Photoelectron Spectroscopy

advertisement
Are you interested in up to 10 test bonus points?
Go to either web site (or both) and write a minimum of a 5
question worksheet that applies to either or both of these
websites. Make sure to include all pertinent information and
email to [email protected] by Friday 10-31 before 5:00 pm.
Links to Photoelectric Effect simulators
http://phet.colorado.edu/en/simulation/photoelectric
http://www.chem.arizona.edu/chemt/Flash/photoelectron.
html
Photoelectron Spectroscopy
(PES)
Unit 6 AP CHEM
What is PES?
´ü▒Photoelectron spectroscopy (PES) is a technique used for determining the ionization
potentials of molecules. Underneath the banner of PES are two separate techniques for
quantitative and qualitative measurements. They are ultraviolet photoeclectron
spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). UPS focuses on
ionization of valence electrons while XPS is able to go a step further and ionize core
electrons and pry them away.
•
When a sample surface is irradiated with photons of energy
hν, electrons are emitted from the sample surface. The figure
below shows the essence of this photoemission process.
This process was first reported by H.Hertz in
1887. The excitation source was a UV lamp.
Nowadays, the excitation source can be X-ray
or synchrotron radiation.
The Photoelectric Effect
• Albert Einstein considered electromagnetic energy to be
bundled into little packets called photons.
•Energy of photon = E = hv
• Where, h = Planck constant ( 6.626 x 10-34 J s )
• v = frequency (Hz) of the radiation
•Photons of light hit surface electrons and transfer their energy
hv = B.E. + K.E.
•The energized electrons overcome their attraction (to the
nucleus) and escape from the surface (binding energy)
• Photoelectron spectroscopy detects the kinetic
energy of the electron escaped from the surface.
PES and AP Chemistry
• Our Studies will be mainly to look at data (spectrums) and try
to explain the findings.
The photoelectron spectra above show the
• For example:
energy required to remove a 1s electron
from a nitrogen atom and from an oxygen
atom. Which of the following statements
best accounts for the peak in the upper
spectrum being to the right of the peak in
the lower spectrum?
(A) Nitrogen atoms have a half-filled p
subshell.
(B) There are more electron-electron
repulsions in oxygen atoms than in
nitrogen atoms.
(C) Electrons in the p subshell of oxygen
atoms provide more shielding than
electrons in the p subshell of nitrogen
atoms.
(D) Nitrogen atoms have a smaller nuclear
charge than oxygen atoms.
More about PES
The diagram opposite shows an energy level diagram
for sodium with approximate binding energies for
the core levels.
If we are using Mg Kα ( hν = 1253.6 eV ) radiation
...
... at what kinetic energy will the Na 1s
photoelectron peak be observed ? (the 1s peak is
that resulting from photoionization of the 1s level)
1253.6 eV – 1072 eV = 181.6 eV
... at what kinetic energy will the Na 2s and 2p
photoelectron peaks be observed ?
1253.6 eV – 64 eV = 1189.6 eV
1253.6 eV – 31 eV = 1222.6 eV
Between 1190 and 1223 eV.
Finally
• The physics behind the PES technique is an application of the photoelectric
effect. For solids, photoelectrons can escape only from a depth on the order of
nanometers, so that it is the surface layer which is analyzed.
• Because of the high frequency of the light, and the substantial charge and
energy of emitted electrons, photoemission is one of the most sensitive and
accurate techniques for measuring the energies and shapes of electronic states
and molecular and atomic orbitals. Photoemission is also among the most
sensitive methods of detecting substances in trace concentrations, provided
the sample is compatible with ultra-high vacuum and the analyte can be
distinguished from background.
• Typical PES (UPS) instruments use helium gas sources of UV light, with photon
energy up to 52 eV (corresponding to wavelength 23.7 nm). The
photoelectrons that actually escaped into the vacuum are collected which
results in a spectrum of electron intensity as a function of the measured
kinetic energy. Because binding energy values are more readily applied and
understood, the kinetic energy values, which are source dependent, are
converted into binding energy values, which are source independent. This is
achieved by applying Einstein's relation.
Explain the data collected for the
1s electron in carbon, oxygen,
nitrogen and fluorine.
Explain the location of the peaks
for sulfur and silicon for the 2p
electron.
What would you explain about this spectrum?
Photoelectron Spectroscopy Conclusions
• It uses quantum mechanics to interpret spectroscopic data and extract
information on atomic structure from such data. In particular, lowresolution PES of atoms provides direct evidence for the shell model.
• Light consists of photons, each of which has energy E = hv, where h is
Planck’s constant and v is the frequency of the light. In the
photoelectric effect, incident light ejects electrons from a material.
This requires the photon to have sufficient energy to eject the
electron.
• PES determines the energy needed to eject electrons from the
material. Measurement of these energies provides a method to
deduce the shell structure of an atom. The intensity of the
photoelectron signal at a given energy is a measure of the number of
electrons in that energy level.
• The electronic structure of atoms with multiple electrons can be
inferred from evidence provided by PES. For instance, both electrons
in He are identical, and they are both roughly the same distance from
the nucleus as in H, while there are two shells of electrons in Li, and
the outermost electron is further from the nucleus than in H.
Download