The Quantum Nature of Matter and Light

Photochemistry is the study of the chemical reactions and physical changes that result from interactions between matter and visible or ultraviolet light. 

The principal aim of this introduction is to familiarise the reader with basic ideas relating to light and matter and the interaction between them. Quantum mechanics underpins an understanding of the nature of both light and matter, but a rigorous treatment of quantum theory involves complex mathematical analysis. In order to make the ideas of quantum mechanics available to a wider readership, conceptually simple models are presented. 

The development of the quantum theory in the early twentieth century allowed predictions to be made relating to the properties and behaviour of matter and light. The electrons in matter have both wavelike and particle-like properties, and quantum theory shows that the energy of matter is quantised that is, only certain specific energies are allowed. 

The quantised energy levels of matter have a separation that is of the same order as the energy of visible or ultraviolet light. Thus the absorption of visible or ultraviolet light by matter can excite electrons to higher energy levels, producing electronically-excited species. 

Lowest energy electronic configuration (Electronic ground state) 

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