# What does photoelectron spectroscopy do?

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).

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:

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 $1 \pi$ orbital is strongly bonding w.r.t. the $\text{O} - 2 p$ and $\text{C} - 2 p$ but has no other interaction with other atomic orbitals.

So, the $1 \pi$ peak is broad.

4sigma)

The $4 \sigma$ orbital is strongly bonding w.r.t the $\text{O} - 2 p$ and $\text{C} - 2 p$, but strongly antibonding w.r.t. the $\text{O} - 2 s$ and slightly bonding w.r.t to the $\text{C} - 2 s$.

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 $1 \pi$ 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 $4 \sigma$ 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 $5 \sigma$ has bonding character w.r.t. the $\text{C} - 2 p$ and $\text{O} - 2 p$, AND antibonding character w.r.t the $\text{C} - 2 s$ and $\text{O} - 2 s$.

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.