D. Menzel

Desorption induced by electronic transitions: Some recent progress

Abstract In analogy to electron impact or photon-induced dissociation of molecules, electronic excitation of adsorbate complexes leads to desorption of ions and neutrals. Both processes require a primary excitation to a repulsive potential curve, by which conversion of electronic excitation to nuclear motion can occur. However, in the surface case strong competition with the desorptive process occurs by transfer of the excitation away from its primary location, because of strong coupling to the many modes of surface layer and substrate; this leads to very strong dependence of cross sections on bonding mode, excitation energy, isotope mass, and coverage. Recently, considerable progress has been made in the understanding of primary, secondary, and tertiary processes by detailed investigations of energy and polarization dependencies of cross sections, and of angle and energy distributions of products, for primary valence as well as core excitations, and for neutral and ionic products. The importance of localization of the excitation and its dependence on many-body interactions becomes obvious. These aspects are reviewed, and some applications sketched.

High resolution vibrational spectroscopy of CO on Ru(001): The importance of lateral interactions

The adsorption of CO on Ru(001) has been investigated in the temperature range 80–400 K with IR reflection-absorption spectroscopy and the results correlated with LEED and thermal desorption measurements. The C-O frequency shifts continuously from 1984 cm−1 to 2061 cm−1 as a function of increasing coverage, which is attributed mainly to dipole-dipole coupling. No new bands were discovered in the spectrum. The frequency versus coverage relation is also clearly affected by the ordering of the adlayer into the 3 × 3 R 30° and 23 × 23 R 30° structures. Likewise the shape and half-width of the absorption band depend on the details of the ordering process. A linear relationship between coverage and integrated absorption intensity exists only below θ = 0.33; thereafter the absorption intensity falls, with the result that at saturation coverage the absorption per adsorbed molecule is only 35–40% of the absorption at θ = 0.33. This effect is also ascribed to strong lateral interactions in the adlayer. The intrinsic high resolution of the IR method is necessary for the careful study of these phenomena associated with position, shape and intensity of the absorption band.

Adsorption of oxygen on silver single crystal surfaces

Abstract The adsorption of oxygen on Ag(110), (111), and (100) surfaces has been investigated by LEED, Auger electron spectroscopy (AES), and by the measurement of work function changes and of kinetics, at and above room temperature and at oxygen pressures up to 10 −5 Torr. Extreme conditions of cleanliness were necessary to exclude the disturbing influences, which seem to have plagued earlier measurements. Extensive results were obtained on the (110) face. Adsorption proceeds with an initial sticking coefficient of about 3 × 10 −3 at 300 K, which drops very rapidly with coverage. Dissociative adsorption via a precursor is inferred. The work function change is strictly proportional to coverage and can therefore be used to follow adsorption and desorption kinetics; at saturation, ΔΦ ≈ 0.85 eV. Adsorption proceeds by the growth of chains of oxygen atoms perpendicular to the grooves of the surface. The chains keep maximum separation by repulsive lateral interactions, leading to a consecutive series of ( n × 1) superstructures in LEED, with n running from 7 to 2. The initial heat of adsorption is found to be 40 kcal/mol. Complicated desorption kinetics are found in temperature-programmed and isothermal desorption measurements. The results are discussed in terms of structural and kinetic models. Very small and irreproducible effects were observed on the (111) face which is interpreted in terms of a general inertness of the close-packed face and of some adsorption at irregularities. On the (100) face, oxygen adsorbs in a disordered structure; from ΔΦ measurements two adsorption states are inferred, between which a temperature-dependent equilibrium seems to exist.

Adsorption of oxygen and oxidation of CO on the ruthenium (001) surface

Abstract The adsorption of oxygen on the ruthenium (001) surface has been studied using a combination of techniques: LEED/Auger, Kelvin probe contact potential changes, and flash desorption mass spectrometry. Oxygen is rapidly adsorbed at 300 K, forming an ordered LEED structure having apparent (2 × 2) symmetry. Two binding states of oxygen are inferred from the abrupt change in surface work function as a function of oxygen coverage. LEED intensity measurements indicate that the oxygen layer undergoes an order-disorder transition at temperatures several hundred degrees below the onset of desorption. The order-disorder transition temperature is a function of the oxygen coverage, consistent with two binding states. A model involving the adsorption of atomic oxygen at θ 0.5 is proposed. The oxidation of CO to form CO2 was found to have the maximum rate of production at a ruthenium temperature of 950 K.

The influence of adsorbate interactions on kinetics and equilibrium for CO on Ru(001). II. Desorption kinetics and equilibrium

A variety of methods [temperature programmed desorption via pressure rise and via work function changes (Δφ); isothermal desorption via Δφ: quasiequilibrium measurements via isobars monitored by Δφ, in combination with sticking coefficients] has been used to obtain detailed data on the coverage dependence of the adsorption equilibrium and desorption kinetics for CO on the basal Ru(001) face. While the deviation from reversibility varies strongly over these methods, no significant influence of the degree of irreversibility on the results has been found. Desorption energies and isosteric heats are constant at 160 kJ/mol for 0<Θ<0.2, then rise slowly up to 175 kJ/mol at Θ=0.33, where they fall abruptly to 120 kJ/mol and more gradually at higher coverage. The ‘‘first order’’ frequency factor (Arrhenius preexponential normalized by the coverage) is 1016 s−1 at Θ=0, rises precipitously, especially in the range 0.2<Θ<0.33, to 1019 s−1 at Θ≊0.33, where it drops abruptly to ≊1014 s−1. The main conclusions drawn ar...

The adsorption of hydrogen on ruthenium (001): Adsorption states, dipole moments and kinetics of adsorption and desorption

Abstract The adsorption and desorption of hydrogen on the basal Ru(001) plane has been investigated by work function changes, thermal desorption and LEED. Two states possessing positive and negative dipole moment, respectively, and different sticking behaviour, can be distinguished which are roughly correlated to the two desorption peaks found in TPD. The coverage dependences of adsorption energy, desorption preexponential and sticking coefficients have been determined; they are characterized by repulsive lateral interactions. No ordered superstructures exist, but evidence of slight expansion of the top Ru layer upon adsorption has been found. The two states discerned may be correlated with occupation of the hcp and fcc types of threefold sites; an overlayer/underlayer conversion as suggested by recent spectroscopic work appears very unlikely. Strong influences of preadsorbed CO, O, and in particular C, on H adsorption are also described which in the last case appear to be due to formation of a CH species.

Simple ways to improve ’’flash desorption’’ measurements from single crystal surfaces

Flash desorption measurements from single crystal surfaces may be improved by following work function changes rather than parameters such as pressure.(AIP)

An example of “fast” desorption: Anomalously high pre-exponentials for CO desorption from Ru (001)

Abstract Using four independent methods, detailed measurements of energies and pre-exponentials of desorption as functions of coverage have been made for CO on Ru(001). Unusual results — in terms of absolute values and coverage dependence — have been obtained. They can be qualitatively understood within the framework of transition state theory, in connection with the characteristics of this system. The findings may have more general importance.

Direct observation of electron dynamics in the attosecond domain

Dynamical processes are commonly investigated using laser pump–probe experiments, with a pump pulse exciting the system of interest and a second probe pulse tracking its temporal evolution as a function of the delay between the pulses. Because the time resolution attainable in such experiments depends on the temporal definition of the laser pulses, pulse compression to 200 attoseconds (1 as = 10-18 s) is a promising recent development. These ultrafast pulses have been fully characterized, and used to directly measure light waves and electronic relaxation in free atoms. But attosecond pulses can only be realized in the extreme ultraviolet and X-ray regime; in contrast, the optical laser pulses typically used for experiments on complex systems last several femtoseconds (1 fs = 10-15 s). Here we monitor the dynamics of ultrafast electron transfer—a process important in photo- and electrochemistry and used in solid-state solar cells, molecular electronics and single-electron devices—on attosecond timescales using core-hole spectroscopy. We push the method, which uses the lifetime of a core electron hole as an internal reference clock for following dynamic processes, into the attosecond regime by focusing on short-lived holes with initial and final states in the same electronic shell. This allows us to show that electron transfer from an adsorbed sulphur atom to a ruthenium surface proceeds in about 320 as.

Photoelectron spectroscopic studies of adsorption of CO and oxygen on Ru(001)

Abstract XPS and UPS have been used for a detailed study of the adsorption and coadsorption of CO and oxygen on a clean Ru(001) single crystal. The measured substrate and adsorbate core level binding energies and valence levels are discussed. The O 1s XPS peak intensity has been used for kinetic studies of adsorption and coadsorption. Some studies of the angular dependence of adsorbate and substrate peak intensity ratios are presented. We also present data on the shifts of XPS peaks and changes in UPS spectra as a function of adsorbate coverage. The data are correlated with the results of earlier measurements with other methods.