S.D. Foulias

The diffusion of carbon to and from W(100)

Abstract The equilibrium segregation in the system W(100) plus two monolayers of total carbon content has been studied in the range 1600 to 2073 K. The energy of segregation is −56(±2) kcal mole −1 . The kinetics of carbon segregation at 1350 and 1420 K were observed. From a semi-empirical treatment an upper limit to the activation energy for volume diffision is deduced to be 59 (±8) kcal mole −1 .

The adsorption of acetylene on W(110) at room temperature: An electron spectroscopic study

The adsorption of acetylene on W(100) at room temperature has been studied by AES, ELS, thermal desorption, mass spectrometry, work function and LEED in one vacuum chamber. AES line profile analysis shows that there are at least two adsorption processes occurring at room temperature. Further, it is possible to explain all the AES results by assuming non-sequential adsorption into just two states, denoted by α and β. This picture was substantiated and embellished by comparison with other standard surface techniques. The α-state comprises either a C2H2 unit with an activation energy for desorption of 2.3 eVmolecule (53 kcal mole−1) or CH units bounded through the carbon of the β-state. Saturation coverage for the α-state is 3 × 1014 molecules cm−2. The β-state is dissociative at low acetylene exposures and comparison between a carbon covered surface and the β-state suggest the latter to be dissociative up to saturation. There also appears to be ca. 1014 hydrogen atoms cm−2 on W(100) on room temperature acetylene saturation, the carbon content of the β-state being 9 × 1014 atoms cm−2. The residual C⋯C bond from the molecule in the β-state remains unknown. No sign of ordering in the adsorbed species was detected, save the possibility of (1 × 1) in the β-state. Acetylene adsorption at 580 K showed hydrogen from the β-state to block acetylene adsorption by 15% at saturation. A two-site adsorption model for the β-state is proposed to explain the results. The α-state is bonded through the carbon of the β-state and it is speculated that the former adsorbs onto “β” domains where there is a critical minimum size for the latter.

The adsorption of nitric oxide on W(110) at 300 K: An electron spectroscopic study

Abstract Nitric oxide adsorbs dissociatively onto W(110) at 300 K. The simplest interpretation requires adsorption into three states. In the first of these N and O penetrate the surface. In the second state N and O form an overlay er on the W(110) surface. The third state also involves surface penetration but this is deeper than for state 1. The adsorbate stoichiometry was 1:1 throughout adsorption at 300 K. The occupancy of each state was similar after a 300 L exposure when the total amount adsorbed exceeded 17 (±2) × 10 18 atoms m −2 .

Thermal desorption using AES: Some examples of adsorbate-adsorbate interaction

Abstract A simple method of analysing step-wise thermal desorption data has been devised which allows the relationship between adsorbate binding energy and coverage to be deduced. The method has been applied to three adsorbates on W(100), each of which is expected to obey first order thermal desorption kinetics. All three gases (C 2 H 2 , Cl 2 , I 2 ) exhibited a substantial variation of binding energy with coverage.

The tungsten Auger transitions : 150–200 eV

Abstract The low-energy Auger spectrum of tungsten, induced by electron bombardment, has been studied. The second-derivative detection mode was employed to improve the resolution of fine structure. A comparison of N ( E ) spectra and yield curves with theoretical Auger rates and line energies showed that N 4,5 N 6,7 X processes dominate the spectrum, where X denotes N 6,7 or O 2,3 . The core—core—valence ( X = O 4,5 ) processes occur with low probability. The N 4 N 5 V transition appears to be energetically allowed in tungsten, and to occur at a sufficiently high rate to alter the initial N 5 : N 4 hole-states ratio for the above Coster—Kronig and super-Coster—Kronig processes. For primary excitation energies above 425 eV, there is also a small contribution (about 12% at maximum) from N 3 N 5 V processes, which occurs around 166 eV.

A model for carburised W(100) and W(110)

Using LEED and AES the surface phases induced on W(100) and W(110) by carbon have been re-examined. A surface carbide, W3C2, is proposed for the W(100) and W(110) surfaces containing the most carbon. The conditions for producing the surface carbide on W(110) that contains the least carbon, possibly W9C2 or W4C, have been defined.

Chemisorption and physisorption of acetylene on W(100) at 80 K

Abstract Adsorption of acetylene on W(100) at 80 and 300 K is similar. The main change is a shift in the relative adsorption rate of the associative (α-C 2 H 2 ) and dissociative (β-C 2 H 2 ) species, in favour of the former. Physisorbed acetylene was obtained by adsorption on an oxygenated W(100) surface at 80 K. Comparison of physisorbed acetylene with α-C 2 H 2 supports the interpretation of the latter being perturbed but intact acetylene molecules bonded to W(100) via the π-d-π ∗ interaction. Analysis of the Auger energy shifts predicts a C(1s) shift of ≳7 eV between physisorbed and chemisorbed acetylene.

A reversible phase transition in carbon segregation to W(110)

The equilibrium segregation of carbon to the (110) faces of a single crystal of tungsten containing the equivalent of two monolayers of carbon has been studied in the temperature range 1500 to 2000K. The enthalpy of segregation is -239 kJ mol-1 for temperatures up to 1660K and -192 kJ mol-1 for temperatures above 1800K. The change in segregation enthalpy between these two temperatures is attributed to a reversible phase transition in the surface. The high-temperature phase can accommodate only one third of the amount of carbon in the low-temperature phase. A semi-empirical analysis of the segregation kinetics in the temperature range 1200 to 1400K gave a value of 250 kJ mol-1 for the activation energy for volume diffusion of carbon in tungsten. This is an upper limit since there may be a barrier just below the surface.

A sensitive technique for detecting chemical changes by AES

Abstract The measurement of Auger currents and their associated peak to peak heights from the first derivative spectra has provided the basis for an analysis of the various stages of nitric oxide absorption onto a W(110) surface at room temperature. This chemisorption system is a test case for the method since the nitrogen and oxygen KLL Auger spectra in first derivative form look similar at all stages of adsorption. The results indicate that the method is sensitive to chemical changes in the adsorbate to an extent comparable with XPS.

A double ionization satellite in the tungsten Auger spectrum

Abstract The electron induced tungsten Auger spectrum exhibits a peak at 210 eV which is on the high energy side of the major Auger emissions, N 4, 5 N 6, 7 X ( X ≡ N 6, 7 or O 2, 3 ). The N 4 N 6, 7 V Coster-Kronig processes should occur around 210 eV but the yield curve for the 210 eV feature shows that most of the intensity arises from a process with threshold behaviour toward N 3 . This process is identified as the second step in an Auger cascade: N 3 single holes decay via an N 5 V state which exhibits a high probability of decaying via a particular N 6, 7 X V three-hole state to yield electron emission at 210 eV.