Question

What does photoelectron spectroscopy do?

Answer 1

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:

• How do we know where the bonding, antibonding, and nonbonding molecular orbitals are compared to the atomic orbitals the atoms start with? Do they always interact?
• Draw the MO diagrams of `"O"_2`, `"O"_2^(2-)`, and `"O"_2^(2+)`?

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

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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.