Electron solvation and solvation-induced crystallization of an ammonia film on Ag(111) studied by 2-photon photoemission

Intermolecular interaction plays a crucial role in electron solvation in the condensed phase. Here, we present a femtosecond time-resolved and angle-resolved 2-photon photoemission (2PPE) study on the dynamics of electron solvation in a 2-dimensional ammonia film on a metal substrate. While the weakly chemisorbed first monolayer (ML) supports delocalized image-potential (IP) states that resemble those of the bare Ag(111) substrate, an additional monolayer localizes the IP state with a larger binding energy obtained through a pre-solvation process. Structural disorder in the metastable ammonia films (>2 ML) leads to a prominent photoelectron peak that is attributed to the long-lived trapped electron state (e(T)) located at 1.5 eV above the Fermi level. Photoinduced crystallization of the metastable phase, verified by the recovery of a delocalized IP state, is suggested to result from inelastic scattering between interfacial electrons and disordered ammonia molecules.     

Photoinduced dissociation of water and transport of hydrogen between silver clusters

Theoretical electronic structure calculations are reported for the dissociation of water adsorbed on a 31-atom silver cluster, Ag31, and subsequent transfer of a H to a second Ag31 cluster leaving OH on the first cluster. Both ground and excited electronic state processes are considered for two choices of Ag cluster separation, 6.35 and 7.94 A, on the basis of preliminary calculations for a range of separation distances. The excited electronic state of interest is formed by photoemission of an electron from one Ag cluster and transient attachment of the photoemitted electron to the adsorbed water molecule. A very large energy barrier is found for the ground-state process (3.53 eV at a cluster separation of 6.35 A), while the barrier in the excited state is small (0.38 eV at a cluster separation of 6.35 A). In the excited state, partial occupancy of an OH antibonding orbital facilitates OH stretch and concomitant movement of the negatively charged OH toward the electron-hole in the metal cluster. The excited-state pathway for dissociation of water and transfer of H begins with the formation of an excited electronic state at 3.59-3.82 eV. Stretch of the OH bond occurs with little change in energy (0.38-0.54 eV up to a stretch of 1.96 A). In this region of OH stretch the molecule must return to the ground-state potential energy surface to fully dissociate and to transfer H to the other Ag cluster. Geometry optimizations are carried out using a simplex algorithm and a semigrid method. These methods allow the total energy to be calculated directly using configuration interaction theory.     

Photochemistry of vinyl chloride physisorbed on Ag(111) through molecular anion formation induced by substrate electron attachment

Pulsed 266 and 355 nm ultraviolet laser irradiation of monolayer vinyl chloride physisorbed on Ag(111) results in molecular dissociation leading to C2H3 and Cl, much of which is adsorbed to the surface. On the basis of observations made on dissociation dependences on chlorine isotope and photon energy, it is deduced that upon excitation vinyl chloride forms a transient negative ion through a substrate mediated, vertical electron attachment mechanism. The anion either dissociates or relaxes through energy transfer to the neutral state causing the neutral molecule to desorb. The threshold for vertical attachment of substrate electron is estimated to be 0.8 eV below the vacuum level, in agreement with the experimentally observed wavelength dependence in photoinduced dissociation. Chemisorbed Cl on the Ag(111) surface inhibits the photodissociation process by increasing the substrate work function and consequently the energy threshold for electron vertical attachment. Upon heating the Ag(111) surface, adsorbed vinyl combines to produce 1,3-butadiene in a first order, diffusion limited, process with an activation energy of 10.4 kcal/mol.     

Direct observation of an intermediate state for a surface photochemical reaction initiated by hot electron transfer

The photoinduced charge transfer that had been suggested to result in the dissociation of phenol on Ag(111) was investigated by two-photon photoemission spectroscopy. An unoccupied intermediate state was positively identified, which was found to be located 3.22 eV above the Fermi level. From the photoelectron energy dispersion, the effective mass of the intermediate state was determined to be (15 +/- 10)m(e) for a 1 ML coverage of phenol. This implies that the excited electron is localized mainly on the adsorbed phenol, forming a molecular resonance state. Polarization dependence of the photoelectron intensity suggested that the initial photoexcitation of the substrate produces hot electrons that scatter into the molecular resonance state, leading ultimately to the dissociation of the adsorbate. These results are the first two-photon photoemission study to characterize the transient anionic state involved in photodissociation of a molecule adsorbed on a metal surface.     

Femtosecond dynamics of electronic excitations of adsorbates studied by two-photon photoemission pulse correlation: CO/Cu(111)

An equal pulse correlation technique based on angle-resolved two-photon photoemission is employed to investigate the lifetime of electronic excitations of an adsorbate on a single crystal metal surface. Photoemission from an occupied surface state on Cu(111) by a non-resonant two-photon process via the sp-band gap is used to characterize directly at the surface the width and shape of 65 fs laser pulses at a photon energy of hv = 3.4 eV. The 2PPE correlation technique allows the establishment of an upper limit of τ < 20 fs for the lifetime of the 2π^* resonance of CO adsorbed on Cu(111), whereas from the spectral width of the 2π^* level a lower limit of ≈1 fs is estimated.     

Theoretical study of the CH2 + O photodissociation of formaldehyde adsorbed on the Ag(111) surface

Configuration interaction calculations of the ground and excited states of the H2CO molecule adsorbed on the Ag(111) surface have been carried out to study the photoinduced dissociation process leading to polymerization of formaldehyde. The metal-adsorbate system has been described by the embedded cluster and multireference configuration interaction methods. The pi electron-attachment H2CO- and n-pi* internally excited H2CO* states have been considered as possible intermediates. The calculations have shown that H2CO* is only very weakly bound on Ag(111), and thus that the dissociation of adsorbed formaldehyde due to internal excitation is unlikely. By contrast, the H2CO- anion is strongly bound to Ag(111) and gains additional vibrational energy along the C-O stretch coordinate via Franck-Condon excitation from the neutral molecule. Computed energy variations of adsorbed H2CO and H2CO- at different key geometries along the pathway for C-O bond cleavage make evident, however, that complete dissociation is very difficult to attain on the potential energy surface of either of these states. Instead, reneutralization of the vibrationally excited anion by electron transfer back to the substrate is the most promising means of breaking the C-O bond, with subsequent formation of the coadsorbed O and CH2 fragments. Furthermore, it has been demonstrated that the most stable state for both dissociation fragments on Ag(111) is a closed-shell singlet, with binding energies relative to the gas-phase products of approximately 3.2 and approximately 1.3 eV for O and CH2, respectively. Further details of the reaction mechanism for the photoinduced C-O bond cleavage of H2CO on the Ag(111) surface are also given.     

State-resolved investigation of the photodesorption dynamics of NO from (NO)2 on Ag nanoparticles of various sizes in comparison with Ag(111)

The translational and internal state energy distributions of NO desorbed by laser light (2.3, 3.5, and 4.7 eV) from adsorbed (NO)(2) on Ag nanoparticles (NPs) (mean diameters, D = 4, 8, and 11 nm) have been investigated by the (1 + 1) resonance enhanced multiphoton ionization technique. For comparison, the same experiments have also been carried out on Ag(111). Detected NO molecules are hyperthermally fast and both rotationally and vibrationally hot, with temperatures well above the sample temperature. The translational and rotational excitations are positively correlated, while the vibrational excitation is decoupled from the other two degrees of freedom. Most of the energy content of the desorbing NO is contained in its translation. The translational and internal energy distributions of NO molecules photodesorbed by 2.3, 3.5, and in part also 4.7 eV light are approximately constant as a function of Ag NPs sizes, and they are the same on Ag(111). This suggests that for these excitations a common mechanism is operative on the bulk single crystal and on NPs, independent of the size regime. Notably, despite the strongly enhanced cross section seen on NP at 3.5 eV excitation energy in p-polarization, i.e., in resonance with the plasmon excitation, the mechanism is also unchanged. At 4.7 eV and for small particles, however, an additional desorption channel is observed which results in desorbates with higher energies in all degrees of freedom. The results are well compatible with our earlier measurements of size-dependent translational energy distributions. We suggest that the broadly constant mechanism over most of the investigated range runs via a transient negative ion state, while at high excitation energy and for small particles the transient state is suggested to be a positive ion.     

Two-photon photoemission spectroscopy study of 1,3-butadiene on Cu(111): electronic structures and excitation mechanism

The characteristics of 1,3-butadiene (C(4)H(6)) adsorbed on Cu(111) were investigated with temperature-programmed desorption (TPD) and two-photon photoemission spectroscopy (2PPE). Dosed at 90 K, the work function drops by 0.4 eV and TPD provides no evidence for dissociation, but there are four coverage-dependent local maxima located at 195, 135, 121, and 115 K. From the 2PPE spectra, three unoccupied electronic states of the C(4)H(6)-Cu(111) system were identified: the LUMO (pi(1)*, 2a(u)), LUMO + 1 (pi(2)*, 2b(g)), and LUMO + 2 (sigma*, 7b(u)), lying 1.3, 3.4, and 4.8 eV above the Fermi level, respectively. Although the excitation mechanisms for the LUMO and LUMO + 1 are substrate mediated, the excitation of the LUMO + 2 is attributed to intramolecular excitation.     

Structure and dynamics of excited electronic states at the adsorbate/metal interface: C6F6/Cu(111)

Excited state electron transfer at the adsorbate/metal interface represents a key step in molecular electronic devices. The dynamics of such processes are governed by ultrafast energy relaxation which can be probed directly by time-resolved two-photon photoemission (2PPE). Using 2PPE spectroscopy we investigate the energetics and lifetimes of the unoccupied electronic states of C6F6 adsorbed on Cu(111) as a model system for electron transfer at organic/metal interfaces. With increasing C6F6 layer thickness we find a pronounced decrease in the energetic position of the lowest unoccupied state, which is accompanied by a strong increase in its lifetime as well as a decrease in the effective electron mass. The frequently employed dielectric continuum model which describes delocalized (quantum well) states within adsorbate layers does not give a consistent explanation of these findings. By adsorption of Xe overlayers onto C6F6/Cu(111) we can show that, even for one monolayer of C6F6, the excited state must be localized predominantly inside the C6F6 layer and thus originates from a molecular state (presumably an antibonding sigma* orbital). With increasing coverage this state becomes more delocalized within the adsorbate layer, which reduces the coupling to the metal substrate and thus enhances the excited state lifetime.     

Bimolecular recombination kinetics and interfacial electronic structures of poly[2-methoxy-5-(2-ethyl-hexyloxy)-p-phenylene vinylene] on gold studied using two-photon photoemission spectroscopy

Photoexcitation kinetics and interfacial electronic structures of poly[2-methoxy-5(2-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) film on gold have been investigated using two-photon photoemission spectroscopy (2PPE). The authors directly probed a fixed intermediate state located at 0.95 eV above the Fermi level (or 2.95 eV below the vacuum level), assigned to a charged polaron. Based on the power law slope and the 2PPE spectra with laser intensity, they found that the polaron follows a second order bimolecular annihilation process. The 2PPE yield dramatically increases with increasing photon energy. They attribute this to an enhanced dissociation of hotter excitons at higher excitation levels. The work function of MEH-PPV/Au is measured to be 3.9 eV, 1.2 eV downshift from the clean gold, attributable to interface dipole effects. The energy gap between the intermediate polaron state and the hole polaron level is estimated to be 2.45 eV.     

Dipole driven bonding schemes of quinonoid zwitterions on surfaces

The permanent dipole of quinonoid zwitterions changes significantly when the molecules adsorb on Ag(111) and Cu(111) surfaces. STM reveals that sub-monolayers of adsorbed molecules can exhibit parallel dipole alignment on Ag(111), in strong contrast with the antiparallel ordering prevailing in the crystalline state and retrieved on Cu(111) surfaces, which minimizes the dipoles electrostatic interaction energy. DFT shows that the rearrangement of electron density upon adsorption is a result of donation from the molecular HOMO to the surface, and back donation to the LUMO with a concomitant charge transfer that effectively reduces the overall charge dipole.     

NbC固体表面能带和电子结构的理论研究

采用DFT/BLYP方法对NbC(001)和(111)面的电子结构进行研究。计算结果表明,对于NbC(001)表面,其表面态主要集中于费米能级(EF)下方约4.5eV附近区域,并以表面Nb原子和C原子为主要成分。O2分子在该表面吸附时,趋向于吸附在表面Nb原子上。对于NbC(111)表面,其表面态集中在EF下方0.02.0eV区域,靠近EF的态具有较高的表面活性,其主要成分为表面Nb原子的4dxz/dyz成分。上述结论与光电子能谱实验结果基本一致;但由于金属原子d电子数的差异导致NbC(111)表面态成分与类似的TiC化合物并不相同。     

Impact of Surface Dipole on Resonant Electron Injection in Scanning Tunneling Spectroscopy

Resonant electron injection and first-principles calculations are utilized to study single-adsorbed selenium (Se) atom on a Si(111)-7×7 surface. Theoretical calculations indicate that a negative dipole of 0.61 eV forms toward the adsorbed Se atom due to electron trans-fer from the associated Si atoms. The formation of surface dipole modifies the effective tunneling barrier height and causes a shift in the energy of the resonant state formed in the vacuum gap between the tip and the sample surface. The experimental data imply that an outward negative surface dipole of 0.61 eV causes a resonant electron injection bias shift to high voltage of about 0.45 V.     

Characteristics of photoexcitations and interfacial energy levels of regioregular poly(3-hexythiophene-2,5-diyl) on gold.

We have studied characteristics of photoexcitations and interfacial electronic structures of regioregular poly(3-hexlythiophene-2,5-diyl) (P3HT) on gold using two-photon photoemission (2PPE) spectroscopy. The vacuum level threshold is decreased by 1.3 eV from that of bare gold, attributable to interface dipole effects. The 2PPE spectral width narrows as the film thickness increases. We tentatively understand that this is due to destabilization of long-lived localized polaron, attributed to strong interchain interactions. On the basis of the analysis of the 2PPE distribution as a function of photon energy and laser power, the polaron level is located at 3.1 eV below the vacuum level. Using this value and a polaron level of 1.75 eV above the HOMO, we indirectly estimate an ionization potential of 4.85 eV for P3HT. An increase in two-photon photoemission yield with increasing photon energy is attributed to an enhanced electron-hole pair dissociation yield at higher photo-excitation levels. The decrease in power law slope with increasing film thickness is understood by Langevin recombination kinetics and saturation of photoexcitations.     

On the adsorption and formation of Pt dimers on the CeO2(111) surface

The direct adsorption of Pt(2) dimers on CeO(2)(111) and their formation from isolated adsorbed Pt atoms have been studied using periodic slab model calculations based on density functional theory and including the so-called on-site Hubbard parameter (GGA + U). In the most stable configuration Pt(2) is found to be almost parallel to the surface; the electronic ground state is closed shell and there is no evidence of charge transfer towards or from the surface. The formation of Pt(2) from two single adsorbed Pt atoms involves a rather small energy barrier of ~0.10 eV only. On the contrary, dissociation of adsorbed Pt(2) requires to overcome a considerable barrier of ~1.43 eV. This indicates that once Pt(2) is formed it will remain on the surface, thus likely triggering the growth of larger supported Pt particles.     

Layer-by-layer growth of polar MgO(111) ultrathin films

By alternate deposition of Mg and exposure of O2, layer-by-layer growth, polar MgO(111) ultrathin films with Mg-terminated or O-terminated surfaces have been successfully fabricated on Mo(110) substrate. The surface geometric structure and electronic structures of the polar MgO(111) films were investigated using surface analysis techniques including low-energy electron diffraction and photoelectron emission and electron energy loss spectroscopies. The results indicate that the O-terminated surface is of an insulating character, while for Mg-terminated surface, a prominent new surface state at 2-3 eV and appreciable density of states near Fermi level have been observed. The polar oxide films provide ideal model surfaces for further investigation of support-particle system.     

Electron kinetic energies from vibrationally promoted surface exoemission: evidence for a vibrational autodetachment mechanism

We report kinetic energy distributions of exoelectrons produced by collisions of highly vibrationally excited NO molecules with a low work function Cs dosed Au(111) surface. These measurements show that energy dissipation pathways involving nonadiabatic conversion of vibrational energy to electronic energy can result in electronic excitation of more than 3 eV, consistent with the available vibrational energy. We measured the dependence of the electron energy distributions on the translational and vibrational energy of the incident NO and find a clear positive correlation between final electron kinetic energy and initial vibrational excitation and a weak but observable inverse dependence of electron kinetic energy on initial translational energy. These observations are consistent with a vibrational autodetachment mechanism, where an electron is transferred to NO near its outer vibrational turning point and ejected near its inner vibrational turning point. Within the context of this model, we estimate the NO-to-surface distance for electron transfer.     

X-ray and filtered UV photoelectron spectroscopic study of CO absorbed on Ni(100) and (111) surfaces

The 4σ, 1π and 5σ orbitals, and possibly the 2π^* orbital, of CO adsorbed on (100) and (111) nickel surfaces, have been detected using both XPS and filtered UPS techniques. The 3σ level was detected only by XPS at ≈ 29 eV with a full width half-maximum of ≈ to 12 eV. The Cls and Ols binding energy shifts exhibit systematic differences between the two surfaces, being larger on the (111) surface.     

Size-dependent surface reactions of Ag nanoparticles supported on highly oriented pyrolytic graphite

Various sizes of Ag particles were grown on highly oriented pyrolytic graphite (HOPG) surfaces, which had previously been modified with nanopits to act as anchoring sites. Surface reactions of O2, CHCl3, and CCl4 on the Ag particles and bulk Ag(111) surfaces were studied by X-ray photoelectron spectroscopy (XPS), and it has been shown that size dependence of O2 and CHCl3 reactions on Ag differs from that of CCl4. Weak reactions of O2 and CHCl3 were observed on the bulk Ag(111) surfaces, while strong reactions occur on Ag particles with medium Ag coverage, suggesting that the reactions are controlled by the number of surface defect sites. On the contrary, the dissociation of CCl4 is mainly determined by the exposed Ag facet area, mainly Ag(111) facet, and strong dissociation reaction happens on the bulk Ag(111) surface. The results suggest that the size effects, which are often discussed in heterogeneous catalysis, are strongly dependent on the reaction mechanism.     

Si-C(N) sigma linkages and N --> Si dative bonding at pyridine/Si(111)-7 x 7

We experimentally demonstrated that pyridine/Si(111)-7 x 7 can act as an electron donor/acceptor pair as a result of the charge transfer from the electron-rich N atom of pyridine to the electron-deficient adatom of the Si surface, evidenced by the upshift of 1.8 eV (state A) for the N(1s) core level upon the formation of a datively bonded complex compared to physisorbed molecules. Another state (B) whose N(1s) binding energy downshifts by 1.2 eV was assigned to an adduct through Si-C and Si-N covalent linkages, formed via a [4 + 2]-like addition mechanism on Si(111)-7 x 7. Binding molecules through the formation of the dative bond resulted from significant electron transfer opens a new approach for the creation of Si-based molecular architectures and modification of semiconductor interfacial properties with unsaturated organic molecules.