PHOTOELECTRON SPECTROSCOPY

(PES)

This is new material for the AP test this year It is not in your book It is not in most books

It is a good bet there will be a couple of multiple choice questions on this material

Ionization energy The energy required to remove an electron. It is easiest to remove electrons from the

valence shell. As we move closer, it becomes more difficult.

However, removing electrons makes the atom more positive. Therefore that impacts the data.

What would the ionization energy be if we didn’t first remove valence electrons?

Ionizing vs. Nonionizing Radiation Ultraviolet, x-rays, and gamma is ionizing. Generally, anything of lower energy is

nonionizing High energy photons are absorbed by atoms. Normally this excites the atom, gets the electrons

to jump up. If they jump high enough, they will reach escape

velocity, they will be going so fast the electromagnetic pull from the nucleus won’t be enough to pull them back.

Photoelecton Spectroscopy PES begins by shooting at atom with a high

energy photon (normally UV or x-ray) that is absorbed by the atom.

An electron is ejected carrying off the excess energy.

The ejected electron is measured in a detector. The energy of the photon absorbed (h), will be

equal to the ionization energy (IE) + kinetic energy (KE) of the electron.

Therefore IE = h - KE

PES

Data from PES We find that all electrons in a given shell

require the same energy to remove them. That is to say all electrons in the 1s require

the same energy to remove, and all in the 2s, and 2p are the same, however, the energy to remove the 2p is different than the 1 s.

This is different from the previous determinations of ionization energy, with IE1, IE2, IE3 etc.

Plotting the data The data from PES experiments is

plotted as peeks. The height of the peeks is proportional

to the number of electrons of equivalent energy ejected during the experiment.

In other words, the height is related to the number of electrons in that energy level.

Peeks

1s

1s

2s