Near edge X-ray absorption fine structure (NEXAFS), also known as X-ray absorp- tion near edge structure (XANES), is the third complementary technique used in this thesis. This technique exploits the absorption of electromagnetic radiation by excitation of core level electrons into unoccupied bound or continuum states [12]. The energy region studied with this technique ranges from slightly below to a few eV (≈50 eV) above the absorption edge. The use of the near edge region is appro- priate for the study of molecular adsorbate species, and in particular, information on the intramolecular bond lengths or bond angles can be obtained. As the photon source for NEXAFS has to be intense, monochromatic, linearly polarised, and tun- able, synchrotron radiation is an ideal light source for NEXAFS. All the NEXAFS experiments presented in this thesis have been carried out in BESSY.
If a photon impinges on the surface with energy greater than the binding energy of a core level [10], this photon can be absorbed, resulting in the excitation of a photoelectron from the core level and the creation of a core hole. The annihilation of the core hole created can be filled in two different ways. The first possibility is that the core hole is filled by an electron from another energy level, causing the emission of an Auger electron, as depicted in Figure 2.15b. The second way is by filling the core hole with an electron of a different energy level, and emission of a fluorescent photon. The probability of these two events to occur depends on the type of atom that is probed. For atoms with atomic number ≤31, Auger electron yields are large for the K edges [10]. In this work, only light elements are probed (C, N), so the NEXAFS experiment shown here has been performed by measuring the Auger emission. A NEXAFS spectrum shows the (photon) energy dependence of the photoabsorption cross-section by measuring the Auger intensity [12].
photoabsorption probability phot on ener gy pot en tial ener gy IP π* σ* σ* π* π σ Rydberg states continuum states Energy Ionisation potential hν photoelectron Auger electron K L1 L2 L3
b)
a)
Figure 2.15: On the top,a, schematic diagram of the effective potential (left) and the corresponding NEXAFS K-shell spectrum (right) of a diatomic molecule adapted from [12]. On the bottom, b, example of the Auger process, where a photon of energyhν impinges into a core level of an adsorbate atom and creates a hole. This hole can be annihilated via Auger emission. One electron of an outer shell can fill the hole, and in this process the electron loses energy. This energy appears as kinetic energy given to another electron from the outer shell.
530 540 550 560 530 540 550 560
σ
π
π
σ
Auger elec tr on yield (ar b. units)Photon energy (eV)
Figure 2.16: Example of O 1s NEXAFS spectra of formate on Cu(110), taken from [40]. The spectra in the left are taken in the [1 1 0] azimuth, and the ones in the right in the [0 0 1] azimuth. The full lines represents the spectra taken at normal incidence, and the dashed lines taken at grazing incidence.The features appearing at lower kinetic energy correspond to the π features and at higher kinetic energy appear theσ features.
In a molecule, the occupied and unoccupied electronic states are discrete; therefore, if the sample is irradiated with X-rays with energy slightly less than the photoionisation energy, discrete excitations to unoccupied bound states will occur [3, 9–12]. The probability for these transitions to occur depends on the incoming photon energy. If this energy is exactly the energy difference between the initial state and an unoccupied molecular state, the transition will occur. This process and the origin of the NEXAFS peaks are depicted in Figure 2.15a for a diatomic molecule. At energies below the ionisation threshold transitions to π∗ (i.e., lowest unoccupied molecular orbital) can occur, whilst at higher energies transitions into the σ∗-orbitals can be observed. The σ∗-orbitals can be found at energies slightly above the ionisation potential (quasi-bound state) whereas theπ∗-orbitals are pulled to lower energies by the electron-hole Coulomb interaction [12]. Only molecules with a π-bonding (double and triple bonds) can exhibit π-transitions. Thus, NEXAFS is very useful to obtain information on the electronic structure of an adsorbate on a surface, e.g., it is useful to check whether a double bond is still intact (see subsection 6.2.1).The width of the features depends on the lifetime of the resonances. The lifetime for theσ-resonances (transitions into σ∗-orbitals) is very small due to the large overlap of these states with the continuum. Therefore, theσ features are much broader than theπ features, as can be seen in Figure 2.16. Figure 2.15a also
Surface Surface
π∗
σ∗
E
E
Ο
Ο
hνatoms
Figure 2.17: Diagram of near edge X-ray absorption fine structure based on an example of [12] for a π-bonded diatomic molecule. The molecular axis is normal to the surface. E denotes the polarisation vector of the incident light, Ephot=h·ν,
andO denotes the direction of the final state orbital. In this case, the π-resonance is maximised at normal incidence (left) while at grazing incidence (right) the σ- resonance is maximised.
depicts the Rydberg orbitals. These are generally located between the ionisation potential and the highest occupied molecular orbital, π∗. The Rydberg orbitals result in sharp resonances at energies slightly below theσ-resonances, but they are usually too weak to be observed [12].
Not only electronic information can be extracted from NEXAFS, but also spatial information about the molecules can be obtained. The fact that bonds and molecular orbitals are highly directional is exploited to determine the orientation of a molecule on a surface (see Figure 2.17). The transition intensities in a NEXAFS spectrum depend on the orientation of the electric field vectorE with respect to the orientation of the molecule [12]. For the case of linearly polarised light and for an initial1s state, the intensity of the transition is proportional to the cosine squared ofδ, whereδ is the angle betweenE and the direction of the final state orbital, O:
I ∝ |E·O|2 ∝cos2δ (2.19) From this equation, is easy to see that the intensity of a resonance is largest when the electric field vector is parallel to the direction of the final state molec- ular orbital and zero when they are perpendicular. It is important to note that theσ∗-orbitals have their maximum molecular orbital amplitude along the bonding direction and theπ∗-orbitals have the maximum amplitude in the direction perpen- dicular to the bond axis. Therefore, by measuring two or more different geometries, for example, grazing and normal incidence, it is possible to determine the spatial
orientation of a molecule on a surface.
One problem in the NEXAFS experiment is the possibility that a photoemis- sion peak appears in the window at the same energy as the Auger peaks, hiding any of the NEXAFS features. This can complicate the analysis of the NEXAFS spectra.