Interaction between Radiation and Matter

Recap from Elements of Life

There are lots of different types of electromagnetic radiation. See the electromagnetic spectrum below.

As the frequency of light increases, its energy also increases. This can be seen in the formula below:
Energy = Planck’s Constant x Frequency of light
Planck’s constant is equal to 6.63 x 10^-34J Hz^-1. The higher the frequency of radiation, the higher its energy.

Electromagnetic radiation can interact with matter, transferring energy to the chemicals involved, making things happen.
The changes that occur depend upon the chemical involved in the reaction, and the energy involved.
A molecule has energy associated with four different aspects of its behaviour:
- Translation- the molecule moving around as a whole.
- Rotation- the rotation of the molecule as a whole.
- Vibration- the vibration of the bonds.
- Electronic- electrons moving from one energy level to another.
These different energy activities require different amounts of energy. The amount of energy required increases down the list.

When the atoms are given energy, their electrons get promoted to a higher energy level. The energy required for the promotion of an electron is specific, it is quantised.
All other energy is also quantised, i.e. it requires a certain amount of energy to vibrate an HCl molecule, the HCl molecule can have different vibrational energies.
When its vibrational energy changes, it moves to a new fixed energy level, in a similar way to how electrons are promoted.
This corresponds to a photon of infrared radiation; we sense radiation energy as heat. The radiation makes the bonds in your skin vibrate more rapidly, and this is why you feel warmer.
To rotate molecules require less energy than needed to vibrate them. Therefore the rotational energy corresponds to a frequency of light lower than that of infrared radiation, mainly the microwave region.
The spacing between translation energy levels is even smaller; we treat translational energy as being continuous.
To make electron changes within the molecule requires a higher energy than is needed to make vibrational changes. Electronic changes involve making electrons shift from a lower energy level to a higher one. Electronic energy changes require energy from the visible and ultraviolet part of the spectrum.

The particular value of energy required depends upon the chemical involved. Different substances have different structures, and this gives them different energy levels.
A C-F bond is stronger than a C-Br bond, so more energy is needed to make it vibrate.

Electrons in atoms occupy definite energy levels. Electrons in molecules similarly occupy definite energy levels.
When the molecule absorbs visible or ultraviolet light one of three things can happen:
1. Electrons become excited so they move up from a lower energy level to a higher one.
2. If higher energy radiation is used, the two atoms can break apart, they dissociate. Radicals are produced:
  Cl₂ Cl + Cl
3. If even higher energy radiation is used, the molecule may lose an electron; it can ionise:
  Cl₂ Cl₂⁺ + e^-