Substrate-mediated mechanism for photoinduced chemical reactions on metal surfaces (original) (raw)
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Adsorbate Specificity in Hot Electron Driven Photochemistry on Catalytic Metal Surfaces
The Journal of Physical Chemistry C, 2014
Visible light driven catalysis on metal surfaces and nanoparticles has attracted significant attention in recent years as a potential route for driving selective chemical reactions that are difficult to achieve with thermal energy. It is most often assumed that photochemistry on metal surfaces occurs through a substrate-mediated process of adsorbate-metal bond photoexcitation, although crucial underlying phenomena controlling the efficiency of this process are still poorly understood. In this work, substrate-mediated photochemistry on metal surfaces was analyzed by combining dynamical models associated with the metal substrate photoexcitation and electron-mediated bond-activation processes. An extended version of two-temperature model was utilized to treat temporal evolution of photoexcited charge carriers in the metal substrate. The electron-induced adsorbate dynamics on the metal surface was modeled using a nonadiabatic, first-principles based inelastic electron scattering model. Photoactivation of three well studied reactions on Pt(111) surfaces, CO and NO desorption and O diffusion, were chosen as model systems. Through our approach, we addressed unresolved issues associated with adsorbate specific reaction time scales and wavelength and temperature-dependent behavior. The results suggest that activating adsorbate-metal bonds with targeted photon wavelengths and at optimal system temperatures could provide an approach to control selectivity in photon-driven reactions on metal surfaces.
Pure and Applied Chemistry, 1986
The photophysics and photochemistry of organic molecules (aromatics , aza-aromatics , diphenylpolyenes , azobenzene , triphenylmethane dyes, thioindigo) adsorbed on metal oxides (alumina, silica, thoria, titania) were investigated by steady-state and time-resolved diffuse ref lectance and luminescence spectroscopy. The fluorescence decay curves of almost all adsorbates are non-exponential. This effect is discussed in terms of different polarizing surface sites, aggregation, and interference between directly emitted and scattered fluorescence radiation. A method is elaborated to determine rate constants and quantum yields of photoreactions in strongly scattering rigid media by photometric measurements using the model of radiative transfer. The method is applied to quantify internal and translational mobilities of adsorbed photoexcited molecules, ring closure reactions, bimolecular photoprocesses, and charge transfer between the adsorbent and the adsorbate.
Photoionization studies of surface layers. Electron spectroscopy and ion desorption
Zeitschrift f�r Physik B Condensed Matter, 1985
By virtue of the continuous tunability, high resolution at good intensities, and welldefined polarization of synchrotron radiation, photoionization processes of surface species-as monitored by photoelectrons, Auger electrons and fragment ions-can be investigated in much greater detail than with line sources. Information about the composition of these species, their geometry (orientation, interatomic distances), their electronic structure and bonding, and many-particle processes in them can be derived from electron spectroscopies. The strong screening of photoholes in surface species, in particular on metals, has the consequence that mainly multiple excitations stay localized sufficiently long for ion desorption to occur. Thus photodesorption is a selective, localized probe. As an example for the application of synchrotron radiation to surface physics, it is shown that correlation of results of electron spectroscopies and of ion yields leads to an improved understanding of all the processes involved.
Electronically induced surface reactions: Evolution, concepts, and perspectives
The Journal of Chemical Physics, 2012
This is a personal account of the development of the title subject which is the broader field encompassing surface photochemistry. It describes the early times when the main interest centered on desorption induced by slow electrons, follows its evolution in experiment (use of synchrotron radiation and connections to electron spectroscopies; use of lasers) and mechanisms, and briefly mentions the many different sub-fields that have evolved. It discusses some practically important aspects and applications and ends with an account of an evolving new subfield, the application to photochemistry on nanoparticles.
Photoinduced desorption of potassium atoms from a two dimensional overlayer on graphite
The Journal of Chemical Physics, 1997
We present an experimental and theoretical investigation of desorption of potassium atoms from the basal plane of graphite induced by photons with energies from 2 to 6 eV. The intensity of the photon flux employed in the measurements is low, and the observed photodesorption is a single-photon, non-thermal event. At monolayer coverage the photon-energy dependence of the cross section has a maximum at 4.8 eV. The experimental observations are interpreted in terms of a hot cartier mechanism, which involves attachment of optically excited substrate hot electrons to the empty 4s state of ionized potassium, and then desorption. The theory predicts a Gaussian line shape of the photoyield versus photon energy. Fitting the model parameters to the experimental data, we determine: (i) the potential energy for Gr + K + and Gr + K°; and (ii) the position (2.4 eV above the Fermi level) and width (0.15 eV) of the potassium 4s resonance, which is in good agreement with independent experimental observations.
Dynamics of photoinduced electron transfer from adsorbed molecules into solids
Ultrafast interfacial electron transfer from the donor orbital of organic chromophores into empty electronic acceptor states of a semiconductor and of a metal was investigated by two-photon photoemission spectroscopy (2PPE). Experimental tools and procedures have been developed for carrying out wet-chemistry preparation of the molecule/solid interface. The organic chromophore perylene was investigated with several different bridge/anchor groups on TiO 2 (110). One perylene compound was investigated for comparison on Ag(110). Angle and polarization dependent 2PPE measurements revealed the orientation of the perylene chromophore on the surface as controlled by the adsorption geometry of the respective anchor group on TiO 2 . UPS measurements gave the position of the HOMO level of the chromophore with respect to the Fermi level of the solid. The donor level of each molecule was found high enough to fulfill the "wide band limit" of heterogeneous electron transfer dynamics. Time constants for heterogeneous electron transfer were extracted from 2PPE transients. A difference by a factor of four was found, 13 fs against 47 fs, when a conjugated bond was exchanged for a saturated bond in the otherwise identical bridge group. The two different contributions to the 2PPE transients arising firstly from the excited state of the chromophore and secondly from the injected electrons were separated by measuring the latter contribution separately in the case of instantaneous interfacial electron transfer realized with catechol as adsorbate. The time scales measured for the electron transfer step and for the subsequent electron escape process from the surface into the bulk of TiO 2 showed both good agreement with recent theoretical predictions of other groups for these systems.
Photodesorption of neutrals from metal surfaces: a wave packet study
Chemical Physics, 1995
By solving the time-dependent Schr6dinger equation for representative wave packets, the desorption dynamics of neutrals from metal surfaces following photoexcitation is studied. Computational parameters are chosen to resemble NO/Pt(Ill). Three different one-dimensional models of increasing complexity are employed to elucidate (a) basic dynamical features, (b) the role of dissipative boundary conditions and (c) non-phenomenological quenching rates. Among the computed quantities are desorption probabilities, density and flux time-of-flight spectra, kinetic energies of the desorbing particles, and "snapshots" of the wave packets in configuration and momentum space. Besides others, effects of the initial vibrational excitation of the molecule-surface bond by temperature or by IR laser pulses, and of coordinate-dependent resonance lifetimes are discussed.
Photochemistry on rough metal surfaces
The Journal of Physical Chemistry, 1984
In this paper the general question of laser-induced photochemistry on metal surfaces is addressed. Specifically, we have studied resonant photodecomposition of a variety of aromatic molecules on roughened silver surfaces in ultrahigh vacuum. A continuous ion laser source at a number of different wavelengths in the region 350-410 nm was used to produce graphitic carbon on the surface which was monitored by Raman spectroscopy at the 1580-cm-' band of surface carbon. Laser po%r-dependence studies of fragmentation rate for several molecules at 406.7 nm indicate that the initial absorption step is a two-photon process. Energetic considerations imply that photochemistry for other molecules studied is also due to multiphoton absorption, except for benzaldehyde fragmentation with 350.7-nm excitation where the photodecomposition rate is linear in intensity. In this case photochemistry apparently occurs directly from the first excited singlet state following one-photon absorption. Distance-dependence studies of photofragmentation rates by use of an inert spacer layer to separate the molecule undergoing photochemistry from the surface indicate that energy transfer to the metal surface is important in determining the reaction rate. A maximum decomposition rate is observed for pyridine 15-20 8, from the surface, resulting from the different distance dependences of enhanced molecular absorption at rough silver surfaces and damping by energy transfer to the surfaces. These competing processes produce a maximum photochemical rate where surface-enhanced absorption is still present but the energy damping rate has decreased substantially. Intermediate species in the decomposition of pyridine and pyrazine to carbon have been observed as CN pea& in the surface Raman spectrum during photolysis of these molecules. The CN stretch frequency of these fragments is shifted from that observed for cyanide ions adsorbed on silver to a frequency typical of a nitrile such as C3H3N. The fragmentation mechanism for these and other surfaces adsorbed molecules studied remains to be established. Evidence indicates that radical fprmation is a likely pathway for some molecules (e g , benzaldehyde) while other molecules may decompose through surface ionic species.
Unfilled levels and excited states of adsorbates on metal surfaces
Journal of Electron Spectroscopy and Related Phenomena, 1986
We discuss the nature and energy of the affinity levels of molecules on metals , on the basis of recent inverse photoemission experiments and ab-initio cluster calculations. We show that the energy and composition of the affinity levels such as the 2a* level of CO are not determined by the interaction of occupied d-band states with the empty adsorbate levels, but that hybridization with nearresonant unoccupied metal states is important. The nature of the bonding of the negative ions produced by the population of the affinity levels in inverse photoemission spectroscopy (IPES), and possible screening mechanisms in IPES are examined.
Chemisorbed-molecule potential-energy surfaces and electronically stimulated processes
Proceedings of SPIE, 1994
Three topics illustrate some central physics of processes produced by UV-laser and low-energy electron stimulation. First, a multi-dimensional ground-state potential energy surface (PES) for NH3:Pd (111), computed using ab initio local-density functional theory, allows dramatically different dynamics depending on poorly-known excited-state forces. We use quantum-resolved experimental data to argue that stimulated desorption is dominated by a direct path off the surface, following placement of the wavepacket on a molecule-surface hard wall accessed by internal molecular motion. This illustrates the questionable relevance of one-dimensional models for understanding molecule-surface dynamics. Second, we study the image-charge model of excited state forces experienced by ions produced, for example, by hot carrier attachment. Ab initio results show that this model totally fails at molecule-surface distances typical of chemisorption. Finally, we present a purely-electronic adiabatic model of excited state PESs and use it to argue that, if significant covalent interactions occur between an adsorbate and a surface, hot carrier attachment does not simply produce singly-charged ions. Instead, attachment creates excitations of the molecule-surface bond occur and, in some cases, may result in multiply-charged ions.