13050
J. Phys. Chem. 1996, 100, 13050-13060
Reactions at Metal Surfaces Induced by Femtosecond Lasers, Tunneling Electrons, and Heating W. Ho Laboratory of Atomic and Solid State Physics and Materials Science Center, Cornell UniVersity, Ithaca, New York 14853-2501 ReceiVed: NoVember 30, 1995; In Final Form: February 12, 1996X
Recent experiments with unprecedented spatial and temporal resolutions are beginning to provide us with a direct, microscopic understanding of the mechanisms and dynamics of reactions at metal surfaces. It is found that a unified picture can be used to describe desorption of molecules from a metal surface by conventional heating, radiation from a femtosecond laser, and current from the tip of a scanning tunneling microscope. By adjusting the excitation source, control of the reaction pathways and product yields can be achieved.
There are three important differences between chemical reactions on metal surfaces and in the gas phase: (1) the adsorbed molecules are in close proximity to one another, (2) the chemical properties of the reactants are modified through bonding to the surface, and (3) the adsorbed molecules are aligned with respect to each other and the surface. These differences in the properties of the reactants which arise naturally upon adsorption provide the basis for the chemistry which is catalyzed on a metal surface and induced by heating, photon irradiation, or an electron beam. How close are the adsorbed molecules from each other? As an example, the nearest-neighbor Pt-Pt distance on the Pt(111) surface is 2.77 Å, and if the number of adsorbed molecules is half the number of surface atoms, the separation between adjacent molecules would be 3.92 Å if the molecules are equally spaced apart. In an ideal gas at 300 K and 1 atm pressure, the average separation of the molecules is 34.5 Å; to reduce this to 3.92 Å, the pressure needs to be increased to 682 atm. Thus, an important function of the surface is to bring the molecules close to each other, which is required for chemical reactions to occur. The adsorption of molecules on the surface involves the formation of a new chemical bond, the molecule-surface bond. The molecule is perturbed, and consequently, its electronic and vibrational states may change. The shift and broadening of the electronic states open up the possibility of surface chemistry with low-energy photons and electrons by making the states energetically accessible. The weakening of selective internal bonds of the adsorbed molecules due to the formation of the molecule-surface bond may facilitate surface reactions induced thermally, photochemically, or by electrons. The formation of chemical bonds depends on the relative orientation of the two reactants. Adsorption leads to oriented molecules on the surface, which may be favorable for certain reactions to occur. The adsorbed molecules are confined to and diffuse on a two-dimensional surface prior to a chemical reaction. By lowering the surface temperature, one can reduce the motion of the molecules (orientation fluctuations and diffusion) on the surface. Thermal reactions are suppressed, which facilitates controlled surface photochemistry and the observation of single-molecule reactions induced by the electron beam from the tunneling current of a scanning tunneling microscope (STM). Extensive studies of chemical reactions on metal surfaces have been carried out in the past 30 years since the introduction X
Abstract published in AdVance ACS Abstracts, June 15, 1996.
S0022-3654(95)03549-0 CCC: $12.00
of ultrahigh-vacuum technology and surface sensitive instrumentation.1 One of the driving forces behind this active research is the connection to heterogeneous catalysis. It would be a formidable task to thoroughly cover the diverse thermal reactions which have been investigated on metal surfaces. While the field of surface photochemistry is relatively young (about 10 years)2 and involves significantly fewer researchers (although rapidly increasing), the number of papers has increased at a rapid rate. The mission of this article is to discuss both thermal and nonthermal reactions on metal surfaces. A feasible approach is to discuss some recent experiments which have led to a unified description of one of the simplest of reactions on metal surfaces, desorption of adsorbed molecules, induced by heating, irradiation with a femtosecond laser, and electrons from the tunneling current of a scanning tunneling microscope. Therefore, it becomes possible to understand conventional thermal reactions from experiments probing the dynamics with femtosecond laser pulses and subnanometer diameter electron beam. The Different Experiments The study of surface chemistry aims at the understanding of the dissociation of the reactants, the formation of new bonds, and the desorption of products. These chemical transformations can be induced by a variety of excitation sources, including heat, photons, and electrons, as shown in Figure 1. By using excitation sources confined temporally or spatially, such as the photons within femtosecond laser pulses or the electrons within a subnanometer diameter beam, unique surface chemistry can be induced via mechanisms which include steps similar to those leading to thermal reactions and desorption. Photochemistry on metal surfaces differs in an important way from that in the gas phase; the substrate can absorb the photons and produce electron-hole pairs. These photoexcited charge carriers can interact with and impart energy to the adsorbed molecules through inelastic resonant electron scattering or negative ion resonant scattering.3 Since the photon energies used in surface photochemistry are predominantly less than about 6 eV (207 nm), the role of the substrate in modifying the electronic states of the adsorbed molecules can make states energetically accessible by the photoexcited electrons for resonant scattering that would be unreachable in a gas phase molecule.4 Similar to electron scattering from molecules in the gas phase, the adsorbed molecules can be vibrationally excited via inelastic electron scattering, thus providing the necessary energy for surface chemistry. When the radiation is from a femtosecond laser, the electron-hole pairs are photoexcited within the laser pulse duration, typically about 100 fs. © 1996 American Chemical Society
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Figure 2. Desorption of O2 desorption yield from O2 on Pt(111) at 80 K as a function of the absorbed fluence for 100-130 fs, 310 nm (O) and 620 nm (b) irradiation, and for 8 ns, 355 nm (2) irradiation. The dashed line shows the linear dependence of the yield on the absorbed fluence for nanosecond irradiation (higher fluences not shown). Reproduced by permission from ref 10. Figure 1. Schematic diagram showing the elementary excitation process in femtochemistry (multiple inelastic resonant electron scatterings from femtosecond laser excited electrons), nanochemistry (multiple inelastic resonant electron scatterings from electrons confined to subnanometer diameter tunneling current), and thermochemistry (multiple scatterings of phonons generated from heating the substrate).
A similar inelastic electron scattering process takes place when an electron beam from the tip of a STM impinges a single molecule on the surface. The continuous electron beam is confined within a subnanometer diameter, thus giving rise to a high current density. The electrons undergo inelastic resonant scatterings from an unoccupied electronic state of the adsorbed molecule and transfer energy in the form of vibrational excitations of the molecule.5,6 This energy then leads to surface chemistry. The vast majority of reactions on metal surfaces are induced by heating the substrate. The microscopic mechanism of heating involves the creation of phonons and the inelastic scatterings of these phonons from the adsorbed molecules. The inelasticity imparts energy to the adsorbed molecule via vibrational excitations. Unlike the chemistry induced by a femtosecond laser or a subnanometer diameter electron beam, thermochemistry has been studied to date on a macroscopic spatial scale and temporal resolution outside the femtosecond regime. The schematic shown in Figure 1 emphasizes the common mode of energy transfer to the vibrational modes of the adsorbed molecules. The surface chemistry induced by the three types of excitation sources (photons, electrons, and phonons) occurs on vastly different temporal or spatial scales. It turns out that distinct processes involved in femtochemistry and nanochemistry reveal directly the underlying mechanisms and dynamics which lead to the observed surface reactions. Characteristic Features Chemical reactions induced by femtosecond lasers exhibit some unique features when the properties of the lasers are varied. The one feature which makes the connection to STM-induced chemistry is the nonlinear dependence of the photoyield on the laser fluence, Y ∝ In, where n lies in the range 3-7.7-9 Figure 2 shows an example of such a dependence for O2 photodesorption yield from O2 adsorbed on Pt(111), O2/Pt(111), using 620 and 310 nm laser pulses.10 The exponent is 6.3 ( 0.1 for the entire range of fluence for which data were taken at 620 nm. In contrast, a change to a linear fluence dependence is
observed below about 0.5 mJ/cm2 absorbed fluence for 310 nm irradiation. Above this absorbed fluence, the photoyield again depends nonlinearly on the absorbed fluence. Within the experimental uncertainty in measuring the laser fluence, there does not appear to be a wavelength dependence of the nonlinear exponent. This crossover from an exponent of 6.3 to 1 for 310 nm irradiation provides clear support for the current understanding of the mechanisms and dynamics of surface photochemistry with femtosecond lasers; a more detailed discussion of the crossover will be given in a later section. Also shown in Figure 2 are the data from nanosecond irradiation at 355 nm; as expected, a linear dependence of the photoyield on the absorbed laser fluence is observed, and the magnitude of the photoyield extrapolates to that of femtosecond laser induced chemistry in the linear regime. The absence of a crossover for 620 nm irradiation is due to the wavelength dependence of the photoyield in the linear fluence regime, which is ∼100 times lower than that at 310 nm.4 The fluence needs to be decreased even further than 0.5 mJ/cm2 in order to observe the crossover for 620 nm irradiation; however, such low cross section (see Figure 2) is below the detection limit. The advancement of the STM has led to surface chemistry on a single adsorbed molecule by using the subnanometer diameter electron beam from the tunneling current. By varying a voltage bias between a polycrystalline tungsten tip and a single crystal Ni(110), a xenon atom was reversibly transferred between the tip and the substrate.11 The rate of Xe transfer was found to depend nonlinearly on the tunneling current, with an exponent of 4.9 ( 0.2, as shown in the top half of Figure 3. This highly nonlinear dependence was also observed for the irreversible desorption of hydrogen atoms, one at a time, from a Hterminated Si(100)-(2 × 1) surface, as shown in the bottom half of Figure 3.12 In this case, the nonlinear dependence of the H desorption yield vs the tunneling current becomes stronger as the bias between the STM tip and the substrate increases. This nonlinear dependence of the yield on the intensity of the excitation provides a link between femtosecond laser and STM induced surface chemistry. The available data for femtosecond laser and STM induced surface chemistry are limited. In contrast, thermally induced reactions are routinely carried out in surface science experiments, and a vast amount of data exists.1 By programming the heating of the substrate, desorption and reactions can be induced; the desorbed molecules are detected by a mass spectrometer as a function of the substrate temperature. Despite the relative simplicity of this experiment, a great deal of understanding of
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Ho
Figure 4. Temperature-programmed desorption spectra of (a) CO2 production from reactions of CO coadsorbed with atomic and molecular oxygen on Pt(111) and (b) O2 desorption from O2 adsorbed alone and CO desorption from CO adsorbed alone on Pt(111). The initial temperature of Pt(111) was 95 K. The heating rate β was 2 K/s. Reproduced by permission from ref 4. Copyright 1993 American Institute of Physics.
Figure 3. (top) Transfer rate of xenon atom from the Ni(110) surface to the polycrystalline tungsten tip as a function of the tunneling current, I, during an applied voltage pulse. Resistance of the tunneling junction was kept fixed. The solid line shows that the transfer rate varies as I4.9(0.2. Reproduced from: Eigler, D. M.; Lutz, C. P.; Rudge, W. E. Nature 1991, 352, 600. Copyright 1991 Macmillan. (bottom) Desorption yield of atomic hydrogen versus the tunneling current for different voltage biases; the tip-surface distance was varied. The solid lines correspond to fits to eq 3, where fin is the inelastic tunneling fraction corresponding to the ratio of vibrationally inelastic to elastic electron scattering cross sections. Reproduced from: Shen, T.-C.; Wang, C.; Abeln, G. C.; Tucker, J. R.; Lyding, J. W.; Avouris, Ph.; Walkup, R. E. Science (Washington, D.C.) 1995, 268, 1590. Copyright 1995 American Association for the Advancement of Science.
surface chemistry has been obtained. Temperature-programmed desorption spectroscopy (TPD) or temperature-programmed reaction spectroscopy (TPRS) is one of the most versatile and informative techniques available in surface science. The increase in the substrate temperature leads to an increase in the population of phonons which inelastically scatter from the adsorbed molecules, thus imparting to the molecules the required energy for desorption and reaction to occur. As an example of the kind of information contained in TPD or TPRS, the top panel of Figure 4 shows the desorption of O2 from O2/Pt(111) initially at 95 K.4 The spectra were obtained with a linear temperature ramp of 2 K/s. A sharp molecular desorption peak is observed at 150 K, followed by a broad peak centered at about 750 K from the recombination of two atomic O to form molecular O2 as the reaction product. The peak of the CO desorption from CO/Pt(111) is centered at about 400 K. The bottom panel of Figure 4 shows the production of CO2 when oxygen is coadsorbed with CO. In the case of O2 + CO/ Pt(111), a sharp low-temperature peak at 150 K and a lower signal peak at 200 K are observed in addition to the dominant peak at 320 K, which is the only peak observed for O + CO/ Pt(111). At 150 K, O2 desorption and dissociation occur with the simultaneous production of CO2. A possible mechanism for explaining these observations involves the creation of hot
O atoms from O2 dissociation, followed by the reaction of hot O atoms with coadsorbed CO to produce CO2. The Universal Picture: Qualitative The connections among the three processes depicted in Figure 1 are made by considering recent results of surface chemistry induced by femtosecond, nanosecond, and continuous wave (CW) lasers. It is, however, with femtosecond lasers that the properties of the excitation source can be varied over the necessary range to reveal the microscopic mechanisms and dynamics. Results from different degrees of excitation can be obtained, e.g., the crossover from nonlinear to linear yield dependences as the laser fluence is decreased, as shown in Figure 2. Femtosecond lasers also allow the possibility of timeresolving reactions on metal surfaces. The connection of femtochemistry to nanochemistry and thermochemistry is revealed via modeling and simulation of results obtained with femtosecond lasers. It has been found that surface chemistry induced by nanosecond lasers differs from that induced by femtosecond lasers;10,13 however, the two can approach each other by reducing the fluence (energy/pulse/unit area, mJ/cm2) or the intensity (energy/pulse/unit time/unit area) of the femtosecond laser pulses, as shown in Figure 2. In principle, the fluence or the intensity of the nanosecond laser pulses can be increased to induce surface chemistry similar to that with femtosecond laser pulses. However, the required increase would have exceeded the threshold for damaging the substrate or photoablation. The observation of a crossover from the femtosecond regime to the nanosecond regime suggests that the mechanisms and dynamics of surface photochemistry induced by femtosecond and nanosecond lasers are related. In fact, the description of photoexcitation and deexcitation with nanosecond laser irradiation (which also holds for CW light) can be extended to that for femtosecond laser irradiation. In Figure 5, an energy diagram depicting desorption induced by electronic transitions (DIET) contains a ground and an excited electronic state. The
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Figure 5. Schematic diagram showing the Franck-Condon transitions, wave packet propagations, the gain in kinetic energy (KE) and the potential energy (vibrational excitation in the ground electronic state), and the translational energy distribution for a fixed lifetime in the excited electronic state. The overall process of a single excitationdeexcitation cycle corresponds to desorption induced by electronic transitions (DIET). The lifetime in the excited electronic state is distributed.
excited state is reached either via direct absorption of a photon by an adsorbed molecule (adsorbate mediated mechanism) or via negative ion resonant electron scattering (substrate mediated mechanism). The excitation process is described by an upward Franck-Condon transition. The molecule then evolves in the excited state prior to de-excitation to the ground state via a downward Franck-Condon transition. While in the excited state, the adsorbed molecule gains kinetic energy, and in the deexcitation process the molecule gains potential energy by making a Franck-Condon transition to the excited vibrational states of the ground state potential curve. If the lifetime in the excited state is longer than a critical time, the molecule gains enough energy during the excitation/ deexcitation cycle for it to overcome the binding to the surface and desorb. Assuming the lifetime in the excited state is exponentially distributed, a Maxwell-Boltzmann distribution of the translational energy of the desorbed molecules is predicted, in agreement with measurements for a large number of adsorbate-substrate systems.14 If the molecule does not gain sufficient energy, it quickly relaxes to the bottom of the ground electronic state via vibrational damping before being excited again. In desorption, frustrated vibrational modes other than the molecule-surface vibration are involved.15 In dissociation leading to surface reaction, the internal vibrational modes need to be considered. In a femtosecond laser pulse, the impingement rate of photons is much higher than in a nanosecond laser pulse of comparable fluence. Thus, instead of a single excitation/deexcitation cycle (the case of nanosecond laser irradiation), there is a finite probability that the adsorbed molecule is reexcited before it relaxes down to the ground vibrational state. This process can be repeated a number of cycles, with a net increase in the probability for desorption. In fact, a significant increase in the photoyield is observed for all the systems studied to date with femtosecond laser irradiation.7-10 Desorption induced by multiple electronic transitions (DIMET)16 is a mechanism unique to femtosecond laser irradiation and is illustrated in the top part of Figure 6. The nonlinear dependence of the photoyield on the laser fluence (shown in Figure 2) is tied to the multiple excitation/ deexcitation cycles. In simulations, the number of cycles
Figure 6. Schematic diagram showing the one-dimensional potential energy curves, the multiple excitations, and vibrational ladder climbing within the ground electronic state. For femtochemistry and nanochemistry, the mechanisms involve desorption induced by multiple electronic transitions (DIMET) from electron scatterings via adsorbate-substrate resonance (represented by the excited electronic state). An asymmetric double well is used to represent transfer of an atom between the substrate and the STM tip in the presence of an electric field bias. Only the ground electronic state is involved in thermochemistry due to the low energies involved in multiple scatterings of phonons generated by heating the substrate.
exceeds the value of the exponent in the fluence dependence of the photoyield,3,16 reflecting the fact that the vibrational Franck-Condon overlap involves a distribution of vibrational states with each transition. The same formulation has been shown to be applicable to desorption induced by a femtosecond laser and the tunneling current of a STM.5,17 The corresponding schematic diagram for STM-induced atom transfer (double potential wells representing the molecule adsorbed on the substrate vs on the tip) and desorption is shown in the middle of Figure 6. The electrons from the tunneling current induce multiple cycles of excitation/deexcitation; this process is possible due to the high current density of the subnanometer diameter electron beam. The process of climbing up the vibrational ladder in the ground electronic state in thermochemistry is driven by thermal fluctuations of the substrate atoms which lead to coupling of the phonons with the vibrational modes of the adsorbed molecules.18 These multiple phonon scatterings become more frequent as the substrate temperature increases. The connection between thermal and photochemical reactions on metal surfaces lies in this common process of vibrational ladder climbing. This connection only exists when the photochemistry is driven by femtosecond lasers and the photoyield depends nonlinearly on the laser fluence. Thus, the time scales for desorption in DIMET are tied to the time scales for vibrational ladder climbing or heating. The Universal Picture: Semiquantitative The connections between photochemistry with femtosecond lasers, STM induced chemistry, and thermochemistry are further clarified by examining the similarity in the equations for
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Ho TABLE 1: Desorption Rates Obtained within the Truncated, One-Dimensional Harmonic Oscillator Model and Considering Only One Quantum Jump for the Up and Down Vibrational Transitions, i.e., the One-Phonon Approximation, for Desorption Induced by a Femtosecond Laser, Electrons from the Tunneling Current of a STM, and Heating the Substrate Arrhenius Equation R ) A exp(-E/kT) Femtochemistry R(t) ≈ (N + 1)Γv(t) exp[-V/kTv(t)]
Figure 7. Schematic diagram of a one-dimensional truncated harmonic oscillator potential showing N bound states within a well depth of V. The one-phonon approximation corresponds to vibrational transitions only between adjacent states.
describing the desorption rates. To obtain analytical expressions, it is necessary to use a simplified model containing the essential features of these three different processes. The rate of desorption, considered as a case in chemical kinetics, is decribed empirically by the Arrhenius equation,
R ) A exp(-E/kT)
(1)
where A is the preexponential factor, E is the activation energy, and T is the temperature of the system. It is generally recognized that E is not simply the height of the energy barrier, and in fact the threshold for desorption is found to be lower than the barrier.18 The transition state theory, however, assumes that the adsorbed molecules and the gas molecules are in equilibrium, which is not the case for desorption in femtochemistry, nanochemistry, and thermochemistry. A nonequilibrium model is required. Equations in the form of eq 1 can be obtained using the truncated, one-dimensional harmonic oscillator model and considering only one quantum jump for the up and down vibrational transitions for the three phenomena, i.e., the onephonon approximation, as schematically illustrated in Figure 7. Only the ground electronic state is shown. (i) Femtochemistry (Desorption by Femtosecond Laser Irradiation). Absorption of photons within each pulse leads to excited carriers which rapidly (
