What does photoelectron spectroscopy do?

1 Answer
Jun 20, 2018

You mean, what do we do with it? Well, we inspect the electronic structures of atoms and molecules with it.

(Abbreviations used:
w.r.t. = with respect to
MO = molecular orbital.)

These are supplementary answers that should help:

Below I go into a MOLECULE example (not atom).


Photoelectron Spectroscopy is simply when one shines a laser (usually UV or X-ray) onto an atom or molecule to eject electrons from it (known as photoelectrons, i.e. those that leave due to photons of the correct minimum frequency).

http://upload.wikimedia.org/

Spectrums from these experiments are typically plotted as electron signal vs. electron kinetic energy (left to right) or ionization energy (right to left, sometimes left to right).

In general:

  • Higher-intensity peaks indicate higher-energy orbitals (because a greater fraction of the atoms/molecules get ionized from such an orbital), and vice versa.

This refers to the peak being tall.

  • Higher ionization energy means the orbital is deeper into the atom, because more energy was imparted to cause the ejection of the electron.

NOTE: that does not mean that it will be a tall peak! In fact, those are usually short.

  • Molecules get vibrational fine structure if the orbital has any bonding or antibonding character overall, although atoms do NOT.

An example MOLECULE is:

http://www.forgottenplanet.com/studyguide/chem210/

Now what we should consider is what kind of information we can get from it regarding the orbitals. Below is an analysis of the above spectrum.

BONDING/ANTIBONDING CHARACTER

This diagram illustrates an example of what the header means.

  • The MO from which the electron left has significant bonding character if the corresponding peak on the spectrum is broad.

#1pi)#

The #1pi# orbital is strongly bonding w.r.t. the #"O"-2p# and #"C"-2p# but has no other interaction with other atomic orbitals.

So, the #1pi# peak is broad.

#4sigma)#

The #4sigma# orbital is strongly bonding w.r.t the #"O"-2p# and #"C"-2p#, but strongly antibonding w.r.t. the #"O"-2s# and slightly bonding w.r.t to the #"C"-2s#.

Overall, this is in general mostly bonding, but not much, so it is not very broad.

  • It also usually results in some vibrational fine structure, i.e. you usually see multiple peaks that have split from one.

#1pi)#

Due to this strongly-bonding character, the #1pi# peak indeed has vibrational fine structure, and has split into about #12# visible peaks in one group.

#4sigma)#

Due to this somewhat-bonding character, the #4sigma# peak has a little bit of vibrational fine structure, and has split into only 3 major peaks in one group.

"NONBONDING" CHARACTER

This diagram illustrates an example of what the header means.

  • The MO from which the electron left has almost no bonding or antibonding character, or both at the same time, if minimal vibrational fine structure is seen.

#5sigma)#

The #5sigma# has bonding character w.r.t. the #"C"-2p# and #"O"-2p#, AND antibonding character w.r.t the #"C"-2s# and #"O"-2s#.

As a result, there is a phase cancellation (so this overall has neither bonding nor antibonding character), and it is a narrow peak instead of a broad one.

It is also the highest-energy MO, so it is the tallest peak.