Question

0 Explain the use of photoelectron spectroscopy (PES) in determining the energies of molecular orbitals. 5 marks (i) Briefly describe the operating principles of a photoelectron spectrometer. Use a diagram to illustrate your answer. 5 marks (İİİ) The highest energy peak in the He(I) PES of 02 is located at 10 eV. From your knowledge of this experiment, what information can be obtained about the molecular orbitals of O2? 5 marks (iv) The PE spectra of N2 and CO are known to be quite similar. Provide a simple MO explanation for this behaviour. 5 marks

Answer 1

Dear student

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).

We can form molecular orbitals by looking at the electrons ejected from bonding orbitals show more vibrational energy than antibonding orbital .
Photoelectron spectroscopv KE(e) hv I Eion KE(e) Eion Measuring the "spectrum" of photoelectron energies provides a map of the quantised energy states of the molecular ion

et or e- Mt te (UV radiation) Mt te (X-rays) 2p Photoelectric Effect (M)

Ultraviolet Photoelectron Spectroscopy UV (hv) CO ejected valence electron (e-) Ek kinetic energy Eb binding energy

Photoelectron Spectroscopy: Principle Detachment continuum M+e M* hv 4 Configuration coordinate Detaching electron from the anionic ground state to access the neutral ground and excited states. This technique depicts the electronic structures of the neutral species.

PES of oxygen o Removal of electron from n orbital of alener coinfiquration leads to two possible electronic states πu 3πα2: three unpaired electrons give either 2TTu or 4II states o Breakdown of Koopman's theorem (no one-to-one correspondence between orbitals and PES bands)

PES wrt C-N Energy Vis C-N dist 1.658 1.656 1.654 1.652 1.65 1.648 .646 1.644 1.642 1.64 1.638 1.636 2.5 1.5 C-N dist [A)

За HOMO HOMO-1 2a HOMO-2 Hydrogen Atomic Orbitals Nitrogen Ammonia Atomic Orbitals Molecular Orbitals

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