Subpicosecond transient infrared spectroscopy of adsorbates. Vibrational dynamics of CO/Pt(111)

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.

Structural dynamics at surfaces by ultrafast reflection high-energy electron diffraction

Many fundamental processes of structural changes at surfaces occur on a pico- or femtosecond timescale. In order to study such ultrafast processes, we have combined modern surface science techniques with fs-laser pulses in a pump-probe scheme. Grazing incidence of the electrons ensures surface sensitivity in ultrafast reflection high-energy electron diffraction (URHEED). Utilizing the Debye-Waller effect, we studied the nanoscale heat transport from an ultrathin film through a hetero-interface or the damping of vibrational excitations in monolayer adsorbate systems on the lower ps-timescale. By means of spot profile analysis, the different cooling rates of epitaxial Ge nanostructures of different size and strain state were determined. The excitation and relaxation dynamics of a driven phase transition far away from thermal equilibrium is demonstrated using the In-induced (8 × 2) reconstruction on Si(111). This Peierls-distorted surface charge density wave system exhibits a discontinuous phase transition of first order at 130 K from a (8 × 2) insulating ground state to (4 × 1) metallic excited state. Upon excitation by a fs-laser pulse, this structural phase transition is non-thermally driven in only 700 fs into the excited state. A small barrier of 40 meV hinders the immediate recovery of the ground state, and the system is found in a metastable supercooled state for up to few nanoseconds.

Molecular dynamics study of plasmon-mediated chemical transformations

Heterogeneous catalysis of adsorbates on metallic surfaces mediated by plasmons has potential high photoelectric conversion efficiency and controllable reaction selectivity. Theoretical modeling of dynamical reaction processes enables in-depth analyses complementing experimental investigations. Especially for plasmon-mediated chemical transformations, light absorption, photoelectric conversion, electron-electron scattering, and electron-phonon coupling occur simultaneously on different timescales, making it very challenging to delineate the complex interplay of different factors. In this work, a trajectory surface hopping non-adiabatic molecular dynamics method is used to investigate the dynamics of plasmon excitation in an Au₂₀-CO system, including hot carrier generation, plasmon energy relaxation, and CO activation induced by electron-vibration coupling. The electronic properties indicate that when Au₂₀-CO is excited, a partial charge transfer takes place from Au₂₀ to CO. On the other hand, dynamical simulations show that hot carriers generated after plasmon excitation transfer back and forth between Au₂₀ and CO. Meanwhile, the C-O stretching mode is activated due to non-adiabatic couplings. The efficiency of plasmon-mediated transformations (∼40%) is obtained based on the ensemble average of these quantities. Our simulations provide important dynamical and atomistic insights into plasmon-mediated chemical transformations from the perspective of non-adiabatic simulations.

Mid-Infrared Scattering in γ-Al2O3 Catalytic Powders

The energy efficiency of heterogeneous catalytic processes may be improved by using mid-infrared light to excite gas-phase reactants during the reaction, since vibrational excitation of molecules has been shown to increase their reactivity at the gas-catalyst interface. A primary challenge for such light-enabled catalysis is the need to ensure close coupling between light-excited molecules and the catalyst throughout the reactor. Thus, it is imperative to understand how to couple infrared light efficiently to molecules near and inside catalytic material. Heterogenous catalysts are often nanoscale metal particles supported on high surface area, porous oxide materials and exhibit feature sizes across multiple scattering regimes with respect to the mid-infrared wavelength. These complex powders make a direct measurement of the scattering properties challenging. Here, we demonstrate that a combination of directional hemispherical measurements along with the in-line transmission measurement allow for a direct measurement of the scattered light signal. We implement this technique to study the scattering behavior of the catalytic support material γ-Al₂O₃ (with and without metal loading) between 1040 and 1220 cm^-1. We first study how both the mean grain size affects the scattering behavior by comparing three different mean grain sizes spanning three orders of magnitude (2, 40, and 900 µm). Furthermore, we study how the addition of metal catalyst nanoparticles, Ru, or Cu, to the support material impacts the light scattering behavior of the powder. We find that the 40 µm grain size scatters the most (up to 97% at 1220 cm^-1) and that the addition of metal nanoparticles narrows the scattering angle but does not decrease the scattering efficiency. The strong scattering of the 40 µm grains makes them the most ideal support material of those studied for the given spectrum because of their ability to distribute light within the reactor. Finally, we estimate that less than 100 mW of laser power is needed to cause significant excitation for testing mid-infrared catalysis in a Harrick Praying Mantis diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) reactor, a magnitude easily available using commercial mid-infrared lasers. Our work also provides a mid-infrared foundation for a wide range of studies of light-enabled catalysis and can be extended to other wavelengths of light or to study the scattering behavior of other complex powders in other fields, including ceramics, biomaterials, and geology.

Inelastic Scattering of H Atoms from Surfaces

We have developed an instrument that uses photolysis of hydrogen halides to produce nearly monoenergetic hydrogen atom beams and Rydberg atom tagging to obtain accurate angle-resolved time-of-flight distributions of atoms scattered from surfaces. The surfaces are prepared under strict ultrahigh vacuum conditions. Data from these experiments can provide excellent benchmarks for theory, from which it is possible to obtain an atomic scale understanding of the underlying dynamical processes governing H atom adsorption. In this way, the mechanism of adsorption on metals is revealed, showing a penetration–resurfacing mechanism that relies on electronic excitation of the metal by the H atom to succeed. Contrasting this, when H atoms collide at graphene surfaces, the dynamics of bond formation involving at least four carbon atoms govern adsorption. Future perspectives of H atom scattering from surfaces are also outlined.

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.

Transient IR spectroscopy as a tool for studying photocatalytic materials

Over the years, a considerable amount of attention has been given to the thermodynamics of photocatalysts, i.e. to the location of their valence and conduction bands on the energy scale. The kinetics of the photoinduced charge carriers at short times (i.e. prior to their surface redox reactions) is no less important. While significant work on the transient electronic spectra of photocatalysts has been performed, the transient vibrational spectra of this class of materials was hardly studied. This manuscript aims to increase the scientific awareness to the potential of transient IR spectroscopy (TRIR) as a complementary tool for understanding the first, crucial, steps of photocatalytic processes in solid photocatalysts. This was done herein first by describing the various techniques currently in use for measuring transient IR signals of photo-excited systems and discussing their pros and cons. Then, a variety of examples is given, representing different types of photocatalysts such as oxides (TiO₂, NaTaO₃, BiOCl, BiVO₄), photosensitized oxides (dye-sensitized TiO₂), organic polymers (graphitic carbon nitride) and organo-metalic photocatalysts (rhenium bipyridyl complexes). These examples span from materials with no IR fingerprint signals (TiO₂) to materials having a distinct spectrum showing well-defined, localized, relatively narrow, vibrational bands (carbon nitride). In choosing the given-above examples, care was made to represent the several pump & probe techniques that are applied when studying transient IR spectroscopy, namely dispersive, transient 2D-IR spectroscopy and step-scan IR spectroscopy. It is hoped that this short review will contribute to expanding the use of TRIR as a viable and important technique among the arsenal of tools struggling to solve the mysteries behind photocatalysis.

Vibrational Relaxation Lifetime of a Physisorbed Molecule at a Metal Surface

Previous measurements of vibrational relaxation lifetimes for molecules adsorbed at metal surfaces yielded values of 1-3 ps; however, only chemisorbed molecules have been studied. We report the first measurements of the vibrational relaxation lifetime of a molecule physisorbed to a metal surface. For CO(v=1) adsorbed on Au(111) at 35 K the vibrational lifetime of the excited stretching mode is 49±3  ps. The long lifetime seen here is likely to be a general feature of physisorption, which involves weaker electronic coupling between the adsorbate and the solid due to bonding at larger distances.

CO Stretch Vibration Lives Long on Au(111)

Measured lifetimes of the CO internal stretch mode on various metal surfaces routinely lie in the picosecond regime. These short vibrational lifetimes, which are actually reproduced by current first-principles nonadiabatic calculations, are attributed to the rapid vibrational energy loss that is caused by the facile excitation of electron-hole pairs in metals. However, this explanation was recently questioned by the huge discrepancy that exists for CO on Au(111) between the experimental vibrational lifetime that is larger than 100 ps and the previous theoretical predictions of 4.8 and 1.6 ps. Here, we show that the state-of-the-art nonadiabatic theory does reproduce the long CO lifetime measured in Au(111) provided the molecule-surface interaction is properly described. Importantly, our new results confirm that the current understanding of the adsorbates' vibrational relaxation at metal surfaces is indeed valid.

Nonadiabatic Vibrational Damping of Molecular Adsorbates: Insights into Electronic Friction and the Role of Electronic Coherence

We present a perturbation approach rooted in time-dependent density-functional theory to calculate electron-hole (e-h) pair excitation spectra during the nonadiabatic vibrational damping of adsorbates on metal surfaces. Our analysis for the benchmark systems CO on Cu(100) and Pt(111) elucidates the surprisingly strong influence of rather short electronic coherence times. We demonstrate how in the limit of short electronic coherence times, as implicitly assumed in prevalent quantum nuclear theories for the vibrational lifetimes as well as electronic friction, band structure effects are washed out. Our results suggest that more accurate lifetime or chemicurrentlike experimental measurements could characterize the electronic coherence.

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.

Electronic Friction-Based Vibrational Lifetimes of Molecular Adsorbates: Beyond the Independent-Atom Approximation

We assess the accuracy of vibrational damping rates of diatomic adsorbates on metal surfaces as calculated within the local-density friction approximation (LDFA). An atoms-in-molecules (AIM) type charge partitioning scheme accounts for intramolecular contributions and overcomes the systematic underestimation of the nonadiabatic losses obtained within the prevalent independent-atom approximation. The quantitative agreement obtained with theoretical and experimental benchmark data suggests the LDFA-AIM scheme as an efficient and reliable approach to account for electronic dissipation in ab initio molecular dynamics simulations of surface chemical reactions.

Ultrafast energy exchange via water-phosphate interactions in hydrated DNA

The ionic phosphate groups in the DNA backbone play a key role for DNA hydration. We study ultrafast vibrational dynamics and local interactions of phosphate groups and water by femtosecond two-color pump-probe spectroscopy. The asymmetric (PO(2))(-) stretching vibration nu(AS)(PO(2))(-) of artificial DNA oligomers containing 23 alternating adenine-thymine base pairs displays a lifetime of 340 fs, independent of the hydration level. For DNA at zero relative humidity, excess energy from the decay of the phosphate excitation is transferred within DNA on a 20 ps time scale. For fully hydrated DNA, the water shells around the phosphates serve as a primary heat sink accepting vibrational excess energy from DNA on a femtosecond time scale. OH stretching excitation of water molecules around fully hydrated DNA induces an ultrafast nu(AS)(PO(2))(-) response which includes rearrangements of the hydration shell and a reduction of the average number of phosphate-water hydrogen bonds.

Insight into action spectroscopy for single molecule motion and reactions through inelastic electron tunneling

We propose a versatile formula that describes action spectra for vibrationally mediated reactions of single molecules with a scanning tunneling microscope. Spectral fitting of the formula to CO hopping and the configurational change of the cis-2-butene molecule on Pd(110) enables us to determine the vibrational energy, reaction order, and transition rate associated with anharmonic coupling between the modes excited by tunneling electrons and the reaction-coordinate modes. The formula proposed here is general and easy to apply to any vibrationally mediated motion and reaction of single molecules.

Applications of 2D IR spectroscopy to peptides, proteins, and hydrogen-bond dynamics

Following a survey of 2D IR principles, this article describes recent experiments on the hydrogen-bond dynamics of small ions, amide-I modes, nitrile probes, peptides, reverse transcriptase inhibitors, and amyloid fibrils.

The determination of an inhomogeneous linewidth for a strongly coupled adsorbate system

We present a set of experiments that provide a complete mapping of coherent and incoherent vibrational relaxation times for a molecule on a metal surface, CO/Ir{111}. Included is the first detection of a midinfrared photon echo from a metallic surface, some 15 years after the analogous measurement on a semiconductor surface, which sets a precedent for the ability to manipulate and rephase polarization on a subpicosecond time scale on surfaces. For the C-O stretch in a strongly dipole-coupled CO layer we obtain a total linewidth of 5.6 cm-1, composed of a homogeneous width of 2.7 cm-1 and an inhomogeneous contribution of 3.0 cm-1. Pure dephasing is negligible at liquid nitrogen temperatures, making CO/Ir{111} an attractive model system for quantum computing.

Laser-Induced Potential Jump at the Electrochemical Interface Probed by Picosecond Time-Resolved Surface-Enhanced Infrared Absorption Spectroscopy

Picosecond time-resolved surface-enhanced infrared absorption spectroscopy (SEIRAS) has been used for the first time to examine the potential jump at the electrochemical interface induced by a visible pulse irradiation. The potential dependent shift of the C-O stretching vibration of CO adsorbed on a Pt electrode was utilized to monitor the potential jump. A 6-cm(-1) red-shift was observed with a time delay of approximately 200 ps with respect to a visible pump-pulse irradiation (532 nm, 35 ps duration, 3 mJ cm(-2)). The observed red-shift is ascribed to the heating of the in-plane frustrated translational mode of CO and the negative shift of potential. These two contributions can be separated with the aid of the transient of the background reflectivity of the electrode surface. The heating of water layers near the surface is mainly responsible for the potential jump through the orientation change of water molecules. This method is promising as a tool to examine ultrafast electrode dynamics.

Frequency- and time-domain femtosecond vibrational sum frequency generation from CO adsorbed on Pt(111)

We have studied the effects of intermolecular and intramolecular coupling on the C-O stretching vibration of CO adsorbed on Platinum (111) by means of femtosecond broadband vibrational sum frequency generation (VSFG). Resonant intermolecular coupling is investigated through the coverage dependence of the VSFG signal. The experimental observations can be accurately modeled as lateral coupling of the molecular transition dipole moments; this coupling is invoked in the nonlinear optical response model as a local field correction. The linear polarizability, which appears in this model, is modified by both the dipole-dipole coupling and the population of bridged adsorption sites. By extending the formalism to include these effects, we deduce a vibrational polarizability of 0.32 A(3) from the data. Intramolecular coupling to the frustrated translational mode is observed as temperature dependence of the C-O stretch. The present data can be described either by pertubative or nonpertubative lineshape models from the literature. Measurements of the temperature dependence of the vibrational free induction decay indicate a population relaxation time T(1) of (0.8+/-0.1) ps, in agreement with the observed low-temperature linewidth. Moreover, the ability of this time-domain method to discriminate spectral inhomogeneity yields clear evidence of the order-disorder transition near 275 K. Above this temperature an inhomogeneous linewidth component of (12+/-3) cm(-1) is observed. This value allows us to estimate the structural heterogeneity of the disordered phase, which result agrees with published Monte Carlo simulations.