Vibrational structure in the carbon 1s ionization of hydrocarbons: Calculation using electronic structure theory and the equivalent-cores approximation

In situ observation of the on-surface thermal dehydrogenation of n-octane on Pt(111)

The catalytic dehydrogenation of alkanes constitutes a key step for the industrial conversion of these inert sp3-bonded carbon chains into other valuable unsaturated chemicals. To this end, platinum-based materials are among the most widely used catalysts. In this work, we characterize the thermal dehydrogenation of n-octane (n-C8H18) on Pt(111) under ultra-high vacuum using synchrotron-radiation X-ray photoelectron spectroscopy, temperature-programmed desorption and scanning tunneling microscopy, combined with ab initio calculations. At low activation temperatures, two different dehydrogenation stages are observed. At 330 K, n-C8H18 effectively undergoes a 100% regioselective single C-H bond cleavage at one methyl end. At 600 K, the chemisorbed molecules undergo a double dehydrogenation, yielding double bonds in their carbon skeletons. Diffusion of the dehydrogenated species leads to the formation of carbon molecular clusters, which represents the first step towards poisoning of the catalyst. Our results reveal the chemical mechanisms behind the first stages of alkane dehydrogenation on a platinum model surface at the atomic scale, paving the way for designing more efficient dehydrogenation catalysts.

Valence-shell ionization of acetyl cyanide: simulation of the photoelectron and infra-red spectra

The vibrational envelopes of the first and second lines of the acetyl cyanide valence photoelectron spectrum [Katsumata et al., J. Electron Spectrosc. Relat. Phenom., 2000, 49, 113] in the gas phase have been simulated considering the Taylor expansion of the dipole moment from zero up to the second order as well as the changes of geometries/frequencies/normal modes between the initial neutral electronic ground state and the final (15a'^(-1), 3a''^(-1)) cationic states. It is shown that the vibrational profile of the first band (A') extending over 3500 cm^-1 with a vibrational spacing of 500 cm^-1 is not due solely to the overtones (v = 0 → v' = 1, 2, 3,…) of the C-CO bending mode as previously suggested but results from a collection of (v = 0 → v' = 1) transitions with frequencies multiple of 500 cm^-1 associated with the CO stretching at 1550 cm^-1, C-C stretching at 1045 cm^-1 and C-CO, C-CN bending modes at 370/500 cm^-1 completed by combination bands. Our calculations also reveal that the structureless and asymmetric shape of the second band (A'') is due to the activation of the torsion mode at low-frequency (ω ≈ 150 cm^-1) induced by the rotation (60 degrees) of the methyl group blurring the main vibrational progression (ω ≈ 1115 cm^-1) corresponding to the cooperative motions of the methyl CH bending and C-CO bending/CO stretching. Infra-red spectra of the fundamental and both the 15a'^(-1) and 3a''^(-1) cationic states were finally simulated. In contrast to the photoemission spectra, the infrared intensity of the CO stretching motion is very weak. The spectra are mainly dominated by the v = 0 → v = 1 transition of the CN stretching and the CH symmetric bending/stretching modes, providing complementary information between photoemission and infra-red spectroscopies to capture the nature of the cationic states in acetyl-cyanide.

Specific chemical bond relaxation unraveled by analysis of shake-up satellites in the oxygen single site double core hole spectrum of CO2

We recently developed [A. Ferté, et al., J. Phys. Chem. Lett., 2020, 11, 4359] a method to compute single site double core hole (ssDCH or K^-2) spectra. We refer to that method as NOTA+CIPSI. In the present paper this method is applied to the O K^-2 spectrum of the CO₂ molecule, and we use this as an example to discuss in detail its convergence properties. Using this approach, theoretical spectra in excellent agreement with the experimental one are obtained. Thanks to a thorough interpretation of the shake-up states responsible for the main satellite peaks and through comparison with the O K^-2 spectrum of CO, we can highlight the clear signature of the two non-equivalent carbon oxygen bonds in the oxygen ssDCH CO₂ dication.

X-ray photochemistry of carbon hydride molecular ions

Ultrafast electronic and nuclear dynamics induced by X-ray absorption in carbon hydride molecular ions are theoretically investigated.

Ethene adsorption and dehydrogenation on clean and oxygen precovered Ni(111) studied by high resolution x-ray photoelectron spectroscopy

The adsorption and thermal evolution of ethene (ethylene) on clean and oxygen precovered Ni(111) was investigated with high resolution x-ray photoelectron spectroscopy using synchrotron radiation at BESSY II. The high resolution spectra allow to unequivocally identify the local environment of individual carbon atoms. Upon adsorption at 110 K, ethene adsorbs in a geometry, where the two carbon atoms within the intact ethene molecule occupy nonequivalent sites, most likely hollow and on top; this new result unambiguously solves an old puzzle concerning the adsorption geometry of ethene on Ni(111). On the oxygen precovered surface a different adsorption geometry is found with both carbon atoms occupying equivalent hollow sites. Upon heating ethene on the clean surface, we can confirm the dehydrogenation to ethine (acetylene), which adsorbs in a geometry, where both carbon atoms occupy equivalent sites. On the oxygen precovered surface dehydrogenation of ethene is completely suppressed. For the identification of the adsorbed species and the quantitative analysis the vibrational fine structure of the x-ray photoelectron spectra was analyzed in detail.

Electronic relaxation effects in condensed polyacenes: A high-resolution photoemission study

We present a high-resolution photoelectron spectroscopy investigation of condensed films of benzene, naphthalene, anthracene, tetracene, and pentacene. High spectroscopic resolution and a systematic variation of the molecular size allow a detailed analysis of the fine structures. The line shapes of the C 1s main lines are analyzed with respect to the different contributions of inhomogeneous broadening, vibronic coupling, and chemical shifts. The shake-up satellite spectra reveal trends, which give insight into the charge redistribution within the molecule upon photoexcitation. In particular, the shake-up between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) increases in intensity and moves closer toward the C 1s main line if the size of the aromatic system is increased. An explanation is given on the basis of the delocalization of the aromatic system and its capability in screening the photogenerated core hole. A comparison of the HOMO-LUMO shake-up position to the optical band gap gives additional insight into the reorganization of the electronic system upon photoexcitation.

Geometry Relaxations After Inner-Shell Excitations and Ionizations

The geometry relaxations due to the inner-shell excitations and ionizations have been studied by the SAC-CI method. The characteristic molecular geometry changes were predicted for the core-hole states of CH4, NH3, H2O and HF: the calculated CH bond length change agrees well with the result simulated by the observed spectrum. The C1s excitation spectrum of CH4 was also investigated for the Rydberg states of the principal quantum numbers n = 3, 4 and 5. The potential energy curves of the dipole-allowed excited states were calculated for the totally symmetric stretching mode. The vibrational structure and Franck-Condon factors for the C1s excitation spectrum were well reproduced, which shows that the equilibrium geometries of the excited states were accurately evaluated. The geometries of the inner-shell π* excited states of N2O and CO2 were also examined. The calculated geometries of these states qualitatively agreed with the experimental values of the corresponding equivalent-core molecules.

Dissociative Photoionization of Methoxycarbonylsulfenyl Chloride, CH3OC(O)SCl, Following Sulfur 2p, Chlorine 2p, and Oxygen 1s Excitations

Total ion yield spectra and photoinduced fragmentations following S 2p, Cl 2p, and O 1s inner shell excitations of methoxycarbonylsulfenyl chloride, CH(3)OC(O)SCl, have been studied in the gaseous phase by using synchrotron radiation and multicoincidence techniques, which include photoelectron-photoion coincidence (PEPICO) and photoelectron-photoion-photoion coincidence (PEPIPICO) time-of-flight (TOF) mass spectrometry. According to the analysis of the partial ion yield spectra the S+ ion signal shows a steep enhancement near the S 2p resonance, which could represent an evidence of state-specific fragmentations promoted by electronic excitations from the S 2p to vacant orbitals with strong antibonding character mainly located at the sulfur atom. The dissociation dynamics for selected PEPIPICO islands have been discussed. Fragmentation channels that involve the extrusion of H+ and CH(x)+ (x = 0, 1, 2, 3) fragments have been predominantly observed for dissociation of doubly charged CH(3)OC(O)SCl(2+).

Anharmonicity and tunneling effects in revisited vibrational O(1s) photoelectron spectrum of water gas phase

The authors have revisited the description of the core-hole ionization dynamics of the oxygen atom in water by re-exploiting the high-resolution, vibrationally resolved, XPS photoelectron spectrum of gas phase at the O(1s) edge. The agreement between theory and experiments is mainly controlled by (i) the description of the tunneling behavior near the barrier top (linear H-O-H conformation) of wave functions with high vibrational quanta, and (ii) the relative displacement of the potential-energy minimum of the O(1s) final state with respect to the ground state one. Accurate change in bond angle between the neutral and core-ionized states is essential to account for the Franck-Condon factors. The O(1s) photoelectron spectrum of water is well reproduced by the molecular ab initio calculations based on density functional theory and Franck-Condon factors calculations in a double-well (2 x W) simulation of the bending motion.

A detailed analysis of vibrational excitations in x-ray photoelectron spectra of adsorbed small hydrocarbons

The vibrational fine structure of x-ray photoelectron (XP) spectra of a number of different small hydrocarbon molecules and reaction intermediates adsorbed on Pt(111) and Ni(111) has been investigated in detail. The data for methyl, methylidyne, acetylene, and ethylene can consistently be analyzed within the linear coupling model. The S factor, i.e., the intensity ratio of the first vibrationally excited to the adiabatic transition, is obtained to be 0.17+/-0.02 per C-H bond; for the deuterated species a value of 0.23+/-0.02 is obtained. Therefore, the vibrational fine structure can be used for fingerprinting in the analysis of XP spectra and for identifying unknown reaction intermediates. From the data, Deltar, the change of the minimum in the potential energy curve upon core ionization, is calculated within the linear coupling model using a first order correction. For all adsorbates, including the deuterated ones, a value of Deltar=0.060+/-0.004 A is obtained. Furthermore, from the binding energy of the adiabatic peak and from the energy of the vibrational excitation in the ionic final state some information on the adsorbate/substrate bond and the adsorption site can be derived.

Lineshapes in carbon 1s photoelectron spectra of methanol clusters

A general protocol for theoretical modeling of inner-shell photoelectron spectra of molecular clusters is presented and applied to C1s spectra of oligomers and medium-sized clusters of methanol. The protocol employs molecular dynamics for obtaining cluster geometries and a polarizable force field for computing site-specific chemical shifts in ionization energy and linewidth. Comparisons to spectra computed from first-principle theories are used to establish the accuracy of the proposed force field approach. The model is used to analyze the C1s photoelectron spectrum of medium-sized clusters in terms of surface and bulk contributions. By treating the surface-to-bulk ratio as an adjustable parameter, satisfactory fits are obtained to experimental C1s spectra of a beam of methanol clusters.

Evidence of Site-Specific Fragmentation on Thioacetic Acid, CH3C(O)SH, Irradiated with Synchrotron Radiation around the S 2p and O 1s Regions

Site-specific fragmentations following S 2p and O 1s photoexcitation of thioacetic acid, CH3C(O)SH, have been studied by means of synchrotron radiation. Total ion yield (TIY) spectra were measured and multicoincidence techniques, which include photoelectron-photoion coincidence (PEPICO) and photoelectron-photoion-photoion coincidence (PEPIPICO) time-of-flight mass spectrometry, were applied. The equivalent-core approximation was employed in order to estimate ionization transition values, and the observed peaks were tentatively assigned. A site-specific fragmentation is moderately observed by comparing the mass spectra collected at resonant energies around the inner and shallow inner shell S 2p and O 1s ionization edges. Beside H+ ion, the most abundant ions observed at the S 2p edge excitation were CH3CO+, SH+, S+, and CH3+. At the O 1s region the large CH3CO+ fragment was depressed, and small CHx+ (x = 0, 1, 2, 3), S+, and SH+ fragments were dominant. The dissociation dynamic for the main ion-pair production has been discussed. Two- and three-body dissociation channels have been observed in the PEPIPICO spectra, and the dissociation mechanisms were proposed.

N-K near-edge x-ray-absorption fine structures of acetonitrile in gas phase

The dynamic processes of N(1s) core-hole excitation in gas-phase CH3CN molecule have been studied at both Hartree-Fock and hybrid density-functional theory levels. The vibrational structure is analyzed for fully optimized core-excited states. Frank-Condon factors are obtained using the linear coupling model for various potential surfaces. It is found that the vibrational profile of the N-K absorption can be largely described by a summation of two vibrational progressions: a structure-rich profile of nu(CN) stretching mode and a large envelope of congestioned vibrational levels related to the strong (-C-CN) terminal bending bond. Excellent agreement between theoretical and experimental spectra is obtained.

Violation of the Franck-Condon principle due to recoil effects in high energy molecular core-level photoionization

Carbon 1s photoelectron spectra of methane are measured over a photon energy range between 480 eV and 1200 eV. Additional components appear between the individual symmetric stretching vibrational components and are attributed to the excitations of asymmetric stretching and bending vibrations due to recoil of the high-energy photoelectron emission. This recoil effect is the evidence for the violation of the Franck-Condon principle which states that neither the positions nor the momenta of the nuclei change during the ionization event.

Toward the spectrum of free polyethylene: linear alkanes studied by carbon 1s photoelectron spectroscopy and theory

Trends in carbon 1s ionization energies for the linear alkanes have been investigated using third-generation synchrotron radiation. The study comprises CH(4), C(2)H(6), C(3)H(8), C(4)H(10), C(5)H(12), C(6)H(14), and C(8)H(18). Both inter- and intramolecular shifts in ionization energy have been determined from gas-phase spectra and ab initio calculations. The shifts are decomposed into initial-state and final-state contributions and are shown to relate to the fundamental chemical properties of group electronegativity and polarizability. By extrapolation, we predict C1s spectra of larger n-alkanes, converging toward isolated strands of polyethylene.

Chemical insights from high-resolution X-ray photoelectron spectroscopy and ab initio theory: propyne, trifluoropropyne, and ethynylsulfur pentafluoride

High-resolution carbon 1s photoelectron spectroscopy of propyne (HC triple bond CCH3) shows a spectrum in which the contributions from the three chemically inequivalent carbons are clearly resolved and marked by distinct vibrational structure. This structure is well accounted for by ab initio theory. For 3,3,3-trifluoropropyne (HC triple bond CCF3) and ethynylsulfur pentafluoride (HC triple bond CSF5), the ethynyl carbons show only a broad structure and have energies that differ only slightly from one another. The core-ionization energies can be qualitatively understood in terms of conventional resonance structures; the vibrational broadening for the fluorinated compounds can be understood in terms of the effects of the electronegative fluorines on the charge distribution. Combining the experimental results with gas-phase acidities and with ab initio calculations provides insights into the effects of initial-state charge distribution and final-state charge redistribution on ionization energies and acidities. In particular, these considerations make it possible to understand the apparent paradox that SF5 and CF3 have much larger electronegativity effects on acidity than they have on carbon 1s ionization energies.

Doppler splitting of In-flight auger decay of dissociating oxygen molecules: the localization of delocalized core holes

By exploiting the core-excitation-induced dissociation of O2, we find that the Auger emission exhibits a Doppler-like energy shift. We show this to be a manifestation of localization of the core hole and propose that the problem of core-hole localization versus delocalization in core-hole spectroscopies may be resolved by considering the nature of the measurement.