O₂/Pd(111). Clarification of the correspondence between thermal desorption features and chemisorption states

The system O2/Pd(111) has been probed with electron energy loss spectroscopy and thermal desorption spectrometry. The coincidence of three peaks in thermal desorption and the O-O region of the EEL spectrum has been noted previously and a one-to-one correspondence between the states resolved in EELS and the desorption features has been assumed. We show that this assumption is incorrect. The lowest binding energy state correlates with the highest frequency vibrational state. The lowest vibrational frequency state acts as a precursor to dissociation. The two higher temperature thermal desorption features both arise from the remaining state

A HREELS investigation of benzenePT(111): the b_2g(π∗) resonance

For benzene chemisorbed on Pt(111) a negative ion resonance has been found at 2.1 eV above the vacuum energy. The resonance is assigned to the b2g(π∗) orbital of benzene. The shift of its energy upon adsorption is discussed in the light of the chemical bonding of the molecule to the surface

Collision induced desorption and dissociation of O2 chemisorbed on Ag(001)

We have investigated desorption and dissociation of O-2 chemisorbed on Ag(001) induced by collision with hyperthermal Xe and Ar atoms by high resolution electron energy loss spectroscopy and supersonic molecular beam technique. The cross section for both processes increases rapidly both as a function of angle of incidence and of total impact energy of the inert gas atom. While the increase with energy is expected, the increase with the angle is somewhat surprising arid is sensibly larger than observed for previously investigated systems. The cross section for desorption decreases moreover with coverage. In the limit of high impact energy and high coverage its value is always larger than the one for dissociation. The branching ratio between the two processes depends thereby on energy and angle of incidence of the inert gas atom. Atomic oxygen is not removed under any impact condition, because of its larger binding energy. In order to explain the experimental results, molecular dynamics simulations have been performed using a simple model including multiple scattering, We find that the angular dependence of the cross section is determined by surface corrugation and by multiple scattering which suppresses desorption at normal incidence while the energetic threshold is determined by energy loss to the substrat

Thermochemical characterization of Ca4La6(SiO4)6(OH)2 a synthetic La- and OH-analogous of britholite: implication for monazite and LREE apatites stability

In this contribution, monazite (LREEPO4) solubility is addressed in a chemical system involving REE-bearing hydroxylapatite, (Ca,LREE)10(PO4,SiO4)6(OH)2. For this purpose, a synthetic (La)- and (OH)-analogous of britholite, Ca4La6(SiO4)6(OH)2, was synthesised and its thermodynamic properties were measured. Formation enthalpy of –14,618.4 ± 31.0 kJ·mol–1 was obtained by high-temperature drop-solution calorimetry using a Tian-calvet twin calorimeter (Bochum, Germany) at 975 K using lead borate as solvent. Heat capacities (Cp) were measured in the 143–323 K and 341–623 K ranges with an automated Perkin-Elmer DSC 7. For calculations of solubility diagrams at 298 K, the GEMS program was used because it takes into account solid solutions. In conditions representative of those expected in nuclear waste disposal, calculations show that La-monazite is stable from pH = 4 to 9 with a minimum of solubility at pH = 7. La-bearing hydroxylapatite precipitates at pH > 7 with a nearly constant composition of 99% hydroxylapatite and 1% La-britholite. Each mineral buffers solution at extremely low lanthanum concentrations (log{La} = 10–10–10–15 mol·kg–1 for pH = 4 to 13). In terms of chemical durability, both La-monazite and La-rich apatite present low solubility, a requisite property for nuclear-waste forms

Electron-Induced Synthesis of Ozone in a Dioxygen Matrix

Ozone (O3) was synthesized in the condensed phase induced by electron bombardment of multilayer films of molecular oxygen condensed at temperatures below 30 K on metal surfaces. O3 formation was demonstrated by the observation of the asymmetric stretching (v3) and bending (v2) normal modes of vibration in a high-resolution electron energy-loss spectroscopy experiment, and by characteristic changes in electron-stimulated desorption of O-. The threshold electron energy for the O3 formation is found at 3.5±0,2 eV. It corresponds to the formation of O(3P) associated with O-(2P) by dissociative electron attachment at condensed O2, followed by the third body reaction O+O2 +O2→O3+O2. Above 5.1 eV bombarding energy, dissociative excitation of the O̊̇̑2̑ (c1Σu -,C3Δu,A3Σu+ ,B3Σu-) states is the main source of atomic oxygen O(3P) or O(1D) involved in the O3 synthesis

Resonant electron scattering of physisorbed O2 on Ag(111)

Physisorbed O-2 on Ag(111) at mono- and multilayer coverages was studied by high-resolution electron energy-loss spectroscopy. As the excitation mechanism, the (4) Sigma(u)(-) negative-ion resonance was chosen. The O-2 stretch mode and its overtones exhibit a strong rail to higher loss energies. For the multilayer, a broad peak around 7 meV is observed. This observation and the lack of any larger intensity on the gain side made it likely that vibrational energy quanta as large as 7 meV are the dominant contribution to the tail

Negative ion resonances of O₂ adsorbed on Ag surfaces

This article gathers together a collection of recent experimental studies of the adsorption of oxygen on (001), (110) and (111) crystal surfaces of silver with special emphasis on the negative ion states of this model system for oxygen adsorption. These investigations were performed in a network entitled ‘Negative ion resonances of adsorbed molecules’ supported financially by the European Union within the ‘Human capital and mobility programme’. The kinetics and thermodynamics of adsorption are investigated by measuring the sticking coefficient and by thermal desorption spectroscopy (TDS). The vibrational spectra provided by high-resolution electron energy loss spectroscopy (HREELS) are used to analyse the adsorbed species (physisorbed and chemisorbed) in the case of O2 on Ag(110) and on Ag(111). The mechanisms of inelastic electron scattering by adsorbed O2 are further investigated with special reference to the negative ion resonances (NIRs), formed by electron capture, which are involved in the electron–molecule collision process

Negative Ion Resonances of O2 adsorbed on Ag Surfaces

This article gathers together a collection of recent experimental studies of the adsorption of oxygen on (001), (110) and (111) crystal surfaces of silver with special emphasis on the negative ion states of this model system for oxygen adsorption. These investigations were performed in a network entitled 'Negative ion resonances of adsorbed molecules' supported financially by the European Union within the 'Human capital and mobility programme'. The kinetics and thermodynamics of adsorption are investigated by measuring the sticking coefficient and by thermal desorption spectroscopy (TDS). The vibrational spectra provided by high-resolution electron energy loss spectroscopy (HREELS) are used to analyse the adsorbed species (physisorbed and chemisorbed) in the case of O-2 On Ag(110) and on Ag(111). The mechanisms of Inelastic electron scattering by adsorbed O-2 are further investigated with special reference to the negative ion resonances (NIRs), formed by electron capture, which are involved in the electron-molecule collision process