Károly Révész

Photoelectron spectroscop1c studies of the adsorption of CO2 on potassium-promoted Rh(111) surface

Abstract Adsorption of CO 2 has been studied on clean and potassium-dosed Rh(111) surfaces by means of UPS and XPS. CO 2 adsorbs molecularly at 90 K on clean Rh without a strong influence on the electronic structure of the adsorbed layer. Adsorption of CO 2 on potassium-dosed Rh(111) (θ k = 0.33) at 100 K and annealing the adsorbed layer at 131 K produced three peaks at 5.2. 8.7 and 10.9 eV in the Hell spectrum and 532.8 for O(1s) and 290.5 eV for C(1s) in XPS. These emissions were attributed to the formation of CO 2 − radical anions. At higher temperatures the CO 2 radical is transformed into carbonate and CO. Carbonate species is characterized by the 3.5. 8.4 and 10.2 eV peaks in UPS, and by C(1s) at 289.0 eV and O(1s) at 531.8 eV levels in XPS. At lower potassium coverage (θ k = 0.1). carbonate formation was not observed, but the CO 2 anion radical dissociated to CO and O at 131–179 K. This process was accompanied by the appearance of photoemission peaks at 8.2, 11.2 and 6.0 eV.

Surface photochemistry: Adsorption and dissociation of CH3Cl on clean and K‐promoted Pd(100) surfaces

The photochemistry of adsorbed CH3Cl on clean and potassium‐dosed Pd(100) surfaces has been studied by illumination with low‐intensity UV light from a high‐pressure Hg lamp. The effects of illumination were established by post‐irradiation thermal desorption, Auger and photoelectron spectroscopic measurements. Evidences are presented for photodissociation of the C–Cl bond to give surface bound methyl group coadsorbed with chlorine. The thermal decomposition of the surface methyl is accompanied by desorption of methane, and small amounts of ethane and ethylene. The presence of preadsorbed potassium greatly enhanced the extent of photodissociation of the C–Cl bond. The efficiency of potassium adatoms depended on its amount, i.e., on the work function of the coadsorbed systems. From the study of the wavelength dependence it is concluded that in the photodissociation of CH3–Cl the optical excitation of the substrate to produce photoelectrons plays a dominant role.

Adsorption of CH3Cl on clean and Cl-dosed Pd(100) surfaces

Abstract The adsorption of methyl chloride on a Pd(100) surface has been investigated by ultraviolet photoelectron spectroscopy (UPS), electron energy loss spectroscopy (in the electronic range, EELS), temperature-programmed desorption (TPD) and work function change. CH3Cl adsorbs with high sticking probability at 80–100 K. UPS and TDS spectra suggest that the adsorption of CH3Cl is molecular at 100 K, with a little distortion of the corresponding gas-phase molecular electronic structure. No dissociation of CH3Cl was observed even up to 550 K. By means of TPD, we distinguished two adsorption states with desorption energies of 46.9 and 33.4 kJ/mol. The formation of a condensed layer at 105–110 K was also observed. Adsorption of CH3Cl caused a significant work function decrease, Δϕ = −0.91 eV, indicating a dipole with positive end pointed away from the surface. The effects of electronegative additives, preadsorbed Cl and O were also examined. Preadsorbed Cl caused a slight destabilization of adsorbed CH3Cl at lower concentration, prevented the adsorption of CH3Cl at higher concentration and facilitated the formation of a condensed layer. No such effect was experienced in the presence of preadsorbed O.

Effects of potassium on the adsorption and dissociation of CH3Cl on Pd(100)

Abstract The effects of potassium on the adsorption and dissociation of CH3Cl on a Pd(100) surface has been investigated by ultraviolet photoelectron spectroscopy (UPS), Auger electron spectroscopy (AES), electron energy loss spectroscopy (in the electronic range EELS), temperature-programmed desorption (TPD) and work function change. In contrast to the clean surface, the adsorption of CH3Cl caused a significant work function increase, 0.9-1.4 eV, of potassium-dosed Pd. Preadsorbed K enhanced the binding energy of CH3C1 to the surface and induced the dissociation of adsorbed molecules. The extent of the dissociation increased almost linearly with the potassium content. The appearance of a new emission in the UPS spectrum at 9.2 eV, attributed to adsorbed CH3 species, and the low-temperature formation of ethane suggest that a fraction of adsorbed CH3Cl dissociates even at 115–125 K on potassium-dosed Pd(100). At the same time, a significant part of adsorbed CH3 radical is stabilized, the reaction of which occurs only at 250–300 K. By means of TPD measurements, H2, CH4, C2H6, C2H4, KCl and K were detected in the desorbing gases. The results are interpreted by assuming a through-metal electronic interaction at low potassium coverage and by a direct interaction of the Cl in the adsorbed CH3Cl with potassium at high potassium coverage. The latter proposal is supported by the electron excited Auger fine structure of the Cl signal and by the formation of KCl in the desorbing gases.

Segregation of boron and its reaction with oxygen on Rh

Abstract The segregation of boron and its reactivity towards oxygen has been investigated by means of AES, XPS, UPS and ELS (in the electronic range) in the temperature range 100–1300 K. The segregation of boron in a Rh foil started from 700 K. The segregated boron produced a peak in XPS for the B(1s) level at 187.8 eV and emissions in UPS at 4.0 and 8.6–9.0 eV for B(2p) and B(2sp2), respectively. Analysis of the results suggested that the segregated boron on Rh foil mainly forms dimers or islands, instead of isolated monomers, without any significant charge transfer between rhodium and boron. Upon oxygen adsorption the B(1s) and O(1s) levels shifted to higher binding energy (to 191.5 and 532.6 eV, respectively) and a new loss in the EELS was produced at 9.4 eV, demonstrating a strong chemical interaction between oxygen and boron. The interaction occurs at as low as 159 K, as indicated by the development of the 9.4 eV loss feature. It is assumed that boron suboxides are formed in which the BB bond is retained. The cleavage of the BB bonds starts above 400 K and is completed at 750 K, when the 2sp2 hybrid state at 8.6–9.0 eV in the UPS, due to the BB bond, is no longer detected. Formation of a polymer-like B2O3 species is proposed which reacts with elemental boron above 900 K to give B2O2.

Photoelectron spectroscopic studies on the dissociation of CO on potassium-dosed Rh(111) surface

Abstract Adsorption of CO has been investigated on clean and potassium-dosed Rh(111) surfaces by means of TDS, UPS, XPS and work function measurements. CO adsorbs molecularly at 90–300 K. on Rh(111) dosed with K up to a monolayer coverage. No spectroscopic evidences were found for the dissociation of CO in the coadsorbed layer heated to near the onset temperature of CO desorption. A well detectable dissociation of CO was observed following electron bombardment of the coadsorbed layer. The effect of potassium on the reaction of adsorbed oxygen and carbon (produced by electron bombardment or by decomposition of ethylene) was also examined. Formation of chemisorbed CO at θ K = 0.33 occurred at 400–500 K, far below the desorption of CO from this surface. A promoting effect of potassium was established. It was concluded that CO desorbs from K-dosed Rh(111) without undergoing a significant dissociation, and the isotopic scrambling between labelled CO proceeds likely via a nondissociative mechanism.

Excimer laser induced surface chemical modification of polytetrafluoroethylene

Abstract Polytetrafluoroethylene has a notoriously non adhesive and non reactive character. Its successful surface photochemical modification was performed by irradiating the polytetrafluoroethylene/liquid triethylamine interface with an ArF excimer laser (λ=193 nm). Due to the photochemical treatment the polytetrafluoroethylene surface became more hydrophilic. The water receding contact angle decreased from 94° to 43°. The reaction cross section was determined from the decrease of the contact angles. It was found to be as high as 6.4×10−18 cm2. XPS measurements evidenced the removal of fluorine from the polytetrafluoroethylene, incorporation of alkyl carbon and nitrogen. Photochemical dissociation path of the triethylamine makes probable that it bonded to the fluoropolymer backbone via the α-carbon atom of an ethyl group. A radical, or a photoinduced electron transfer mechanism was suggested to describe this reaction. A selective area electroless plating of silver was performed after pretreating the sample with patterned photomodification. The increased adhesion of the sample was proved by gluing with epoxy resin. As a result of the surface modification the tensile strength of gluing increased by 210× and reached 24% of the value characteristic for the bulk material.

A comparative study of the thermal stability and reactions of CH2, CH3 and C2H5 species on the Pd(100) surface

Adsorbed CH2, CH2 and C2H5 moieties were produced on Pd(100) at 90 K by photoinduced dissociation of the corresponding iodo compounds, and their thermal reactions were established.

Formation of the surface structure of polyethylene-terephtalate (PET) due to ArF excimer laser ablation

Abstract The development process of the surface structure on polyethylene-terephtalate (PET) has been investigated. It was found that the average dimension and shape of its unit cells depend on the excimer laser fluence, the incident angle of the ablating laser beam (the longitudinal dimension is proportional tocons × tan(α) + D formula, whereD means the average dimension of the unit cell atα = 0°) and the number of shots (the average dimension and height proportional to the logarithm of the number of shots). A dye laser based arrangement was constructed to investigate the temporal dependence of the scattered probe light intensity from the ablated polymer surface. It was found that the formation of the surface structure takes place in the time range of 5–10 μs. We used a heat diffusion melting model to explain the development of the surface structure. A simple 1D simulation of the heat transfer shows that the lifetime of the liquid phase (∼ 1–7 μs) is comparable with the time scale mentioned above.

Interaction of NO with clean and K-dosed Rh(111) surfaces. II. EELS and PES studies

The interaction of NO with clean and potassium covered Rh(111) surfaces has been investigated by means of EELS (in the electronic range), UPS and XPS. The appearance of the O 2p signal at 6.0 eV in UPS indicates the dissociation of NO at low coverage on the clean surface at 300 K. At high exposures the adsorption is preferentially molecular. In the presence of a potassium adlayer the thermal stability of adsorbed NO depended on the potassium coverage. At low K coverage the potassium promotes the dissociation due to enhanced back-donation from the d-orbitals of Rh into the 2π orbital of NO. At monolayer K coverage, the UPS and XPS spectra suggest an appreciable stabilization of NO; the dissociation started at 400–422 K. Above 422 K, the appearance of new photoemission signals at 4.0, 9.8 and 11.7 eV in UPS, and at 403.8 and 533.7 eV in XPS suggest the formation of NO2 species stabilized by potassium. This surface complex decomposes on Rh(111) at around 664 K.