Photoelectron spectroscopy Electromagnetic radiation has the properties of both a wave and a particle .The radiation is usually described in terms of its energy, E, or its wavelength, . The relation between these units is expressed in following equation: E=hv=hc/ where h is Plancks constant, v is the frequency of the wave, c is the speed of light in vacuum and is the wavelength of the light . Photons are particles of light and with the right energy they can interact with matter .If an atom or molecule absorbs a photon, the electronic structure of the atom or molecule will adjust itself to the added quantum of energy .This process can be used to explore the different energy levels of atoms or molecules, and one can obtain useful chemical information about different species. X-ray photoelectron spectroscopy (XPS) is the most widely used surface

analysis technique to provide both quantitative atomic concentration and

chemical state information of the detected elements .X-ray irradiation

of surfaces results in the emission of photoelectrons whose energies are

characteristic of the elements .The information depth is approximately

57 nm. Angle-resolved XPS offers non-destructive resolution of

structures within the XPS sampling depth, e .g .layer ordering, composition and thickness can be determined.

Moreover, XPS can be utilized for sputter depth profiling to characterize

thin films and multi-layer systems by quantifying matrix-level elements as a function of depth.

Principles of X-RAY Photoelectron Spectroscopy

Briefly, XPS is based on the principle that, when a surface is irradiated with X-rays, i will be ejected. If X-ray lines of sufficiently narrow widths are used, the photoelectrons have characteristic energies related directly to the atomic levels from which they came; XPS commonly uses either the AlK(1486.6 eV) or the MgK(1253.6 eV ) lines .With such low -

energy excitation, the photoelectrons must originate in the outer few monolayers only if they are to escape without energy loss .The resultant energetic electrons are then collected and counted, after dispersion, by an electrostatic analyzer .A photoelectron energy spectrum then consists of a plot of counts as a function of kinetic energy .Such spectra may be used

for analytical purposes or to gain insight into the chemical bonding of the elements present .The technique is fully quantitative insofar as the area under a characteristic peak can be related directly to the concentration of the corresponding atomic species in the surface layer .The sensitivity of XPS is of the order of 0.1 at% for most elements. Chemical information can be extracted from the spectrum by detailed considerations of the position (with a typical resolution of 0.1 eV) and shape of peak envelopes .This information can be interpreted readily by comparison with spectra of standard compounds, recorded in the same experiment,

or by consulting the voluminous and mature literature on XPS investigations of various compound.

The basic requirements for a photoemission experiment (XPS) are:

1. a source of fixed-energy radiation

2. an electron energy analyzer (which can disperse the emitted electrons according to their kinetic energy, and thereby measure the flux of emitted electrons of a particular energy)

3. a high vacuum environment (to enable the emitted photoelectrons to be analyzed without interference from gas phase collisions)

XPS can provide typical information such as:

determining the composition of a surface )elemental and chemical composition quantification.

mapping the spatial distribution of the surface constituents.

in-depth profiling these constituents into the bulk of the material.

determining an over-layer or thin film thickness

identifying particles

quantifying light element impurities

Limitations

All elements are detectable except for H and He

Sample has to be a solid at RT and stable under vacuum conditions, powders are possible

Depending on the chemical composition samples might be sensitive

to X-ray irradiation

Basic Instrument

n For each and every element, there will be a characteristic binding energy associated with each core atomic orbital i.e. each element will give rise to a characteristic set of peaks in the photoelectron spectrum at kinetic energies determined by the photon energy and the respective binding energies.

n The presence of peaks at particular energies therefore indicates the presence of a specific element in the sample under study - furthermore, the intensity of the peaks is related to the concentration of the element within the sampled region. Thus, the technique provides a quantitative analysis of the surface composition and is sometimes known by the alternative acronym , ESCA (Electron Spectroscopy for Chemical Analysis).

n The most commonly employed x-ray sources are those giving rise to :

Mg K radiation : hv = 1253.6 eV

Al K radiation : hv = 1486.6 eV

References

1- L .J .Sathre, T .D .Thomas, S .Svensson, J .Chem .Soc., Perkin Trans .2, 749, 1997

2- 15 C .D .Wagner, J .Electron Spectrosc .Relat .Phenom .47, 283 1988

3- J .C .Vickerman, Surface analysis the principal techniques, John Wiley & Sons 1997

4- D .Briggs and M .Seah, Practical surface analysis, John Wiley & Sons 1997 O .Svren, Electron Spectroscopy and Chemical Reactivity, University of Bergen, June. 1997.

5- T .Fujikawa, R .Suzuki, H .Arai, H .Shinotsuka, L .Kvr, J .Electron Spectrosc .Relat .Phenom., in press; T .Fujikawa, R .Suzuki, L .Kvr, ibid, 151,2006,170

6- L .Kvr, M .Novk, S .Egri, I .Cserny, Z .Bernyi, J .Tth, D .Varga, W .Drube, F .Yubero, S .Tougaard, W .S .M .Werner, Surf .Interface Anal .38,2006,569.