Question

Which radiations stimulate which energy transitions? (e.g. electronic, vibrational, rotational)

Answer 1

Electromagnetic radiation is a form of energy that is produced by oscillating electric and magnetic disturbance. The electric and magnetic fields come at right angles to each other and combined wave moves perpendicular to both magnetic and electric oscillating fields with a constant velocity. In vacuum, electromagnetic radiation travels at the speed of light but when it travels through a medium other than vacuum its velocity is less than the speed of light. The behaviour of electromagnetic radiation is described by the properties of both waves and particles. It has certain characteristics like amplitude, wavelength, and frequency. The electromagnetic radiation is grouped into categories based on its wavelength or frequency into the electromagnetic spectrum. The different types of electromagnetic radiation in the electromagnetic spectrum consists of radio waves, microwaves, infrared waves, visible light, ultraviolet radiation, X-rays and gamma rays.

The absorption of a photon by a chemical species results in electromagnetic energy being transferred to the chemical species. But for a given molecular species the energy is the sum of the electronic, vibrational, and rotational energies. However, an atomic species cannot undergo vibrational or rotational excitations since there are no chemical bonds, thus only electronic energy levels. The energy transitions by the absorption or emission of a photon by a chemical or atomic species is strictly based on particular wavelength of the electromagnetic radiation. A particular type of transition occurs when the energy difference between the energy levels exactly match the wavelength of the particular type of electromagnetic radiation (ie the energy of the photon). A molecule or a set of molecules characterised by the absorption of microwave radiation leads to transitions between rotational energy levels. If the molecules absorbs infrared radiation it facilitates the transitions between vibrational levels followed by transitions between rotational energy levels. Similarly when molecules absorbs visible and ultraviolet radiation transitions between electronic energy levels follows by simultaneous transitions between vibrational and rotational levels.