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.     

A double rainbow interpretation of rotational energy transfer in energetic NO/Ag(111) collisions

The mechanism of rotational energy transfer in energetic (E ? eV) NO/Ag(111) collisions is investigated in quantal rigid-rotor/static-surface calculations. Reasonable agreement with recent experimental results is obtained for the case of a strongly asymmetric interaction potential which leads to two well-separated rotational rainbows at low and at high E_rot, respectively.     

苯乙烯在Ag(111)和Ag(110)表面环氧化反应结构敏感性的理论研究

采用密度泛函理论(DFT)对苯乙烯在Ag(110)表面和Ag(111)表面的环氧化反应进行了计算研究. 经计算, 在Ag(110)表面预吸附氧原子更易吸附在3 重穴位(3h), 吸附能为-3.59 eV; 在Ag(111)表面预吸附氧原子的最稳定吸附位是fcc 位, 吸附能为-3.69 eV. 苯乙烯的环氧化反应过程首先经过一个金属中间体, 然后再进一步反应变为产物, 其中经过直链中间体较支链中间体更加有利. Ag(110)面的反应活化能一般大于Ag(111)面的, 并且微观动力学模拟结果表明, Ag(111)表面生成环氧苯乙烷的选择性要明显高于Ag(110)表面(0.38 与 0.003), 原因是Ag(111)面环氧化反应活化能小于苯乙醛及燃烧中间体的活化能, 而在Ag(110)上正相反.     

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.     

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.     

Two-photon dissociation of the NO dimer in the region 7.1-8.2 eV: excited states and photodissociation pathways

A study of excited states of the NO dimer is carried out at 7.1-8.2 eV excitation energies. Photoexcitation is achieved by two-photon absorption at 300-345 nm followed by (NO)(2) dissociation and detection of electronically excited products, mostly in n=3 Rydberg states of NO. Photoelectron imaging is used as a tool to identify product electronic states by using non-state-selective ionization. Photofragment ion imaging is used to characterize product translational energy and angular distributions. Evidence for production of NO(A (2)Sigma(+)), NO(C (2)Pi), and NO(D (2)Sigma(+)) Rydberg states of NO, as well as the valence NO(B (2)Pi) state, is obtained. On the basis of product translational energy and angular distributions, it is possible to characterize the excited state(s) accessed in this region, which must possess a significant Rydberg character.     

Interfacial electron dynamics and hot-electron-driven surface photochemistry of carbon tetrachloride on Ag(111)

We used time-resolved two-photon photoemission (2PPE) spectroscopy to investigate the photochemical behavior, the interfacial electronic structure, and the fate of photogenerated hot electron for carbon tetrachloride adsorbed on Ag(111). The photodissociation cross section was determined over a wide range of photon energy from 1.62 to 5.69 eV, which suggested a low-lying electron affinity level of adsorbed CCl4. A CCl4-derived unoccupied state located at 3.41 eV above the Fermi level was attributed to an image potential (IP) state based on its binding energy and effective mass. Polarization dependence of the 2PPE signal revealed that the IP state was populated by an indirect excitation process involving scattering of photoexcited hot electrons rather than direct electronic transition from a bulk band. The lifetime of the IP state was much shorter on the CCl4-covered Ag(111) surface than on the clean one, implying that the electron in the IP state is scavenged effectively by CCl4, probably through dissociative attachment to it. These results are significant in the sense that they provide dynamical evidence for a new relaxation pathway of the IP state in addition to the more common pathway involving back transfer of electron to the substrate.     

State-to-state dynamics at the gas-liquid metal interface: rotationally and electronically inelastic scattering of NO[2Π(1/2)(0.5)] from molten gallium

Jet cooled NO molecules are scattered at 45° with respect to the surface normal from a liquid gallium surface at E(inc) from 1.0(3) to 20(6) kcal/mol to probe rotationally and electronically inelastic scattering from a gas-molten metal interface (numbers in parenthesis represent 1σ uncertainty in the corresponding final digits). Scattered populations are detected at 45° by confocal laser induced fluorescence (LIF) on the γ(0-0) and γ(1-1) A(2)Σ ← X(2)Π(Ω) bands, yielding rotational, spin-orbit, and λ-doublet population distributions. Scattering of low speed NO molecules results in Boltzmann distributions with effective temperatures considerably lower than that of the surface, in respectable agreement with the Bowman-Gossage rotational cooling model [J. M. Bowman and J. L. Gossage, Chem. Phys. Lett. 96, 481 (1983)] for desorption from a restricted surface rotor state. Increasing collision energy results in a stronger increase in scattered NO rotational energy than spin-orbit excitation, with an opposite trend noted for changes in surface temperature. The difference between electronic and rotational dynamics is discussed in terms of the possible influence of electron hole pair excitations in the conducting metal. While such electronically non-adiabatic processes can also influence vibrational dynamics, the γ(1-1) band indicates <2.6 × 10(-4) probability for collisional formation of NO(v = 1) at surface temperatures up to 580 K. Average translational to rotational energy transfer is compared from a hard cube model perspective with previous studies of NO scattering from single crystal solid surfaces. Despite a lighter atomic mass (70 amu), the liquid Ga surface is found to promote translational to rotational excitation more efficiently than Ag(111) (108 amu) and nearly as effectively as Au(111) (197 amu). The enhanced propensity for Ga(l) to transform incident translational energy into rotation is discussed in terms of temperature-dependent capillary wave excitation of the gas-liquid metal interface.     

NO(2) dissociation on Ag(111) revisited by theory

NO(2) dissociation on Ag(111) is investigated with first-principles calculations. For single NO(2) molecules, a high adsorption potential energy is found to prohibit dissociation. This result is surprising as experiments indicate dissociation at low temperatures. Neither entropy effects nor irregularities in the potential energy surface can remedy the discrepancy. Instead it is proposed that collective Eley-Rideal type of reaction mechanisms can drive the dissociation.     

Photochemistry on ultrathin metal films: strongly enhanced cross sections for NO2 on Ag/Si(100)

The surface photochemistry of NO(2) on ultrathin Ag(111) films (5-60 nm) on Si(100) substrates has been studied. NO(2), forming N(2)O(4) on the surface, dissociates to release NO and NO(2) into the gas phase with translational energies exceeding the equivalent of the sample temperature. An increase of the photodesorption cross section is observed for 266 nm light when the film thickness is decreased below 30 nm despite the fact that the optical absorptivity decreases. For 4.4 nm film thickness this increase is about threefold. The data are consistent with a similar effect for 355 nm light. The reduced film thickness has no significant influence on the average translation energy of the desorbing molecules or the branching into the different channels. The increased photodesorption cross section is interpreted to result from photon absorption in the Si substrate producing electrons with no or little momenta parallel to the surface at energies where this is not allowed in Ag. It is suggested that these electrons penetrate through the Ag film despite the gap in the surface projected band structure.     

The photodissociation of NO(2) by visible and ultraviolet light

We present velocity map images of the NO, O((3)P(J)) and O((1)S(0)) photofragments from NO(2) excited in the range 7.6 to 9.0 eV. The molecule was initially pumped with a visible photon between 2.82-2.95 eV (440-420 nm), below the first dissociation threshold. A second ultraviolet laser with photon energies between 4.77 and 6.05 eV (260-205 nm) was used to pump high-lying excited states of neutral NO(2) and/or probe neutral photoproducts. Analysis of the kinetic energy release spectra revealed that the NO photofragments were predominantly formed in their ground electronic state with little kinetic energy. The O((3)P(J)) and O((1)S(0)) kinetic energy distributions were also dominated by kinetically 'cold' fragments. We discuss the possible excitation schemes and conclude that the unstable photoexcited states probed in the experiment were Rydberg states coupled to dissociative valence states. We compare our results with recent time-resolved studies using similar excitation and probe photon energies.     

Microcanonical unimolecular rate theory at surfaces. III. Thermal dissociative chemisorption of methane on Pt(111) and detailed balance

A local hot spot model of gas-surface reactivity is used to investigate the state-resolved dynamics of methane dissociative chemisorption on Pt(111) under thermal equilibrium conditions. Three Pt surface oscillators, and the molecular vibrations, rotations, and the translational energy directed along the surface normal are treated as active degrees of freedom in the 16-dimensional microcanonical kinetics. Several energy transfer models for coupling a local hot spot to the surrounding substrate are developed and evaluated within the context of a master equation kinetics approach. Bounds on the thermal dissociative sticking coefficient based on limiting energy transfer models are derived. The three-parameter physisorbed complex microcanonical unimolecular rate theory (PC-MURT) is shown to closely approximate the thermal sticking under any realistic energy transfer model. Assuming an apparent threshold energy for CH(4) dissociative chemisorption of E(0)=0.61 eV on clean Pt(111), the PC-MURT is used to predict angle-resolved yield, translational, vibrational, and rotational distributions for the reactive methane flux at thermal equilibrium at 500 K. By detailed balance, these same distributions should be observed for the methane product from methyl radical hydrogenation at 500 K in the zero coverage limit if the methyl radicals are not subject to side reactions. Given that methyl radical hydrogenation can only be experimentally observed when the CH(3) radicals are kinetically stabilized against decomposition by coadsorbed H, the PC-MURT was used to evaluate E(0) in the high coverage limit. A high coverage value of E(0)=2.3 eV adequately reproduced the experimentally observed methane angular and translational energy distributions from thermal hydrogenation of methyl radicals. Although rigorous application of detailed balance arguments to this reactive system cannot be made because thermal decomposition of the methyl radicals competes with hydrogenation, approximate applicability of detailed balance would argue for a strong coverage dependence of E(0) with H coverage--a dependence not seen for methyl radical hydrogenation on Ru(0001), but not yet experimentally explored on Pt(111).     

Luminescence from 3,4,9,10-perylenetetracarboxylic dianhydride on Ag(111) surface excited by tunneling electrons in scanning tunneling microscopy

The electronic excitations induced with tunneling electrons into adlayers of 3,4,9,10-perylenetetracarboxylic dianhydride (PTCDA) on Ag(111) have been investigated by in situ fluorescence spectroscopy in scanning tunneling microscopy (STM). A minute area of the surface is excited by an electron tunneling process in STM. Fluorescence spectra strongly depend on the coverage of PTCDA on Ag(111). The adsorption of the first PTCDA layer quenches the intrinsic surface plasmon originated from the clean Ag(111). When the second layer is formed, fluorescence spectra are dominated by the signals from PTCDA, which are interpreted as the radiative decay from the manifold of first singlet excited state (S(1)) of adsorbed PTCDA. The fluorescence of PTCDA is independent of the bias polarity. In addition, the fluorescence excitation spectrum agrees with that by optical excitation. Both results indicate that S(1) is directly excited by the inelastic impact scattering of electrons tunneling within the PTCDA adlayer.     

Adsorption Energetics of NO and CO on Pt(111)

The adsorption energetics of NO and CO on Pt(111) are studied using an ab initio embedding theory. The Pt(111) surface is modeled as a three-layer, 28-atom cluster with the Pt atoms fixed at bulk lattice sites. Molecular NO is adsorbed at high symmetry sites on Pt(111), with the fcc threefold site energetically more favorable than the hcp threefold and bridge sites. The calculated adsorption energy at the fcc threefold site is 1.90 eV, with an N-surface distance of 1.23 Å. The NO molecular axis is perpendicular to the Pt(111) surface. Tilting the O atom away from the surface normal destablizes adsorbed NO at all adsorption sites considered. On-top Pt adsorption has been ruled out. The Pt(111) potential surface is very flat for CO adsorption, and the diffusion barriers from hcp to fcc sites are 0.03 eV and less than 0.06 eV across the bridge and the atop sites, respectively. Calculated adsorption energies are 1.67, 1.54, 1.51, and 1.60 eV at the fcc threefold, hcp threefold, bridge, and atop sites, respectively. Calculated C-surface distances are 1.24 Å at the fcc threefold site and 1.83 Å at the atop site. It is concluded that NO and CO adsorption energetics and geometries are different on Pt(111).     

The mechanism of N2O formation via the (NO)2 dimer: a density functional theory study

Catalytic formation of N(2)O via a (NO)(2) intermediate was studied employing density functional theory with generalized gradient approximations. Dimer formation was not favored on Pt(111), in agreement with previous reports. On Pt(211) a variety of dimer structures were studied, including trans-(NO)(2) and cis-(NO)(2) configurations. A possible pathway involving (NO)(2) formation at the terrace near to a Pt step is identified as the possible mechanism for low-temperature N(2)O formation. The dimer is stabilized by bond formation between one O atom of the dimer and two Pt step atoms. The overall mechanism has a low barrier of approximately 0.32 eV. The mechanism is also put into the context of the overall NO + H(2) reaction. A consideration of the step-wise hydrogenation of O(ads) from the step is also presented. Removal of O(ads) from the step is significantly different from O(ads) hydrogenation on Pt(111). The energetically favored structure at the transition state for OH(ads) formation has an activation energy of 0.63 eV. Further hydrogenation of OH(ads) has an activation energy of 0.80 eV.     

Electron-induced isomerisation of dichlorobenzene on Cu(111) and Ag(111)

The constitutional isomerisation of single dichlorobenzene molecules adsorbed on the surfaces of Ag(111) and Cu(111) between their meta- and para-isomers is induced and investigated by means of a low temperature scanning tunneling microscope. On both substrates similar isomerisation thresholds are found indicating that the excitation mechanism of this reaction does not depend significantly on the underlying substrate. The isomerisation threshold voltage of (170 +/- 7) meV most likely corresponds to excitation of a C-C stretch mode whose gas-phase energies we calculated ab initio to lie at 174 and 172 meV for meta- and para-isomers respectively. Though the reaction is found to be localized on the submolecular scale, it depends heavily on the second substituent both in terms of excitation energy and reaction outcome.     

Investigation of the electronic spectra and excited-state geometries of poly(para-phenylene vinylene) (PPV) and poly(para-phenylene) (PP) by the symmetry-adapted cluster configuration interaction (SAC-CI) method

The symmetry-adapted cluster-configuration interaction (SAC-CI) method has been used to investigate the optical and geometric properties of the oligomers of poly(para-phenylene vinylene) (PPV) and poly(para-phenylene) (PP). Vertical singlet and triplet absorption spectra and emission spectra have been calculated accurately; the mean average deviation from available experimental results lies within 0.2 eV. The chain length dependence of the transition energies has been improved in comparison to earlier TDDFT and MRSDCI calculations. The present analysis suggests that conventional TDDFT with the B3LYP functional should be used carefully, as it can provide inaccurate estimates of the chain length dependence of the excitation energies of these molecules with long pi conjugation. The T1 state was predicted to be at a lower energy, by 1.0-1.5 eV for PPV and by 0.9-1.7 eV for PP, than the S1 state, which indicates a localized T1 state with large exchange energy. By calculating the SAC-CI electron density difference between the ground and excited states, the geometry relaxations due to excitations can be analyzed in detail using electrostatic force theory. For trans-stilbene, the doubly excited 21Ag state was studied, and the calculated transition energy of 4.99 eV agrees very well with the experimental value of 4.84 eV. In contrast to previous ab initio calculations, we predict this doubly excited 21Ag state to lie above the 11Bu state.     

Vibrational effects on the reaction of NO(2)(+) with C(2)H(2): effects of bending and bending angular momentum

NO(2)(+) in six different vibrational states was reacted with C(2)H(2) over the center-of-mass energy range from 0.03 to 3.3 eV. The reaction, forming NO(+)+C(2)H(2)O and NO+C(2)H(2)O(+), shows a bimodal dependence on collision energy (E(col)). At low E(col), the reaction is quite inefficient (<2%) despite this being a barrierless, exoergic reaction, and is strongly inhibited by E(col). For E(col)> approximately 0.5 eV, a second mechanism turns on, with an efficiency reaching approximately 27% for E(col)>3 eV. The two reaction channels have nearly identical dependence on E(col) and NO(2)(+) vibrational state, and identical recoil dynamics, leading to the conclusion that they represent a single reaction path throughout most of the collision. All modes of NO(2)(+) vibrational excitation enhance both channels at all E(col), however, the effects of bend (010) and bend overtone (02(0)0) excitation are particularly strong (factor of 4). In contrast, the asymmetric stretch (001), which intuition suggests should be coupled to the reaction coordinate, leads to only a factor of approximately 2 enhancement, as does the symmetric stretch (100). Perhaps the most surprising effect is that of the bending angular momentum, which strongly suppress reaction, even though both the energy and angular momentum involved are tiny compared to the collision energy and angular momentum. The results are interpreted in light of ab initio and Rice-Ramsperger-Kassel-Marcus calculations.     

The detection of the 2π* orbital of CO and NO chemisorbed on Ni(111) by surface penning ionization electron spectroscopy

We use surface Penning ionization spectroscopy (SPIES) to study the electronic properties of CO and NO adsorbed on Ni(111). In this experiment an atomic beam containing ground-state and 2 ¹S He atoms collides with the adsorbate covered Ni(111) surface. This causes the transfer of ≈20.6 eV energy into the electronic degrees of freedom of the adsorbate molecules, forcing them to emit electrons. The SPIE spectrum is obtained by analysing the energy of these electrons. The high surface sensitivity of this method allows us to measure the binding energy of the partly filled 2π* orbitals of CO and NO.     

D atom loss in the photodissociation of the DNCN radical: implications for prompt NO formation

The photodissociation of DNCN following excitation of the C 2A"<--X 2A" electronic transition was studied using fast beam photofragment translational spectroscopy. Analysis of the time-of-flight distributions reveals a photodissociation channel leading to D+NCN competitive with the previously observed CD+N2 product channel. The translational energy distributions describing the D+NCN channel are peaked at low energy, consistent with internal conversion to the ground state followed by statistical decay and the absence of an exit barrier. The results suggest a relatively facile pathway for the reaction CH+N2-->H+NCN that proceeds through the HNCN intermediate and support a recently proposed mechanism for prompt NO production in flames.