A bonding study of c-C5H8 adsorption on Pt(1 1 1)

The chemisorption of cyclopentane (c-C₅H₈) on Pt(1 1 1) has been studied using a qualitative band-structure calculations in the framework of tight-binding implementation with the YAeHMOP package. We modeled the metal surface by a two-dimensional slab of finite thickness with an overlayer of c-C₅H₈, in a (3 × 3) di-σ geometry. The c-C₅H₈ molecule is attached to the surface with its CC atoms bonded mainly with two Pt atoms while the opposite CH₂ bends towards the surface. The PtPt bonds in the underlying surface and the CC bonds of c-C₅H₈ are weakened upon the chemisorption. A noticeable Pt-H and Pt-C interactions has been observed. We found that of Pt band plays an important role in the bonding between c-C₅H₈ and the surface, as do the Pt 6s and 6p_z bands. The HOMO-LUMO bands of c-C₅H₈ are very dispersed, indicative of a strong interaction with the metal surface.     

Adsorption of organic molecules by photochemical reaction on Cl:Si(1 1 1) and H:Si(1 1 1) evaluated by HREELS

The covalent attachment of alkyl groups to silicon surfaces, via carbon-silicon bond formation, has been attempted using gas-surface reactions starting from Cl-terminated Si(1 1 1) or H:Si(1 1 1) under ultraviolet light irradiation. The formation of Cl-terminated Si(1 1 1) and its resulting stability were examined prior to deposition of organic molecules. High-resolution electron energy loss spectroscopy (HREELS) was utilized for detecting surface-bound adsorbates. The detection of photo-deposited organic species on Cl:Si(1 1 1) from gas-phase CH₄ or CH₂CH₂ was not significant. On H:Si(1 1 1), it was evident that after the photoreaction with gas-phase C₂H₅Cl, C₂H₅ groups were chemically bonded to the surface Si atoms through single covalent bonds. The C₂H₅ groups were thermally stable at temperatures below 600 K. Alkyl monolayers prepared on silicon surfaces by dry process will lead to a new prospective technology of nanoscale fabrication and biochemical applications.     

A theoretical study of c-C5H8 adsorption on Ge (0 0 1)-2 × 1 and on dimer vacancies on the surface: Electronic structure and bonding

In this work we analyzed the geometry and the chemical interactions for c-C₅H₈ adsorption on Ge (0 0 1), using density functional theory calculations (DFT). We examined the changes in the atomic interactions using a slab model. We considered two cases, the cyclopentene adsorption on Ge(0 0 1) and on dimer vacancies on the surface. We found an average distance H-Ge, -C-Ge and C-Ge of 1.50, 1.70 and 1.65 Å, respectively, on dimer vacancies; and an average C-Ge distance of 2.05 Å on Ge-Ge dimer. We also computed the density of states (DOS) and the DOS weighted overlap populations (OPDOS) corresponding to C-C, C-Ge, C-H, and Ge-Ge bonds. During adsorption the main contribution are the CC double bond in both cases, and the next C and the H's belonging to this bonds in the case of adsorption on dimer vacancies. The orbital contribution includes participation of the 2p_y and 2p_z orbitals corresponding to unsaturated C atoms, 2p_z corresponding to side saturated C, and the 4p orbitals of Ge for the adsorption on dimer vacancies; 2s and 2p_z orbitals corresponding to double bond C atoms, 4s and 4p_z orbitals of Ge for the adsorption on Ge(0 0 1).     

c-C5H5 on a Ni(1 1 1) surface: Theoretical study of the adsorption, electronic structure and bonding

In the present work the ASED-MO method is applied to study the adsorption of cyclopentadienyl anion on a Ni(1 1 1) surface. The adsorption with the centre of the aromatic ring placed above the hollow position has been identified to be energetically the most favourable. The aromatic ring remains almost flat, the H atoms are tilted 17° away from the metal surface. We modelled the metal surface by a two-dimensional slab of finite thickness, with an overlayer of c-C₅H₅⁻, one c-C₅H₅⁻ per nine surface Ni atoms. The c-C₅H₅⁻ molecule is attached to the surface with its five C atoms bonding mainly with three Ni atoms. The NiNi bond in the underlying surface and the CC bonds of c-C₅H₅⁻ are weakened upon adsorption. We found that the band of Ni 5dz² orbitals plays an important role in the bonding between c-C₅H₅⁻ and the surface, as do the Ni 6s and 6p_z bands.     

Site-blocking effects of preadsorbed H on Pt(1 1 1) probed by 1,3-butadiene adsorption and reaction

The influence of hydrogen coadsorption on hydrocarbon chemistry on transition metal surfaces is a key aspect to an improved understanding of catalytic selective hydrogenation. We have investigated the effects of H preadsorption on adsorption and reaction of 1,3-butadiene (H₂CCHCHCH₂, C₄H₆) on Pt(1 1 1) surfaces by using temperature-programmed desorption (TPD) and Auger electron spectroscopy (AES). Preadsorbed hydrogen adatoms decrease the amount of 1,3-butadiene chemisorbed on the surface and chemisorption is completely blocked by the hydrogen monolayer (saturation) coverage (θ_H = 0.92 ML). No hydrogenation products of reactions between coadsorbed H adatoms and 1,3-butadiene were observed to desorb in TPD experiments over the range of θ_H investigated (θ_H = 0.6-0.9 ML). This is in strong contrast to the copious evolution of ethane (CH₃CH₃, C₂H₆) from coadsorbed hydrogen and ethylene (CH₂CH₂, C₂H₄) on Pt(1 1 1). Hydrogen adatoms effectively (in a 1:1 stoichiometry) remove sites from interaction with chemisorbed 1,3-butadiene, but do not affect adjacent sites. The adsorption energy of coadsorbed 1,3-butadiene is not affected by the presence of hydrogen on Pt(1 1 1). The chemisorbed 1,3-butadiene on hydrogen preadsorbed Pt(1 1 1) completely dehydrogenates to H₂ and surface carbon upon heating without any molecular desorption detected, which is identical to that observed on clean Pt(1 1 1). In addition to revealing aspects of site blocking that should have broad implications for hydrogen coadsorption with hydrocarbon molecules on transition metal surfaces in general, these results also provide additional basic information on the surface science of selective catalytic hydrogenation of butadiene in butadiene-butene mixtures.     

Chemisorption of 1,1-dichloroethene on the Si(1 1 1)-7 × 7 surface

Chemisorption of 1,1-dichloroethene (Cl₂CCH₂) to a Si(1 1 1)-7 × 7 surface was studied by means of X-ray photoelectron spectroscopy using synchrotron radiation, recording chlorine 2p and carbon 1s spectra. For carbon 1s, spectral assignment of the chemisorbed species is based on quantum chemical calculations of chemical shifts in model compounds.The results confirm the identity of covalently bonded 1-chlorovinyl (-CClCH₂) and vinylidene (CCH₂) adspecies. Upon chemisorption at room temperature it was found that about one-third of the molecules break one C-Cl bond while about two-thirds of the adsorbates break two C-Cl bonds. We do not, however, find evidence for isomerization of CCH₂ to di-bonded vinylene (-CHCH-).     

Formamide reactions on rutile TiO2(0 1 1) surface

The reaction of formamide over the (0 1 1) faceted TiO₂(0 0 1) surface has been studied by Temperature Programmed Desorption (TPD) and X-ray Photoelectron Spectroscopy (XPS). Two main reactions were observed: dehydration to HCN and H₂O and decomposition to NH₃ and CO. The dehydration reaction was found to be three to four times larger than the decomposition at all coverages. Each of these reactions is found to occur in two temperature domains which are dependent upon surface coverage. The low temperature pathway (at about 400 K) is largely insensitive to surface coverage while the high temperature pathway (at about 500 K) shifts to lower temperatures with increasing surface coverage. These two temperature pathways may indicate two adsorption modes of formamide: molecular (via an η¹(O) mode of adsorption) and dissociative (via an η²(O,N) mode of adsorption). C1s and N1s XPS scans indicated the presence of multiple species after formamide absorption at 300 K. These occurred at ca. 288.5 eV (-CONH-) and 285 eV (sp³/sp² C) for the C1s and 400 eV-(NH₂), 398 eV (-NH) and 396 eV (N) for the N1s and result from further reaction of formamide with the surface.     

Isomeric effects on room-temperature chemisorption and thermal evolution of iso-, cis- and trans-dichloroethylene on Si(1 1 1)7 × 7

The room-temperature adsorption and thermal evolution of iso-, cis- and trans-dichloroethylene (DCE) on Si(1 1 1)7 × 7 have been studied by vibrational electron energy loss spectroscopy and thermal desorption spectrometry (TDS). The presence of the Si-Cl stretch at 510 cm⁻¹ suggests that, upon adsorption, all three isomers dissociate via C-Cl bond breakage on the 7 × 7 surface to form mono-σ bonded chlorovinyl , which could, in the case of iso-DCE, further dechlorinate to vinylidene (:CCH₂) upon insertion into the back-bond. The higher saturation exposure for the Si-Cl stretch at 510 cm⁻¹ observed for cis- and trans-DCE than iso-DCE suggests that Cl dissociation via the CHCl group in the cis and trans isomers is less readily than the CCl₂ group in iso-DCE. Our TDS data show remarkable similarities in both molecular desorption near 360 K and thermal evolution of the respective adstructures for all three isomers on Si(1 1 1)7 × 7. In particular, upon annealing to 450 K, the mono-σ bonded chlorovinyl adspecies is found to further dechlorinate to either vinylene di-σ bonded to the Si surface or acetylene to be released from the surface. Above 580 K, vinylene could also become gaseous acetylene or undergo H abstraction to produce hydrocarbon or SiC fragments. All three DCE isomers also exhibit TDS features attributable to an etching product SiCl₂ at 800-950 K and recombinative desorption products HCl at 700-900 K and H₂ at 650-820 K. The stronger Cl-derived TDS signals and Si-Cl stretch at 510 cm⁻¹ over 450-820 K for trans-DCE than those for cis-DCE indicate stronger dechlorination for trans-DCE than cis-DCE, which could be due to less steric hindrance resulting from the formation of the chlorovinyl adspecies for trans-DCE during the initial adsorption/dechlorination process. Finally, our density functional calculations qualitatively support the thermodynamic feasibility and relative stabilities of the proposed adstructures involving chlorovinyl, vinylidene, and vinylene adspecies.     

Reactivity of 3-hexyne on oxygen modified Ru(0 0 1) surfaces: Observation of oxametallacycles by RAIRS

The chemical behaviour of 3-hexyne on oxygen modified Ru(0 0 1) surfaces has been analysed under ultrahigh-vacuum, using reflection-absorption infrared spectroscopy (RAIRS). The effects of oxygen coverage, 3-hexyne exposure and adsorption temperature were studied. Two modified Ru(0 0 1) surfaces were prepared: Ru(0 0 1)-(2 × 2)-O and Ru(0 0 1)-(2 × 1)-O that correspond to oxygen coverages (θ_O) of 0.25 and 0.5 ML, respectively. The striking result is the direct bonding to an O atom when the modified surfaces are exposed to a very low dose (0.2 L) of 3-hexyne at low temperature (100 K). For θ_O = 0.25 ML, an unsaturated oxametallacycle [Ru-O-C(C₂H₅)C(C₂H₅)-Ru] is proposed, identified by RAIRS for the first time, through the νCC and νCO modes. Further decomposition at 110 K yields smaller oxygenated intermediates, such as acetyl [μ₃-η²(C,O)-CH₃CO], co-adsorbed with a small amount of carbon monoxide and non-dissociated species. The temperature at which a fraction of molecules undergoes complete C-C and C-H bond breaking is thus much lower than on clean Ru(0 0 1). The ultimate decomposition product observed by RAIRS at 220 K is methylidyne [CH]. Another key observation was that the adsorption temperature is not determinant of the reaction route, contrarily to what occurs on clean Ru(0 0 1): even when 3- hexyne strikes the surface at a rather high temperature (220 K), the multiple bond does not break completely. For θ_O = 0.5 ML, a saturated oxametallacycle [Ru-O-CH(C₂H₅)-CH(C₂H₅)-Ru] is also proposed at 100 K, identified by the ν_asO-C-C (at 1043 cm⁻¹) and ν_sO-C-C (at 897 cm⁻¹) modes, showing that some decomposition with C-H bond breaking occurs. For this oxygen coverage, the reaction temperatures are lower, and the intermediate surface species are less stable.     

The chemistry of C2 perfluoroalkyl iodide on the Cu(1 1 1) single crystal surface

The thermal chemistry of perfluoroethyl iodide (C₂F₅I) adsorbed on Cu(1 1 1) has been investigated by temperature-programmed reaction/desorption (TPR/D), reflection-absorption infrared spectroscopy (RAIRS), and X-ray photoelectron spectroscopy (XPS). I 4d and F 1s XPS spectra show that dissociative adsorption of C₂F₅I to form the surface-bound perfluroethyl (Cu-C₂F₅) moieties occurs at very low temperature (T < 90 K), while the C-F bond cleavage in adsorbed perfluroethyl (Cu-C₂F₅) begins at ca. 300 K. XPS and TPR/D studies further reveal that the reactions of ^βCF₃^αCF_2(ad) on Cu(1 1 1) are strongly dependent on the surface coverage. At high coverages (?0.16 L exposure), the adsorbed perfluroethyl (Cu-C₂F₅) evolves, via α-F elimination, into the surface-bound tetrafluoroethylidene moieties (CuCF-CF₃) followed by a dimerization step to form octafluoro-2-butene (CF₃CFCFCF₃) at 315 K as gas product. The surface-bound (Cu-C₂F₅) decomposes preferentially, at low coverages (?0.04 L), via consecutive α-F abstraction to afford intermediate, trifluoroethylidyne (CuCCF₃), resulting in the final coupling reaction to yield hexafluoro-2-butyne (CF₃CCCF₃) at 425 K. However, at middle coverages (ca. 0.08-0.16 L exposure), the adsorbed perfluroethyl (Cu-C₂F₅) first experiences an α-F elimination and then prefers to loss the second F from β position to yield the intermediate of Cu-CF₂-CFCu (μ-η,η-perfluorovinyl), which may further evolve into hexafluorocyclobutene (CF₂CFCFCF₂) at 350 K through cyclodimerization reaction. Our results have also shown that the surface reactions to yield the products, CF₃CFCFCF₃ and CF₃CCCF₃, obey first-order kinetics, whereas the formation of CF₂CFCFCF₂ follows second-order kinetics.     

Low temperature adsorption of oxygen on reduced V2O3(0 0 0 1) surfaces

Well ordered V₂O₃(0 0 0 1) films were prepared on Au(1 1 1) and W(1 1 0) substrates. These films are terminated by a layer of vanadyl groups under typical UHV conditions. Reduction by electron bombardment may remove the oxygen atoms of the vanadyl layer, leading to a surface terminated by vanadium atoms. The interaction of oxygen with the reduced V₂O₃(0 0 0 1) surface has been studied in the temperature range from 80 to 610 K. Thermal desorption spectroscopy (TDS), infrared reflection absorption spectroscopy (IRAS), high resolution electron energy loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) were used to study the adsorbed oxygen species. Low temperature adsorption of oxygen on reduced V₂O₃(0 0 0 1) occurs both dissociatively and molecularly. At 90 K a negatively charged molecular oxygen species is observed. Upon annealing the adsorbed oxygen species dissociates, re-oxidizing the reduced surface by the formation of vanadyl species. Density functional theory was employed to calculate the structure and the vibrational frequencies of the O₂ species on the surface. Using both cluster and periodic models, the surface species could be identified as η²-peroxo () lying flat on surface, bonded to the surface vanadium atoms. Although the O-O vibrational normal mode involves motions almost parallel to the surface, it can be detected by infrared spectroscopy because it is connected with a change of the dipole moment perpendicular to the surface.     

Thermal chemistry of diiodomethane on Ni(1 1 0) surfaces II. Effect of coadsorbed oxygen

The effect of coadsorbed oxygen on the thermal chemistry of diiodomethane on Ni(1 1 0) single-crystal surfaces was studied by temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). I 3d and C 1s XPS data indicated that adsorbed diiodomethane undergoes two sequential C-I bond scission steps to ultimately produce methylene surface species, the same as on clean Ni(1 1 0). Moreover, significant amounts of methane and other heavier hydrocarbons are produced after further thermal activation of those chemisorbed methylene groups. The production of alkanes and alkenes, which is accounted for by a chain growth mechanism where the initial hydrogenation of some adsorbed methylene to methyl moieties is followed by a rate-limiting methylene insertion step to yield ethyl intermediates, is inhibited but not fully blocked by the coadsorbed oxygen. New reaction pathways are also opened up by the presence of oxygen in this system, including a direct coupling of two methylene groups to ethene, the insertion of an oxygen atom into a nickel-methylene group to produce formaldehyde, and a parallel methylene insertion chain growth sequence starting from a CH₂I_ads intermediate to ultimately yield C₃H₅ and C₄H₇ unsaturated gas-phase radicals.     

Adsorption of phenylalanine on single crystal rutile TiO2(1 1 0) surface

The adsorption of the aromatic amino acid, phenylalanine on a TiO₂ rutile (1 1 0) single crystal surface has been investigated with photoemission and NEXAFS (near edge X-ray absorption fine structure) spectroscopy. The results indicate initial adsorption via the carboxylate group in a bidentate configuration with the phenyl ring oriented at approximately 25° to the surface normal. The amino group remains as NH₂. Subsequent layers of phenylalanine appear to adsorb as neutral molecules with H-bonding between NH₂ and CO groups.     

The electronic structure and reactivity of the oxygen-modified Mo2C(0 0 0 1) surface

Oxygen adsorption on Mo₂C(0 0 0 1) has been investigated with angle-resolved photoemission spectroscopy (ARPES). When the surface is reacted with O₂, the O 2p-induced states are formed at 4.1 and 5.3 eV at the point. The emissions around the Fermi level are also intensified by oxygen adsorption, which is due to the formation of a partially filled state. It is found that the reactivity of the surface toward H₂O adsorption is much enhanced by pre-adsorption of oxygen. The reactivity is found to be maximized at θ_O ∼ 0.2.     

Adsorption and reaction of bicyclic hydrocarbons at Pt(1 1 1) and Sn/Pt(1 1 1) surface alloys: trans-decahydronaphthalene (C10H18) and bicyclohexane (C12H22)

Adsorption and desorption of trans-decahydronaphthalene (C₁₀H₁₈) and bicyclohexane (C₁₂H₂₂) can be used to probe important aspects of non-specific dehydrogenation leading to surface carbon accumulation and establish better estimates of activation energies for C-H bond cleavage at Pt-Sn alloys. This chemistry was studied on Pt(1 1 1) and the (2 × 2)-Sn/Pt(1 1 1) and (√3 × √3)R30°-Sn/Pt(1 1 1) surface alloys by using temperature programmed desorption (TPD) mass spectroscopy and Auger electron spectroscopy (AES). These hydrocarbons are reactive on Pt(1 1 1) surfaces and fully dehydrogenate at low coverages to produce H₂ and surface carbon during TPD. At monolayer coverage, 87% of adsorbed C₁₀H₁₈ and 75% C₁₂H₂₂ on Pt(1 1 1) desorb with activation energies of 70 and 75 kJ/mol, respectively. Decomposition of C₁₀H₁₈ is totally inhibited during TPD on these Sn/Pt(1 1 1) alloys and decomposition of C₁₂H₂₂ is reduced to 10% of the monolayer coverage on the (2 × 2)-Sn/Pt(1 1 1) alloy and totally inhibited on the (√3 × √3)R30°-Sn/Pt(1 1 1) alloy. C₁₀H₁₈ and C₁₂H₂₂ are more weakly chemsorbed on these two alloys compared to Pt(1 1 1) and these molecules desorb in narrow peaks characteristic of each surface with activation energies of 65 and 73 kJ/mol on the (2 × 2) alloy and 60 and 70 kJ/mol on the (√3 × √3)R30°-Sn/Pt(1 1 1) alloy, respectively. Alloyed Sn has little influence on the monolayer saturation coverage of these two molecules, and this is decreased only slightly on these two Sn/Pt(1 1 1) alloys. The use of these two probe molecules enables an improved estimate of the activation energy barriers E* to break aliphatic C-H bonds in alkanes on Sn/Pt alloys; E* = 65-73 kJ/mol on the (2 × 2)-Sn/Pt(1 1 1) alloy and E* ? 70 kJ/mol on the (√3 × √3)R30°-Sn/Pt(1 1 1) alloy.     

Adsorption of N@C60 on Si(1 0 0)

The interactions between endohedrally doped N@C₆₀ molecules and the Si(1 0 0) surface have been explored via ab initio total energy calculations. Configurations which have the cage located upon the dimer row bonded to two dimers (r2) and within the dimer trench bonded to four dimers (t4) have been investigated, as these have previously been found to be the most stable for the C₆₀ molecule. We have investigated the differences between the adsorption of the C₆₀ and N@C₆₀ molecules upon the Si(1 0 0) surface and found that there are only minimal differences. Two interesting cases are the r2g and t4d configurations, as they both exhibit differences that are not present in the other configurations. These subtle differences have been explored in-depth. It is shown that the effects on the endohedral nitrogen atom, due to its placement within the fullerene cage, are small. Bader analysis has been used to explore differences between the C₆₀ and N@C₆₀ molecules.     

Bond polarization and image-potential screening in adsorbed C6F6 on Cu(1 1 1)

The orientation of hexafluorobenzene (C₆F₆) on the Cu(1 1 1) surface has been determined for different coverages with the help of near edge X-ray absorption fine structure (NEXAFS) spectroscopy and X-ray photoelectron spectroscopy (XPS). The adsorption geometry and the bonding mode of C₆F₆ differ significantly in comparison to its hydrocarbon analog C₆H₆. C₆F₆ is found to adsorb on Cu(1 1 1) with the ring plane parallel to the surface for coverages below 10 ML. Next to the distinct multilayer, bilayer and monolayer phases we also present evidence of sub-monolayer (i.e., 1/2 ML) coverage with different electronic structure. These findings are explained in a phenomenological model based on fluorine’s property as a σ-acceptor and a π-donor and the resulting bond polarization within the molecule, which is stabilized by image-potential screening within the substrate.     

An investigation of the surface chemistry of methyl pyruvate on Cu(1 1 1)

The surface chemistry of an α-ketoester, methyl pyruvate, has been studied on a model Cu(1 1 1) single crystal surface. Monolayers of methyl pyruvate at 180 K consist predominately (ca. 66%) of a chemisorbed methyl pyruvate moiety, with its keto-carbonyl bonded to the surface in a η² configuration, this moiety desorbs intact at 365 K. The rest of the monolayer contains weakly adsorbed methyl pyruvate, which desorbs at 234 K, which interacts with the surface through the lone pair electrons of the oxygen atoms of the CO groups, adopting a η¹ configuration. Previous studies of simple ketones on model noble metal surfaces have only observed weakly bonded η¹ configurations. The observation of a strongly chemisorbed moiety in the present study is attributed to the activation of the keto-carbonyl by the electron withdrawing ester group. This behaviour is consistent with the homogeneous inorganic chemistry of ketones. Given both the formation of a η² bonded methyl pyruvate moiety on Cu(1 1 1) and the known activity of Cu as a selective hydrogenation catalyst, it is suggested that it maybe worthwhile considering the possibility of testing the effectiveness of chirally modified supported Cu as an enantioselective catalyst.     

High-temperature measurements of the rates of the reactions CH2O + Ar → Products and CH2O + O2 → Products

The two-channel thermal decomposition of formaldehyde [CH₂O], (1a) CH₂O + Ar → HCO + H + Ar, and (1b) CH₂O + Ar → H₂ + CO + Ar, was studied in shock tube experiments in the 2258-2687 K temperature range, at an average total pressure of 1.6 atm. OH radicals, generated on shock heating trioxane-O₂-Ar mixtures, were monitored behind the reflected shock front using narrow-linewidth laser absorption. 1,3,5 trioxane [C₃H₆O₃] was used as the CH₂O precursor in the current experiments. H-atoms formed upon CH₂O and HCO decomposition rapidly react with O₂ to produce OH via H + O₂ → O + OH. The recorded OH time-histories show dominant sensitivity to the formaldehyde decomposition pathways. The second-order reaction rate coefficients were inferred by matching measured and modeled OH profiles behind the reflected shock. Two-parameter fits for k_1a and k_1b, applicable in this temperature range, are:

     

Resonant Auger spectroscopy study of charge transfer phenomena in N 1s core-excited acetonitrile adsorbates on Si(0 0 1)-2 × 1

Acetonitrile (CH₃CN) adsorbs on Si(0 0 1)-2 × 1 at room temperature under two forms, a cycloaddition-like adduct (Si-CN-Si) and a pendent cyano (Si-CH₂-CN) resulting from the decomposition of the molecule. Resonant Auger spectroscopy has been used to study the excited-state-dependent electron transfer from the N 1s core-excited molecular adsorbate to the silicon substrate, using the core-hole lifetime (∼6 fs) as an internal clock. It is shown that the π_CN^∗ NEXAFS state lies within the silicon bandgap because of a core-excitonic effect. Therefore no charge transfer of the excited electron to the substrate is observed. On the other hand the π_CN^∗ NEXAFS state is placed within the silicon conduction band. Excitation to this orbital leads to valence/Auger spectra in which both resonant and normal Auger contributions are observed. Therefore there is evidence for a charge transfer from the pendent CN to the silicon surface, on a timescale estimated to tens of femtoseconds.