of nanomaterials within the framework of on-surface synthesis based on the photolytic activation of specific bonds.

Time-dependent band position difference between vibrational sum and difference frequency generation: a phenomenon originating from dispersion in the visible pulse

Vibrational spectroscopy is significant for identifying chemical specification. Here, the spectral band frequencies corresponding to the same molecular vibration in sum frequency generation (SFG) and difference frequency generation (DFG) spectra present delay-dependent deviation. Through numerical analysis of time resolved SFG and DFG spectra with a frequency marker in the incident IR pulse, the frequency ambiguity was not caused by any structure and dynamic variation on the surface, but from the dispersion in the incident visible pulse. Our results provide a helpful method to correct the vibrational frequency deviation and improve the assignment accuracy for SFG and DFG spectroscopies.

All-experimental analysis of doubly resonant sum-frequency generation spectra for Franck–Condon and Herzberg–Teller vibronic modes

The transform technique applied to the analysis of doubly resonant sum-frequency generation (DR-SFG) spectra is extended to include Herzberg–Teller (HT) vibronic modes. The experimentally measured overlap spectral function generates all the energy resonant amplitudes of the DR-SFG excitation function for both Franck–Condon (FC) and HT modes. When FC modes dominate the DR-SFG spectra, a methodology is provided to perform efficient curve fitting and orientation analysis in order to extract FC activities of the various vibration modes from experimental spectra with the help of a molecular model. Determination of the FC or HT natures of the vibration modes from DR-SFG data is also shown to be possible through their visible line shapes with an appropriate choice of polarizations. As an example, experimental DR-SFG data suggest that a known HT-active mode in the vibronic structure of Rhodamine 6G monomers exhibits a FC behavior in molecular aggregates.

Novel Quinolizine AIE System: Visualization of Molecular Motion and Elaborate Tailoring for Biological Application

Molecular motions are ubiquitous in nature and they immutably play intrinsic roles in all actions. However, exploring appropriate models to decipher molecular motions is an extremely important but very challenging task for researchers. Considering aggregation-induced emission (AIE) luminogens possess their unique merits to visualize molecular motions, it is particularly fascinating to construct new AIE systems as models to study molecular motion. Herein, a novel quinolizine (QLZ) AIE system was constructed based on the restriction intramolecular vibration (RIV) mechanism. It was demonstrated that QLZ could act as an ideal model to visualize single-molecule motion and macroscopic molecular motion via fluorescence change. Additionally, further elaborate tailoring of this impressive core achieved highly efficient reactive oxygen species production and realized fluorescence imaging-guided photodynamic therapy applications, which confirms the great application potential of this new AIE-active QLZ core. Therefore, this work not only provides an ideal model to visualize molecular motion but also opens a new way for the application of AIEgens.

Time-Variable Chiroptical Vibrational Sum-Frequency Generation Spectroscopy of Chiral Chemical Solution

Vibrational sum-frequency generation (VSFG) spectroscopy, a surface-specific technique, was shown to be useful even for characterizing the vibrational optical activity of chiral molecules in isotropic bulk liquids. However, accurately determining the spectroscopic parameters is still challenging because of the spectral congestion of chiroptical VSFG peaks with different amplitudes and phases. Here, we show that a time-variable infrared-visible chiroptical three-wave-mixing technique can be used to determine the spectroscopic parameters of second-order vibrational response signals from chiral chemical liquids. For varying the delay time between infrared and temporally asymmetric visible laser pulses, we measure the chiral VSFG, achiral VSFG, and their interference spectra of bulk R-(+)-limonene liquid and perform a global fitting analysis for those time-variable spectra to determine their spectroscopic parameters accurately. We anticipate that this time-variable VSFG approach will be useful for developing nearly background-free chiroptical characterization techniques with enhanced spectral resolution.

Ultrafast Adsorbate Excitation Probed with Subpicosecond-Resolution X-Ray Absorption Spectroscopy

We use a pump-probe scheme to measure the time evolution of the C K-edge x-ray absorption spectrum from CO/Ru(0001) after excitation by an ultrashort high-intensity optical laser pulse. Because of the short duration of the x-ray probe pulse and precise control of the pulse delay, the excitation-induced dynamics during the first picosecond after the pump can be resolved with unprecedented time resolution. By comparing with density functional theory spectrum calculations, we find high excitation of the internal stretch and frustrated rotation modes occurring within 200 fs of laser excitation, as well as thermalization of the system in the picosecond regime. The ∼100  fs initial excitation of these CO vibrational modes is not readily rationalized by traditional theories of nonadiabatic coupling of adsorbates to metal surfaces, e.g., electronic frictions based on first order electron-phonon coupling or transient population of adsorbate resonances. We suggest that coupling of the adsorbate to nonthermalized electron-hole pairs is responsible for the ultrafast initial excitation of the modes.

All-experimental analysis of doubly resonant sum-frequency generation spectra: Application to aggregated rhodamine films

A new method is proposed to analyze Doubly Resonant infrared-visible Sum-Frequency Generation (DR-SFG) spectra. Based on the transform technique, this approach is free from assumptions about vibronic modes, energies, or line widths and accurately captures through the overlap spectral function all required aspects of the vibronic structure from simple experimental linear absorption spectra. Details and implementation of the method are provided along with three examples treating rhodamine thin films about one monolayer thick. The technique leads to a perfect agreement between experiment and simulations of the visible DR-SFG line shapes, even in the case of complex intermolecular interactions resulting from J-aggregated chromophores in heterogeneous films. For films with mixed H- and J-aggregates, separation of their responses shows that the J-aggregate DR-SFG response is dominant. Our analysis also accounts for the unexplained results published in the early times of DR-SFG experiments.

Transient CO desorption from thin Pt films induced by mid-IR pumping

Resonant and off-resonant mid-infrared pump-probe spectroscopy is used to measure the vibrational dynamics of CO adsorbed to thin (0.2 nm, 2 nm, and 10 nm) heterogeneous Pt layers in an aqueous solution. The transient signals observed with resonant pumping are dominated by vibrational relaxation of the CO internal stretch vibration with a lifetime of T₁ ∼ 3 ps in all cases. Off-resonant pumping suppresses that contribution to the signal and singles out a signal, which is attributed to heating of the metal layer as well as transient desorption of the CO molecules. Due to the small photon energy (0.2 eV) used as pump pulses, the mechanism of desorption must be thermal, in which case the desorption yield depends exclusively on the fluence of absorbed light and not its wavelength. The thin Pt layers facilitate CO desorption, despite a relatively low pump pulse fluence, as they concentrate the absorbed energy in a small volume.

Polarization- and Wavelength-Dependent Shell-Isolated-Nanoparticle-Enhanced Sum-Frequency Generation with High Sensitivity

Sum-frequency generation (SFG) spectroscopy is a highly versatile tool for surface analysis. Improving the SFG intensity per molecule is important for observing low concentrations of surface species and intermediates in dynamic systems. Herein, Shell-Isolated-Nanoparticle-Enhanced SFG (SHINE-SFG) was used to probe a model substrate. The model substrate, p-mercaptobenzonitrile adsorbed on a Au film with SHINs deposited on top, provided an enhancement factor of up to 10^{5}. Through wavelength- and polarization-dependent SHINE-SFG spectroscopy, the majority of the signal enhancement was found to come from both plasmon enhanced emission and chemical enhancement mechanisms. A new enhancement regime, i.e., the nonlinear coupling of SHINE-SFG with difference frequency generation, was also identified. This novel mechanism provides insight into the enhancement of nonlinear coherent spectroscopies and a possible strategy for the rational design of enhancing substrates utilizing coupling processes.

On-Surface Chemistry Induced by Long-Lived Excitons: (NO)2 Dissociation on C60

Solid-state excitonic excitations play an increasingly important role in optoelectronic and light harvesting processes due to their ubiquitous presence in dipolar two-dimensional materials. Here we show that long-lived solid-state excitons induce chemical reactions in adsorbed molecules and thus convert light into chemical energy. For the model system (NO)₂ dimer adsorbed on ordered c(4×4) C₆₀ films, time-of-flight measurements following UV laser excitation reveal a slow and a fast dissociative desorption channel, which are assigned to intersystem crossing and internal conversion, respectively, by time-dependent density functional theory calculations.

Time-resolved observation of transient precursor state of CO on Ru(0001) using carbon K-edge spectroscopy

The transient dynamics of carbon monoxide (CO) molecules on a Ru(0001) surface following femtosecond optical laser pump excitation has been studied by monitoring changes in the unoccupied electronic structure using an ultrafast X-ray free-electron laser (FEL) probe. The particular symmetry of perpendicularly chemisorbed CO on the surface is exploited to investigate how the molecular orientation changes with time by varying the polarization of the FEL pulses. The time evolution of spectral features corresponding to the desorption precursor state was well distinguished due to the narrow line-width of the C K-edge in the X-ray absorption (XA) spectrum, illustrating that CO molecules in the precursor state rotated freely and resided on the surface for several picoseconds. Most of the CO molecules trapped in the precursor state ultimately cooled back down to the chemisorbed state, while we estimate that ∼14.5 ± 4.9% of the molecules in the precursor state desorbed into the gas phase. It was also observed that chemisorbed CO molecules diffused over the metal surface from on-top sites toward highly coordinated sites. In addition, a new "vibrationally hot precursor" state was identified in the polarization-dependent XA spectra.

The Prevailing Role of Hotspots in Plasmon-Enhanced Sum-Frequency Generation Spectroscopy

The plasmonic amplification of nonlinear vibrational sum frequency spectroscopy (SFG) at the surfaces of gold nanoparticles is systematically investigated by tuning the incident visible wavelength. The SFG spectra of dodecanethiol-coated gold nanoparticles chemically deposited on silicon are recorded for 20 visible wavelengths. The vibrational intensities of thiol methyl stretches extracted from the experimental measurements vary with the visible color of the SFG process and show amplification by coupling to plasmon excitation. Because the enhancement is maximal in the orange-red region rather than in the green, as expected from the dipolar model for surface plasmon resonances, it is attributed mostly to hotspots created in particle multimers, in spite of their low surface densities. A simple model accounting for the longitudinal surface plasmons of multimers allows the recovery of the experimental spectral dispersion.

Fundamentals of TiO2 Photocatalysis: Concepts, Mechanisms, and Challenges

Photocatalysis has been widely applied in various areas, such as solar cells, water splitting, and pollutant degradation. Therefore, the photochemical mechanisms and basic principles of photocatalysis, especially TiO₂ photocatalysis, have been extensively investigated by various surface science methods in the last decade, aiming to provide important information for TiO₂ photocatalysis under real environmental conditions. Recent progress that provides fundamental insights into TiO₂ photocatalysis at a molecular level is highlighted. Insights into the structures of TiO₂ and the basic principles of TiO₂ photocatalysis are discussed first, which provides the basic concepts of TiO₂ photocatalysis. Following this, details of the photochemistry of three important molecules (oxygen, water, methanol) on the model TiO₂ surfaces are presented, in an attempt to unravel the relationship between charge/energy transfer and bond breaking/forming in TiO₂ photocatalysis. Lastly, challenges and opportunities of the mechanistic studies of TiO₂ photocatalysis at the molecular level are discussed briefly, as well as possible photocatalysis models.

Abnormal spectral bands in broadband sum frequency generation induced by bulk absorption and refraction

In this paper, the time-resolved broadband sum frequency generation (BB-SFG) spectra from a bare Au surface with a distorted infrared (introduced with a 10 µm polyethylene film in the IR light path) and principal component generalized projection (PCGP) algorithm were used to investigate the bulk distortion on the measured BB-SFG spectra. Besides the SFG intensity reduction from the bulk absorption, the frequency dependent refraction of the bulk layer led to misleading SFG features at the positive delay times beyond the Au dephasing time. These results suggest that SFG spectroscopy is not entirely 'bulk-free' for the buried interfaces because of the bulk absorption and refraction of the incident pulses.

Accessing a Charged Intermediate State Involved in the Excitation of Single Molecules

Intermediate states are elusive to many experimental techniques due to their short lifetimes. Here, by performing single-electron alternate charging scanning tunneling microscopy of molecules on insulators, we accessed a charged intermediate state involved in the rapid toggling of individual metal phthalocyanines deposited on NaCl films. By stabilizing the transient species, we reveal how electron injection into the lowest unoccupied molecular orbital leads to a pronounced change in the adsorption geometry, characterized by a different azimuthal orientation. This observation allows clarifying the nature of the toggling process, unveiling the role of transient ionic states involved into fundamental processes occurring at interfaces.

Ultrafast dynamics of the dipole moment reversal in a polar organic monolayer

Pyridine layers on Cu(110) possess a strong electric field due to the large dipole of adsorbed pyridine. This electric field is visible as an enhanced sum frequency response from both the copper surface electrons and the aromatic C-H stretch of pyridine via a third order susceptibility. In response to a visible pump pulse, both surface electron and C-H stretch sum frequency signals are reduced on a subpicosecond time scale. In addition, the relative phase between the two signals changes over a few hundred femtoseconds, which indicates a change in the electronic structure of the adsorbate. We explain the transients as a consequence of the previously observed pyridine dipole field reversal when the pump pulse excites electrons into the pyridine π^* orbital. The pyridine anions in the pyridine layer cause a large-scale structural change which alters the pyridine-copper bond, reflected in the altered sum frequency response.

Ultrafast Vibrational Dynamics of CO Ligands on RuTPP/Cu(110) under Photodesorption Conditions

We have studied CO coordinated to ruthenium tetraphenylporphyrin (RuTPP)/Cu(110) and directly adsorbed to Cu(110), using femtosecond pump-sum frequency probe spectroscopy, to alter the degree of electron-vibration coupling between the metal substrate and CO. We observe the facile femtosecond laser-induced desorption of CO from RuTPP/Cu(110), but not from Cu(110). A change in the vibrational transients, in the first few picoseconds, from a red- to blue-shift of the C–O stretching vibration under photodesorption conditions, was also observed. This drastic change can be explained, if the cause of the C–O frequency redshift of Cu(110) is not the usually-assumed anharmonic coupling to low frequency vibrational modes, but a charge transfer from hot electrons to the CO 2π* state. This antibonding state shifts to higher energies on RuTPP, removing the C–O redshift and, instead, reveals a blueshift, predicted to arise from electron-mediated coupling between the coherently excited internal stretch and low frequency modes in the system.

Ultrafast Transient Dynamics of Adsorbates on Surfaces Deciphered: The Case of CO on Cu(100)

Time-resolved vibrational spectroscopy constitutes an invaluable experimental tool for monitoring hot-carrier-induced surface reactions. However, the absence of a full understanding of the precise microscopic mechanisms causing the transient spectral changes has limited its applicability. Here we introduce a robust first-principles theoretical framework that successfully explains both the nonthermal frequency and linewidth changes of the CO internal stretch mode on Cu(100) induced by femtosecond laser pulses. Two distinct processes engender the changes: electron-hole pair excitations underlie the nonthermal frequency shifts, while electron-mediated vibrational mode coupling gives rise to linewidth changes. Furthermore, the origin and precise sequence of coupling events are finally identified.

Determination of Specific Electrocatalytic Sites in the Oxidation of Small Molecules on Crystalline Metal Surfaces

The identification of active sites in electrocatalytic reactions is part of the elucidation of mechanisms of catalyzed reactions on solid surfaces. However, this is not an easy task, even for apparently simple reactions, as we sometimes think the oxidation of adsorbed CO is. For surfaces consisting of non-equivalent sites, the recognition of specific active sites must consider the influence that facets, as is the steps/defect on the surface of the catalyst, cause in its neighbors; one has to consider the electrochemical environment under which the "active sites" lie on the surface, meaning that defects/steps on the surface do not partake in chemistry by themselves. In this paper, we outline the recent efforts in understanding the close relationships between site-specific and the overall rate and/or selectivity of electrocatalytic reactions. We analyze hydrogen adsorption/desorption, and electro-oxidation of CO, methanol, and ammonia. The classical topic of asymmetric electrocatalysis on kinked surfaces is also addressed for glucose electro-oxidation. The article takes into account selected existing data combined with our original works.

Understanding the Enhancement of Surface Diffusivity by Dimerization

Beyond dilute coverage, the collective diffusion of molecules might enhance material transport. We reveal an enhanced mobility of molecular dimers by separating two motions, diffusion and rotation, of CO dimers on elemental Ag(100) as well as on a dilute Cu alloy of Ag(100). From time-lapsed scanning tunneling microscopy movies recorded between 15 and 25 K, we determine the activation energy of dimer diffusion on elemental Ag(100) to be, at (40±2)  meV, considerably smaller than the one for monomer diffusion, at (72±1)  meV. The alloyed Cu atoms reduce the dimer mobility facilitating to determine their rotational barrier separately to be (39±3)  meV. Disentangling different degrees of freedom suggests that a rotational motion is at the origin of enhanced dimer diffusivity.

Quantum-Sized Metal Catalysts for Hot-Electron-Driven Chemical Transformation

Hot-electron-driven chemical transformation (HEDCT) represents an emerging research area in utilizing photoresponsive nanoparticles to enable efficient solar-to-chemical conversion. The unique properties of quantum-sized metal nanoparticles (QSMNPs) make them a class of photocatalysts that can generate hot electrons to drive surface chemical reactions with high quantum efficiency. Compared to the conventional thermal-driven chemical reactions, HEDCT offers the advantages of accelerating reaction rate, improving reaction selectivity, and possibly enabling the occurrence of thermodynamically endergonic reactions. Despite its embryonic stage of development, using QSMNPs for HEDCT shows great promise. Herein, a timely overview on the research progress is provided with a focus on the fundamental quantum processes involved in the photoexcitation of hot electrons and the following HEDCT on the surface of QSMNPs. The last section discusses the challenges, which also represent the opportunities for the materials research community, in designing robust QSMNP photocatalysts and understanding the fundamental quantum phenomena in HEDCT.

Approaching the forbidden fruit of reaction dynamics: Aiming reagent at selected impact parameters

Collision geometry is central to reaction dynamics. An important variable in collision geometry is the miss-distance between molecules, known as the "impact parameter." This is averaged in gas-phase molecular beam studies. By aligning molecules on a surface prior to electron-induced dissociation, we select impact parameters in subsequent inelastic collisions. Surface-collimated "projectile" molecules, difluorocarbene (CF2), were aimed at stationary "target" molecules characterized by scanning tunneling microscopy (STM), with the observed scattering interpreted by computational molecular dynamics. Selection of impact parameters showed that head-on collisions favored bimolecular reaction, whereas glancing collisions led only to momentum transfer. These collimated projectiles could be aimed at the wide variety of adsorbed targets identifiable by STM, with the selected impact parameter assisting in the identification of the collision geometry required for reaction.

Identification of Active Sites in Oxidation Reaction from Real-Time Probing of Adsorbate Motion over Pd Nanoparticles

Obtaining insight into the type of surface sites involved in a reaction is essential to understand catalytic mechanisms at the atomic level and a key for understanding selectivity in surface-catalyzed reactions. Here we use ultrafast broad-band vibrational spectroscopy to follow in real-time diffusion of CO molecules over a palladium nanoparticle surface toward an active site. Site-to-site hopping is triggered by laser excitation of electrons and followed in real-time from subpicosecond changes in the vibrational spectra. CO photoexcitation occurs in 400 fs and hopping from NP facets to edges follows within ∼1 ps. Kinetic modeling allows to quantify the contribution of different facet sites to the catalytic reaction. These results provide useful insights for understanding the mechanism of chemical reactions catalyzed by metal NPs.

Molecular Coupling between Organic Molecules and Metal

Molecular couplings at interfaces play important roles in determining the performance of nanophotonics and molecular electronics. In this Letter, using femtosecond sum frequency generation to trace free-induction decay of vibrationally excited aromatic thiol molecules immobilized on metal with and without the bridged methylene group(s), metal surface free electron-coupled and uncoupled phenyl C-H stretching vibrational modes were identified, with dephasing times of ∼0.28 and ∼0.60 ps, respectively. For thiols on Au with the bridged methylene group(s) (benzyl mercaptan and phenylethanethiol), both the coupled and uncoupled modes were observed; for thiol on Au without the bridged methylene group (thiophenol), only the coupled mode was observed. This indicates that the bridged methylene group(s) serving as a spacer can be used to adjust the molecular coupling between the phenyl vibration and surface free electrons. The experimental approach can be used to tune molecular couplings in advanced nanophotonics and molecular electronics.

Electron-Mediated Phonon-Phonon Coupling Drives the Vibrational Relaxation of CO on Cu(100)

We bring forth a consistent theory for the electron-mediated vibrational intermode coupling that clarifies the microscopic mechanism behind the vibrational relaxation of adsorbates on metal surfaces. Our analysis points out the inability of state-of-the-art nonadiabatic theories to quantitatively reproduce the experimental linewidth of the CO internal stretch mode on Cu(100) and it emphasizes the crucial role of the electron-mediated phonon-phonon coupling in this regard. The results demonstrate a strong electron-mediated coupling between the internal stretch and low-energy CO modes, but also a significant role of surface motion. Our nonadiabatic theory is also able to explain the temperature dependence of the internal stretch phonon linewidth, thus far considered a sign of the direct anharmonic coupling.

Characterization of the Platinum-Hydrogen Bond by Surface-Sensitive Time-Resolved Infrared Spectroscopy

The vibrational dynamics of Pt-H on a nanostructured platinum surface has been examined by ultrafast infrared spectroscopy. Three bands are observed at 1800, 2000, and 2090 cm^-1, which are assigned to Pt-CO in a bridged and linear configuration and Pt-H, respectively. Lifetime analysis revealed a time constant of (0.8 ± 0.1) ps for the Pt-H mode, considerably shorter than that of Pt-CO because of its stronger coupling to the metal substrate. Two-dimensional attenuated total reflection infrared spectroscopy provided additional evidence for the assignment based on the anharmonic shift, which is large in the case of Pt-H (90 cm^-1), in agreement with the density functional theory calculations. The absorption cross section of Pt-H is smaller than that of the very strong Pt-CO vibration by only a modest factor of ∼1.5-3. Because Pt-H is transiently involved in catalytic water splitting on Pt, the present spectroscopic characterization paves the way for in-operando kinetic studies of such reactions.

Vibrational coupling in plasmonic molecules

Plasmon hybridization theory, inspired by molecular orbital theory, has been extremely successful in describing the near-field coupling in clusters of plasmonic nanoparticles, also known as plasmonic molecules. However, the vibrational modes of plasmonic molecules have been virtually unexplored. By designing precisely configured plasmonic molecules of varying complexity and probing them at the individual plasmonic molecule level, intramolecular coupling of acoustic modes, mediated by the underlying substrate, is observed. The strength of this coupling can be manipulated through the configuration of the plasmonic molecules. Surprisingly, classical continuum elastic theory fails to account for the experimental trends, which are well described by a simple coupled oscillator picture that assumes the vibrational coupling is mediated by coherent phonons with low energies. These findings provide a route to the systematic optical control of the gigahertz response of metallic nanostructures, opening the door to new optomechanical device strategies.

Real-Time Elucidation of Catalytic Pathways in CO Hydrogenation on Ru

The direct elucidation of the reaction pathways in heterogeneous catalysis has been challenging due to the short-lived nature of reaction intermediates. Here, we directly measured on ultrafast time scales the initial hydrogenation steps of adsorbed CO on a Ru catalyst surface, which is known as the bottleneck reaction in syngas and CO₂ reforming processes. We initiated the hydrogenation of CO with an ultrafast laser temperature jump and probed transient changes in the electronic structure using real-time X-ray spectroscopy. In combination with theoretical simulations, we verified the formation of CHO during CO hydrogenation.

Electron to Adsorbate Energy Transfer in Nanoparticles: Adsorption Site, Size, and Support Matter

Confinement of hot electrons in metal nanoparticles (NPs) is expected to lead to increased reactivity in heterogeneous catalysis. NP size as well as support may influence molecule-NP coupling. Here, we use ultrafast nonlinear vibrational spectroscopy to follow energy transfer from hot electrons generated in Pd NP/MgO/Ag(100) to chemisorbed CO. Photoexcitation and photodesorption occur on an ultrashort time scale and are selective according to adsorption site. When the MgO layer is thick enough, it becomes NP size-dependent. Hot electron confinement within NPs is unfavorable for photodesorption, presumably because its dominant effect is to increase relaxation to phonons. An avenue of research is open where NP size and support thickness, photon energy, and molecular electronic structure will be tuned to obtain either molecular stability or reactivity in response to photon excitation.

Adlayer structure dependent ultrafast desorption dynamics in carbon monoxide adsorbed on Pd (111)

We report our ultrafast photoinduced desorption investigation of the coverage dependence of substrate-adsorbate energy transfer in carbon monoxide adlayers on the (111) surface of palladium. As the CO coverage is increased, the adsorption site population shifts from all threefold hollows (up to 0.33 ML), to bridge and near bridge (>0.5 to 0.6 ML) and finally to mixed threefold hollow plus top site (at saturation at 0.75 ML). We show that between 0.24 and 0.75 ML this progression of binding site motifs is accompanied by two remarkable features in the ultrafast photoinduced desorption of the adsorbates: (i) the desorption probability increases roughly two orders magnitude, and (ii) the adsorbate-substrate energy transfer rate observed in two-pulse correlation experiments varies nonmonotonically, having a minimum at intermediate coverages. Simulations using a phenomenological model to describe the adsorbate-substrate energy transfer in terms of frictional coupling indicate that these features are consistent with an adsorption-site dependent electron-mediated energy coupling strength, ηel, that decreases with binding site in the order: three-fold hollow > bridge and near bridge > top site. This weakening of ηel largely counterbalances the decrease in the desorption activation energy that accompanies this progression of adsorption site motifs, moderating what would otherwise be a rise of several orders of magnitude in the desorption probability. Within this framework, the observed energy transfer rate enhancement at saturation coverage is due to interadsorbate energy transfer from the copopulation of molecules bound in three-fold hollows to their top-site neighbors.

Experimental and Theoretical Investigation of the Restructuring Process Induced by CO at Near Ambient Pressure: Pt Nanoclusters on Graphene/Ir(111)

The adsorption of CO on Pt nanoclusters grown in a regular array on a template provided by the graphene/Ir(111) Moiré was investigated by means of infrared-visible sum frequency generation vibronic spectroscopy, scanning tunneling microscopy, X-ray photoelectron spectroscopy from ultrahigh vacuum to near-ambient pressure, and ab initio simulations. Both terminally and bridge bonded CO species populate nonequivalent sites of the clusters, spanning from first to second-layer terraces to borders and edges, depending on the particle size and morphology and on the adsorption conditions. By combining experimental information and the results of the simulations, we observe a significant restructuring of the clusters. Additionally, above room temperature and at 0.1 mbar, Pt clusters catalyze the spillover of CO to the underlying graphene/Ir(111) interface.

Mass Transport in Surface Diffusion of van der Waals Bonded Systems: Boosted by Rotations?

Mass transport at a surface is a key factor in heterogeneous catalysis. The rate is determined by excitation across a translational barrier and depends on the energy landscape and the coupling to the thermal bath of the surface. Here we use helium spin-echo spectroscopy to track the microscopic motion of benzene adsorbed on Cu(001) at low coverage (θ ∼ 0.07 ML). Specifically, our combined experimental and computational data determine both the absolute rate and mechanism of the molecular motion. The observed rate is significantly higher by a factor of 3.0 ± 0.1 than is possible in a conventional, point-particle model and can be understood only by including additional molecular (rotational) coordinates. We argue that the effect can be described as an entropic contribution that enhances the population of molecules in the transition state. The process is generally relevant to molecular systems and illustrates the importance of the pre-exponential factor alongside the activation barrier in studies of surface kinetics.

Disentangling Multidimensional Nonequilibrium Dynamics of Adsorbates: CO Desorption from Cu(100)

Hot carriers at metal surfaces can drive nonthermal reactions of adsorbates. Characterizing nonequilibrium statistics among various degrees of freedom in an ultrafast time scale is crucial to understand and develop hot carrier-driven chemistry. Here we demonstrate multidimensional vibrational dynamics of carbon monoxide (CO) on Cu(100) along hot-carrier induced desorption studied by using time-resolved vibrational sum-frequency generation with phase-sensitive detection. Instantaneous frequency and amplitude of the CO internal stretching mode are tracked with a subpicosecond time resolution that is shorter than the vibrational dephasing time. These experimental results in combination with numerical analysis based on Langevin simulations enable us to extract nonequilibrium distributions of external vibrational modes of desorbing molecules. Superstatistical distributions are generated with mode-dependent frictional couplings in a few hundred femtoseconds after hot-electron excitation, and energy flow from hot electrons and intermode anharmonic coupling play crucial roles in the subsequent evolution of the non-Boltzman distributions.

Frequency-domain nonlinear regression algorithm for spectral analysis of broadband SFG spectroscopy

The resonant spectral bands of the broadband sum frequency generation (BB-SFG) spectra are often distorted by the nonresonant portion and the lineshapes of the laser pulses. Frequency domain nonlinear regression (FDNLR) algorithm was proposed to retrieve the first-order polarization induced by the infrared pulse and to improve the analysis of SFG spectra through simultaneous fitting of a series of time-resolved BB-SFG spectra. The principle of FDNLR was presented, and the validity and reliability were tested by the analysis of the virtual and measured SFG spectra. The relative phase, dephasing time, and lineshapes of the resonant vibrational SFG bands can be retrieved without any preset assumptions about the SFG bands and the incident laser pulses.

Two distinctive energy migration pathways of monolayer molecules on metal nanoparticle surfaces

Energy migrations at metal nanomaterial surfaces are fundamentally important to heterogeneous reactions. Here we report two distinctive energy migration pathways of monolayer adsorbate molecules on differently sized metal nanoparticle surfaces investigated with ultrafast vibrational spectroscopy. On a 5 nm platinum particle, within a few picoseconds the vibrational energy of a carbon monoxide adsorbate rapidly dissipates into the particle through electron/hole pair excitations, generating heat that quickly migrates on surface. In contrast, the lack of vibration-electron coupling on approximately 1 nm particles results in vibrational energy migration among adsorbates that occurs on a twenty times slower timescale. Further investigations reveal that the rapid carbon monoxide energy relaxation is also affected by the adsorption sites and the nature of the metal but to a lesser extent. These findings reflect the dependence of electron/vibration coupling on the metallic nature, size and surface site of nanoparticles and its significance in mediating energy relaxations and migrations on nanoparticle surfaces.

Energy migrations at metal nanomaterial surfaces are fundamentally important to heterogeneous reactions. Here, the authors employ ultrafast vibrational spectroscopy to show two distinctive energy migration pathways of monolayer adsorbate molecules on differently sized metal nanoparticle surfaces.

A RAIRS, TPD and femtosecond laser-induced desorption study of CO, NO and coadsorbed CO + NO on Pd(111)

Here we describe novel RAIRS, TPD and LID studies of CO, NO and coadsorbed CO and NO on Pd.

Multimodal Broadband Vibrational Sum Frequency Generation (MM-BB-V-SFG) Spectrometer and Microscope

A broadband sum frequency generation (BB-SFG) spectrometer with multimodal (MM) capabilities was constructed, which could be routinely reconfigured for tabletop experiments in reflection, transmission, and total internal reflection (TIR) geometries, as well as microscopic imaging. The system was constructed using a Ti:sapphire amplifier (800 nm, pulse width = 85 fs, repetition rate = 2 kHz), an optical parameter amplification (OPA) system for production of broadband IR pulses tunable between 1000 and 4000 cm(-1), and two Fabry-Pérot etalons arranged in series for production of narrowband 800 nm pulses. The key feature allowing the MM operation was the nearly collinear alignment of the visible (fixed, 800 nm) and infrared (tunable, 1000-4000 cm(-1)) pulses which were spatially separated. Physical insights discussed in this paper include the comparison of spectral bandwidth produced with 40 and 85 fs pump beams, the improvement of spectral resolution using etalons, the SFG probe volume in bulk analysis, the normalization of SFG signals, the stitching of multiple spectral segments, and the operation in different modes for air/liquid and adsorbate/solid interfaces, bulk samples, as well as spectral imaging combined with principle component analysis (PCA). The SFG spectral features obtained with the MM-BB-SFG system were compared with those obtained with picosecond-scanning-SFG system and high-resolution BB-SFG system (HR-BB-SFG) for dimethyl sulfoxide, α-pinene, and various samples containing cellulose (purified commercial products, Cladophora cell wall, cotton and flax fibers, and onion epidermis cell wall).

How Atomic Steps Modify Diffusion and Inter-adsorbate Forces: Empirical Evidence from Hopping Dynamics in Na/Cu(115)

We followed the collective atomic-scale motion of Na atoms on a vicinal Cu(115) surface within a time scale of pico- to nanoseconds using helium spin echo spectroscopy. The well-defined stepped structure of Cu(115) allows us to study the effect that atomic steps have on the adsorption properties, the rate for motion parallel and perpendicular to the step edge, and the interaction between the Na atoms. With the support of a molecular dynamics simulation we show that the Na atoms perform strongly anisotropic 1D hopping motion parallel to the step edges. Furthermore, we observe that the spatial and temporal correlations between the Na atoms that lead to collective motion are also anisotropic, suggesting the steps efficiently screen the lateral interaction between Na atoms residing on different terraces.

Differences between thermal and laser-induced diffusion

A combination of femtosecond laser excitation with a low-temperature scanning tunneling microscope is used to study long-range interaction during diffusion of CO on Cu(111). Both thermal and laser-driven diffusion show an oscillatory energy dependence on the distance to neighboring molecules. Surprisingly, the phase is inverted; i.e., at distances at which thermal diffusion is most difficult, it is easiest for laser-driven diffusion and vice versa. We explain this unexpected behavior by a transient stabilization of the negative ion during diffusion as corroborated by ab initio calculations.

Interpretation of surface diffusion data with Langevin simulations: a quantitative assessment

Diffusion studies of adsorbates moving on a surface are often analyzed using 2D Langevin simulations. These simulations are computationally cheap and offer valuable insight into the dynamics, however, they simplify the complex interactions between the substrate and adsorbate atoms, neglecting correlations in the motion of the two species. The effect of this simplification on the accuracy of observables extracted using Langevin simulations was previously unquantified. Here we report a numerical study aimed at assessing the validity of this approach. We compared experimentally accessible observables which were calculated using a Langevin simulation with those obtained from explicit molecular dynamics simulations. Our results show that within the range of parameters we explored Langevin simulations provide a good alternative for calculating the diffusion procress, i.e. the effect of correlations is too small to be observed within the numerical accuracy of this study and most likely would not have a significant effect on the interpretation of experimental data. Our comparison of the two numerical approaches also demonstrates the effect temperature dependent friction has on the calculated observables, illustrating the importance of accounting for such a temperature dependence when interpreting experimental data.