Weak Chemisorption System Research Articles

Structure and electronic structure of van der Waals interfaces at a Au(1 1 1) surface covered with a well-ordered molecular layer of n-alkanes

The structure and electronic structure of a system comprising molecular layers of n-alkane adsorbed on Au(111) were investigated to understand the origin of the interfacial electric dipole layer. These alkane molecules are inert and thus van der Waals (vdW) interaction is the primary adsorptive force. Although these vdW interfaces are regarded as ideal physical adsorption systems unlikely to chemically interact with the metal surface, a large interfacial electric dipole layer over 1 eV is formed. It is important to elucidate the mechanism underlying the generation of such large dipole layers to understand the electrode interface in organic semiconductor devices. Thus, this paper elucidates the details of the vdW interface, in particular between tetratetracontane (TTC) and Au(111). First, the mechanism of growth of the TTC layer up to monolayer thickness was revealed using a low-energy electron-diffraction method. Second, the interface states were newly formed, as determined using angle-resolved photoelectron spectroscopy, X-ray photoemission spectroscopy, and near-edge absorption fine-structure spectroscopy. Our findings indicate, unexpectedly, that a relatively strong inter-orbital interaction governs this vdW interface, similar to a weak chemisorption system rather than a physisorption system.

The structure of the Ni(1 0 0)c(2 × 2)–N 2 surface: a chemical-state-specific scanned-energy mode photoelectron diffraction determination

Using the chemical shift in the N 1s photoemission peak from the two inequivalent N atoms of N 2 adsorbed on Ni(1 0 0) we have performed a scanned-energy mode photoelectron diffraction (PhD) structure determination of the Ni(1 0 0)c(2 × 2)–N 2 weak chemisorption system. The N 2 is found to adsorb atop surface Ni atoms with the N–N axis perpendicular to the surface at a Ni–N nearest-neighbour distance of 1.81 ± 0.02 Å. This is very significantly shorter than the value (2.25 Å) found in an earlier published study. An independent density-functional theory slab calculation yields a value of 1.79 Å, in excellent agreement with the results of the current experiment. Analysis of the PhD modulations of the N 1s photoemission satellite peak show that these are consistent with this comprising separable components localised at the two N atoms as has previously been assumed in an earlier investigation based on (angle-scan) X-ray photoelectron diffraction. Both experiment and theory indicate a small extension of the N–N distance due to the adsorption (0.03 ± 0.03 Å and 0.02 Å respectively).

Multiple-scattering approach to near-edge X-ray absorption fine structure of CO/NiO(100) and NO/NiO(100)

The carbon and nitrogen 1s near-edge X-ray absorption fine structure (NEXAFS) of CO/NiO(100) and NO/NiO(100) were calculated by the multiple-scattering cluster (MSC) method. The calculation shows that the shape resonance in C 1s NEXAFS spectra is attributed to the photoelectron scattered between the carbon atom and the substrate, and the weak features near the π-resonance are caused by the shake-up effect. R-factor calculation demonstrates for the first time that the CO molecule is adsorbed at the asymmetric bridge site of the NiO(100) surface; the adsorption height of the carbon atom in the CO molecule is equal to 0.31±0.01 nm. Our MSC calculation supports former ab initio studies and the experimental results, i.e., CO/NiO(100) is a physisorption system, while NO/NiO(100) is a weak chemisorption system.

Strong dynamical screening in weak chemisorption systems

Green's function calculations have been carried out on the N 1s XPS of a linear NiN 2 cluster used as a model of the N 2/Ni(100) system. A good agreement with the experimental spectra is obtained. A new method, based on exact effective Hamiltonians (EEH) is used for a detailed analysis of adsorbate core hole screening. Two lowest energy states in the spectrum, split by 1.47 eV are classified as π→ π* shake-down satellites, whereas two states dominating the region of the so-called “giant satellite” are attributed to the main states. The EEH technique has disclosed a dominant role of dynamical screening in the formation of the strong shake-down satellites in the spectrum.

Dynamical core-hole screening in weak chemisorption systems

Adsorbate core-hole screening in weak chemisorption systems is studied by the example of the ${N}_{2}/Ni(100)$ system. Fourth-order Green's-function calculations have been carried out on the $\mathrm{N}1s$ x-ray photoelectron spectra (XPS) of a linear ${\mathrm{NiN}}_{2}$ cluster used as a model of the chemisorption system. Good agreement with the experimental spectra is obtained. A powerful method, based on exact effective Hamiltonians (EEH's) constructed by means of a technique for a unique block diagonalization of Hermitian matrices [J. Phys. A 22, 2427 (1989)] is used for a detailed analysis of many-electron core-hole states. Two lowest-energy states in the $\mathrm{N}1s$ XPS of the ${\mathrm{NiN}}_{2}$ cluster, split by 1.47 eV (the experimental splitting in the chemisorption system is 1.3 eV), are classified as $\stackrel{\ensuremath{\rightarrow}}{\ensuremath{\pi}}{\ensuremath{\pi}}^{*}$ shakedown satellites, whereas two states dominating the region of the so-called ``giant satellite'' are attributed to the main states. The EEH technique allows us to study in detail the dynamical screening of core holes, which is shown to be responsible for the formation of the strong shakedown satellites. A comparative EEH analysis of the core-hole states of the ${\mathrm{NiN}}_{2}$ and NiCO clusters explains in simple terms the origin of the very different intensities of the giant satellites observed experimentally in adsorbate spectra of weak and strong chemisorption systems.

Surface plasmon satellites in the XPS core level spectra of physisorbed species: fiction or reality?

We analyze the possibility of detecting experimentally the character of the mechanism of screening of core holes in photoionized adsorbates weakly bound to the substrate surface. On noticing that both the image-like (extra-adsorbate) and charge transfer (intra-adsorbate) screening models have been rather successful in describing the spectral features of the XPS core level spectra of the prototype weak chemisorption system CO/Cu, we conclude that this type of system cannot provide a clear cut answer as to the character of screening of adsorbate core holes. On the other hand, by selecting systems in which, first, the substrate surface electronic response may exhibit pronouncedly distinct features, such as surface plasmons, and second, the adsorbate valence orbitals would remain “passive” as regards the charge transfer effects, we arrive at the conclusion that the XPS core level spectra of Ar atoms physisorbed on Ag or Al surfaces can provide experimental evidence for the operation of the image-like or extra-adsorbate screening mechanism. These conjectures are completed by calculations of the corresponding core level spectra and a discussion of the physical conditions for detection of the extra-adsorbate screening effects.

Extra-adsorbate screening in XPS of C 1s core levels of CO/Cu

Recent high resolution measurements of the XPS C 1s and O 1s core level ionization and corresponding NEXAFS excitation energies of CO/Cu adsorption complexes have revived the discussion about the nature of relaxation and screening of adsorbate core holes in XPS and NEXAFS of these systems. The presently available experimental data, as well as the theoretical estimates of the magnitude of the intra-adsorbate core hole-valence electron interactions, call for a reassessment of the efficiency of charge transfer processes in screening of C 1s core holes in weak chemisorption systems CO/Cu. Hence, we have reexamined the role which the complementary mechanism of extra-adsorbate or dynamic image screening may play in the XPS of C 1s levels in CO/Cu(100) by using a realistic surface response function of the substrate. A good agreement between the calculated and experimental spectral shapes indicates that the extra-adsorbate screening mechanism may indeed give an important contribution to the overall final state relaxation following the C 1s core ionization of CO/Cu.

Extra-adsorbate screening effects in XPS core level spectra of chemisorbed CO revisited

Recent high resolution XPS and NEXAFS measurements of the electronic spectra of CO chemisorbed on Cu and Ni have revived the interest in the mechanisms of screening and relaxation of core holes in adsorbates. In particular, controversial experimental findings regarding the positions of the XPS 1 s core level relaxed thresholds relative to the maxima of the corresponding 1 s → 2π ∗ NEXAFS absorption resonance in CO/Cu systems have been interpreted as indicative of a relatively slow (on the scale of core hole lifetime) screening processes via the charge transfer mechanism. This may imply that in XPS of adsorbate core levels some other screening effects, e.g. the extra-adsorbate or dynamic image screening, can become responsible for the core hole spectral shapes. In order to verify this we have re-examined the role of extra-adsorbate screening effects in the XPS core level spectra of CO/Cu and CO/Ni by using realistic surface response functions of the respective substrates. The comparison of our results with the experimental data shows that this relaxation mechanism may account for most of the core spectral features of the weak chemisorption system CO/Cu. On the other hand, for a strong chemisorption system CO/Ni such an agreement is lacking. This indicates that in CO/Ni the charge transfer screening mechanism is likely to provide a better description of the XPS spectra, as has already been emphasized earlier.

The low temperature CO/H coadsorption state on NiP{100}: X-ray absorption and photoemission studies

Angle-resolved photoemission and X-ray absorption spectroscopy have been used to re-examine the electronic structure and orientation of the low-temperature CO/H-coadsorption state observed on the N{100} surface. Although the C-O axis remains perpendicular to the surface, the photoemission spectrum is different from that of the pure Ni{100} system, showing some evidence for satellite structure. A comparison is made with CO on Cu{100}, a similar weak chemisorption system.

Substrate induced specificities in atom-adsorbate van der Waals scattering

We investigate the substrate induced specificities of the van der Waals potentials acting between gas phase atoms and chemisorbed CO molecules, and the effect of these specificities on the enhancement of the magnitude of the total scattering cross section in He → CO/metal collisions. The role of these effects in the relative enhancement of the cross section can be quantitatively estimated for prototype systems of weak and strong CO chemisorption, viz. CO/Cu and CO/Ni, respectively, for which other electronic and structural properties are known. Since the strengths of the atom-adsorbate van der Waals potentials depend on the spectra of virtual electronic excitations in each electronic subsystem involved, the specificity of the cross section arises from the particularities of the screening properties of the substrate surface, on the one hand, and on the strength of the adsorbate-substrate chemisorptive bond, on the other. Using the response functions of Cu and Ni surfaces and the valence electronic properties of the 2π ∗ derived resonances of CO chemisorbed on Cu and Ni substrates, we are able to calculate and discuss qualitatively the scattering potentials and the corresponding cross sections for He → CO/Cu and He → CO/Ni collision events. The results of our calculations demonstrate that the strong chemisorption system CO/Ni represents also a stronger scattering centre for thermal He beam atoms than the weak chemisorption system CO/Cu, and that for equal kinetic parameters of the collision the total cross sections may in these sysems differ by up to 10%.

Substrate induced specificities in atom-adsorbate van der Waals scattering

We investigate the substrate induced specificities of the van der Waals potentials acting between gas phase atoms and chemisorbed CO molecules, and the effect of these specificities on the enhancement of the magnitude of the total scattering cross section in He → CO/metal collisions. The role of these effects in the relative enhancement of the cross section can be quantitatively estimated for prototype systems of weak and strong CO chemisorption, viz. CO/Cu and CO/Ni, respectively, for which other electronic and structural properties are known. Since the strengths of the atom-adsorbate van der Waals potentials depend on the spectra of virtual electronic excitations in each electronic subsystem involved, the specificity of the cross section arises from the particularities of the screening properties of the substrate surface, on the one hand, and on the strength of the adsorbate-substrate chemisorptive bond, on the other. Using the response functions of Cu and Ni surfaces and the valence electronic properties of the 2π∗ derived resonances of CO chemisorbed on Cu and Ni substrates, we are able to calculate and discuss qualitatively the scattering potentials and the corresponding cross sections for He → CO/Cu and He → CO/Ni collision events. The results of our calculations demonstrate that the strong chemisorption system CO/Ni represents also a stronger scattering centre for thermal He beam atoms than the weak chemisorption system CO/Cu, and that for equal kinetic parameters of the collision the total cross sections may in these sysems differ by up to 10%.

Chemisorption induced effects in He → CO/metal collisions

The contribution of the chemisorption induced 2π ∗ intraresonance virtual excitations to the polarizability of adsorbed CO modifies the strengths of the direct and substrate mediated van der Waals forces as well as the reference plane positions of the substrate mediated interactions. We considered such effect on the total He-adsorbate scattering cross sections for the strong and weak chemisorption systems of CO/Ni and CO/Cu, respectively, and found that the total cross sections can change by up to 22% for CO/Ni and 8% for CO/Cu.

2 pi resonance features in the electronic spectra of chemisorbed CO.

The electronic spectra of CO chemisorbed on transition metals and copper as obtained with photoelectron spectroscopies, surface Penning spectroscopy, inverse photoemission spectroscopy, and x-ray absorption spectroscopy are discussed in regard to inherent features pointing to the existence of a CO 2${\ensuremath{\pi}}^{\mathrm{*}}$--derived resonance in these adsorption systems. Since the various spectroscopies probe the CO-metal systems predominantly in the excited final state, the spectra of one and the same line, however, are also affected by differences in the dynamic relaxation processes and nonequilibrium final-state charge configurations which are specific to each particular experimental method. In order to arrive at a unified interpretation of some seemingly controversial features in the electronic spectra of chemisorbed CO, we have invoked the notion of the CO 2${\ensuremath{\pi}}^{\mathrm{*}}$--derived resonance. Our model includes both initial-state chemical (i.e., backdonation-induced bonding) effects and final-state dynamical screening (relaxation) effects. Assuming a very small fractional occupation of the CO 2${\ensuremath{\pi}}^{\mathrm{*}}$ resonance in the case of weak chemisorption systems (e.g., CO/Cu), and notably larger occupation in the case of strong chemisorption (e.g., CO/Ni), it is possible to give a unified interpretation of the experimental features such as the threshold energies, level shifts, line shapes, etc., observed with CO adsorbed on transition metals and copper. The great success of the concept of a resonance in interpreting these experimental spectral features as obtained with the various spectro- scopies, provides, in turn, strong support for the actual existence of a CO 2${\ensuremath{\pi}}^{\mathrm{*}}$-- derived resonance in these adsorption systems.

An IR study of the Σ-CO CO-adsorption state in the system CO/H/Ni(100)

The weakly bound Σ-CO co-adsorption state in the system CO/H/Ni(100) has been investigated by IR reflection-absorption spectroscopy (IRAS). Its CO stretching frequency changes from 2095 to 2115 cm −1 as a function of increasing coverage. Experiments with 12CO and 13CO mixtures show that this shift is composed of an increase in frequency of 48 cm −1 due to dipole-dipole coupling and a decrease of 24 cm −1 due to “static” or chemical effects. The negative chemical shift can be correlated with the appearance of satellites in the photoelectron spectrum as in other weak chemisorption systems.

An IR-study of the -CO CO-adsorption state in the system CO/H/Ni(100)

Abstract The weakly bound Σ-CO co-adsorption state in the system CO/H/Ni(100) has been investigated by IR reflection-absorption spectroscopy (IRAS). Its CO stretching frequency changes from 2095 to 2115 cm −1 as a function of increasing coverage. Experiments with 12 CO and 13 CO mixtures show that this shift is composed of an increase in frequency of 48 cm −1 due to dipole-dipole coupling and a decrease of 24 cm −1 due to “static” or chemical effects. The negative chemical shift can be correlated with the appearance of satellites in the photoelectron spectrum as in other weak chemisorption systems.

A correlation between anomalous electronic and vibrational properties of Chemisorbed molecules

The photoelectron spectra of weakly chemisorbed CO and N 2 molecules exhibit a multitude of satellite lines due to multi-electron excitations, i.e., the simple single particle picture of photoionization does not apply to these systems. In all chemisorption systems for which data are presently available, the observation of multi-electron excitations in the valence spectrum is always accompanied by an anomalous behavior for the energy of the molecular vibration as a function of coverage. The energy of such a vibrational mode decreases or remains constant with increasing coverage in these weak chemisorption systems, in contrast to what is seen in “ordinary adsorbates” or predicted by theory for adsorbate dipole-dipole coupling. The correlation between the vibrational spectral data and photoemission data is so consistent that there would appear to be a common explanation for these electronic and vibrational properties. We offer a possible explanation based on: (a) recent ab initio theoretical calculations, which incorporate important electronic correlation effects, and (b) a model previously proposed by Pritchard for the infrared data. The proposed model consists of the following main features. In weakly chemisorbed diatomic systems the 2π orbital (unoccupied in the free molecule) is not involved in the bonding to the substrate in the neutral system. The screening of the hole produced by photoionization is then a non-adiabatic process in which the unoccupied 2π-like orbital (slightly above the Fermi energy) of the neutral ground state falls below the Fermi level and becomes occupied in the ion state. This leads to strong satellite lines in the valence spectra. If the 2π orbital in adsorbed CO or N 2 is not occupied in the ground state, then the 5σ orbital is primarily responsible for the bond to the substrate. Since this orbital is slightly antibonding with respect to the two atoms of the diatomic, the molecular stretching frequency can decrease as the adsorbate coverage increases and the adsorbate-substrate bond weakens (Pritchard's model).