surface spectroscopy

1. INVERSE PHOTOEMISSION
SPECTROSCOPY (IPES)
By:
ABUBAKAR YAKUBU (PhD)
Universiti Putra Malaysia

2. OUTLINE
• INTRODUCTION
• WORKING PRINCIPLES
• EXPERIMANTAL SETUP
• RESULT AND DISCUSSION
• CONCLUSSION

3. INTRODUCTION
TERMS RELATED TO IPES
Photoemission: This is the emission of
photoelectrons (especially from a metallic surface).
Emission mechanism photoelectric effects
Photoemission spectroscopy (PES): This refers to
energy measurement of electrons emitted from
solid, liquid or gases by photoelectric effects.

4. Physical principles(PES):
1. The binding energy of the
electrons in the material is
obtained.
2.The technique depend on
ionization energy by X-ray
photons.

5. Types PES
1.Angle-resolve photoemission spectroscopy
(ARPES).
2.Laser-based angle-resolved photoemission
spectroscopy (LBARPES).
3.Inverse Photoemission Spectroscopy (IPES):
This is the modem appellation of experiment
which analyze the photons produced by
electrons hitting a solid samples. It is also the
surface science to study the unoccupied
electronic surf.

6. WORKING PRINCIPLES
A well-collimated beam of electrons of defined
energy <20eV is directed at the sample.
The electrons couple to high-lying unoccupied
electronic state and decay to low-lying unoccupied
states.
The photons emitted in decay process are detected
and an energy spectrum, photon count Vs. incident
electron energy is generate
Due to the low energy of the incident electrons,
their penetration depth is only a few atomic layers.
Making inverse photoemission a particularly surface
sensitive technique.

7. THEORY
The energy of photons (hv) emitted
when electrons incident on a
substance using electron beam Ei
relax to a lower energy unoccupied
state Ef is giving below.
Ei= Ef + hv
Which is the conservation of energy.

8. TWO MODES FOR IPES
ISOCHROMAT: in this the incident electron energy
is scanned and keeps the detected photon energy
constant
SPECTROGRAPH: in this way the incident electron
energy is kept constant and the distribution of the
detected photon is measured. A diffraction grating
is used to dispersed the emitted photons that are in
turn detected with a two dimensional position.
Spectrograph has advantages of acquiring IPES
spectra over wide range of photon energies
simultaneously.

9. The setup shows a schematic overview of IPES containing the followings;
1. Photon detector: works in proportional region, negligible dead time,
operates at 730V, 4mbar acetone pressure to measure the emitted
photons.
2. Electron gun: focus at 25mm and spot size 1.4mm diameter, with energy
range of 5 – 40ev
General setup of IPES

10. Inverse
photoemission
(IPES)
UPS give information
about occupied states
IPES gives information
about unoccupied
states.
e- hν
e-hν

11. EXAMPLES
Example 1 : Time dependence of IPES spectra showing the radiation
damage of copper phthalocyanine samples. No significant spectral
changes are observed even after the 14 hours in this new method,
whereas apparent spectral changes due to the sample deterioration
are observed only after 10 min in the previous method.

12. Example 2 : shows the x dependence of IPES for FeSe1−x Tex measured with
an incident electron energy of 40 eV. The normalization was performed at
10 eV above∼ EF. The spectrum of FeSe comprises two features around 1.5
and 6 eV, which were denoted as α and β. Similarly for the remaining
samples applies .

13. Effects of temperature on energy of photon is
shown on the plot below
2 mm thick SrF2 crystal

14. Conclusion
Inverse photoemission spectroscopy has been
implemented as a method of studying the
unoccupied states of materials. IPES measures
the energy range between the Fermi energy EF
and the vacuum level Evac
Further applications of this setup are promising.
The IPES technique can be applied to many
materials and interfaces. Interfaces between
different materials can be analyzed as well and
the band offset can be determined by giving
information about the electron and hole
injection barriers.

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