INVERSE PHOTOEMISSION SPECTROSCOPY (IPES)

By:

ABUBAKAR YAKUBU (PhD)

Universiti Putra Malaysia

OUTLINE

• INTRODUCTION

•WORKING PRINCIPLES

• EXPERIMANTAL SETUP

• RESULT AND DISCUSSION

• CONCLUSSION

INTRODUCTION

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

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.

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.

WORKING PRINCIPLESA 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.

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.

TWO MODES FOR IPESISOCHROMAT: 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.

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

Inverse photoemission (IPES)

UPS give information about occupied states

IPES gives information about unoccupied states.

e- hν

e-hν

EXAMPLESExample 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.

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 .

Effects of temperature on energy of photon is shown on the plot below

2 mm thick SrF2 crystal

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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