Transitions Associated with Radiation (College Board AP® Chemistry)

Transitions Associated with Microwave Radiation

• Molecules and atoms can absorb light particles (photons)

• When they absorb light, the molecule or atom is promoted to an excited state

• Infrared photons have low energy

• When photons of microwave radiation are absorbed the rotational behavior of a molecule is stimulated

  • Therefore, the molecule is promoted to an excited rotational state

• The different rotational axis of a molecule can be analyzed by thinking atoms as points in a 3-dimensional system of coordinates

• A rotation will change the orientation of the molecule

  • The orientation will depend on the frequency of the photon with microwave radiation that is absorbed

Rotational axis in a water molecule

Different rotational axis in a water molecule

• One of the most used examples regarding excited rotational states is the use of the microwave oven

  • The microwave oven transfer rotational energy to the water molecules in your food

Transitions Associated with Infrared Radiation

• Molecules and atoms can absorb low energy infrared photons

• When they absorb a photon, they are promoted to an excited state

• When photons of infrared radiation are absorbed, the vibrational behavior of a molecule is stimulated

  • Therefore, the molecule is promoted to an excited vibrational state

• The modes of vibration of molecules can be analyzed by understanding the covalent bonds as springs

• In the same ways as springs, there are multiple ways in which a molecule can vibrate

  • This will depend on the frequency of the photon with infrared radiation that is absorbed

Modes of vibration for the CO₂ molecule

Different modes of vibration in a CO₂ molecule. Each mode has a characteristic frequency of vibration

•  The analytical technique that exploits the changes in vibrations of atoms to identify compounds is called infrared spectroscopy 

Transitions Associated with Ultraviolet/Visible Radiation

• Molecules and atoms can absorb photons in the visible and the ultraviolet region of the spectrum

• When they absorb a photon, they are promoted to an excited state

• The energy of visible and ultraviolet photons is high enough to stimulate electronic transitions

  • Therefore, the electrons are promoted from low energy orbitals to high energy orbitals

  • When electrons return to the ground state, energy is emitted

Absorption and Emission diagram

The difference between absorption and emission depends on whether electrons are jumping from lower to higher energy levels or the other way around

• When an electronic transition occurs, it is usually followed by changes in the rotational and vibrational states in a molecule

• The image below shows a summary that connects the specific regions of the electromagnetic spectrum and the types of molecular and electronic transitions associated with them

  • Photons with higher energy are needed to reach electronic excited states.

  • On the other hand, lower energy photons (infrared and microwave) can be absorbed to reach vibrational and rotational excited states

Summary of the molecular and electronic transitions

Electronic, vibration and rotational excited states associated with an specific region of the electromagnetic spectrum