Xray Photoelectron Spectroscopy (XPS or ESCA)

Question 1

What is the basic principle of XPS?

Answer 1

XPS provides surface chemical state and quantitative compositional information for all elements from lithium to uranium.

Question 2

How does the XPS operate?

Answer 2

1. Sample irradiated with beam of low energy monochromatic X-Rays.
2. X-Ray photons interact with sample atoms to generate photoelectrons.
3. Photoelectrons generated from top 1-3 nm of surface detected, detected.

Question 3

What are the properties of XPS operation?

Answer 3

Photoelectrons: energies related to binding energies of atom and orbitals from which they originated.
Spectrum: Surface specific

Question 4

What are the instruments used in XPS?

Answer 4

1. Sources - X-ray tubes (Mg or Al)
2. Elaborate Vacuum Systems (10^-5 to 10^-10 torr)
3. Monochromators (improve S/N ratio, allow smaller spots on surface to be examined)
4. Sample holder
5. Analyzers
6. Curvature

Question 5

What are the properties of the sample holder?

Answer 5

1. Solid samples mounted close to source
2. Sample compartment evacuated (10^-5 to 10^-10 torr)
3. avoid contamination of sample surface by water or oxygen absorbed on surface

Question 6

What are properties of analyzers?

Answer 6

1. Hemispherical where electron beam deflected by an electrostatic magnetic field
2. Electrons travel in curved path

Question 7

What is a property of the curvature in XPS?

Answer 7

Radius is dependent on KE of electron and magnitude of field.

Question 8

Why UHV preferred for surface analysis?

Answer 8

1. Remove adsorbed gases
2. Eliminate adsorption of contaminants
3. Prevent arcing and high voltage breakdown
4. Increase mean free path for electrons, ions, photons

Question 9

What affects the peaks of the spectrum?

Answer 9

When one peak examined under conditions of higher energy resolution, position of maximum depends on chemical environment.

Question 10

What affects the binding energies, and what outcomes are observed?

Answer 10

1. variations in valence electrons
2. types of bonds
3. oxidation state

• as BE increases, oxidation state becomes more positive

Question 11

What is the concept of chemical shifts?

Answer 11

Attraction of the nucleus for a core electron is diminished by presence of outer electrons. When one electron is removed, effective charge sensed by core electron increased => BE increases