Chapter 1
Introduction to Photochemistry
Abstract This chapter summarizes some general concepts in photochemistry, with
two aims: to provide an overview of phenomena and empirical rules that will be discussed on theoretical grounds in the next chapters, and to present a language and some
physical laws concerning light and its interaction with matter. We shall introduce the
main differences between thermal chemistry that takes place in the ground electronic state, and photochemistry, that involves optical excitation. The first overview
of elementary photoinduced events will highlight the different ways the excitation
energy can be disposed of. We shall distinguish primary and secondary processes
and define their quantum yields. Excitation rates will be introduced in connection
with the Lambert and Beer law. Some elementary examples of photochemical kinetics will be discussed. Much more extended introductions to photochemistry can be
found in well-renowned textbooks by Wayne (Principles and Applications of Photochemistry. Oxford University Press, Oxford, 1988 [1]), Wardle (Principles and Applications of Photochemistry. Wiley, Chichester, 2009 [2]), Turro (Modern Molecular
Photochemistry of Organic Molecules. University Science Books, Sausalito, 2010
[3]), Balzani (Photochemistry and Photophysics: Concepts Research, Applications.
Wiley, Chichester, 2014 [4]), Rohatgi-Muckerjee (Fundamentals of Photochemistry.
New Age International, New Delhi, 2017 [5]), and others.
Keywords Light · Photon · Primary and Secondary processes · Quantum yeild
Photochemical kinetics
1.1 What Is Photochemistry?
Photochemistry deals with the chemical reactions and other physicochemical
phenomena induced by the absorption of light. This definition stems from a basic
rule formulated by Grotthuss in 1917 and independently by Draper in 1842. The
Grotthuss–Draper law states that only the light absorbed by a material sample can
induce chemical transformations. In fact, if the sample is transparent light goes
through it without being attenuated, i.e., no light is absorbed (see Sect. 1.6.1). Then,
the sample can be temporarily affected; for instance, it can experience periodic oscillations of its charge distribution in response to the electric field of light, but no
© Springer International Publishing AG, part of Springer Nature 2018
M. Persico and G. Granucci, Photochemistry, Theoretical Chemistry
and Computational Modelling, https://doi.org/10.1007/978-3-319-89972-5_1
1
Introduction to Photochemistry
Abstract This chapter summarizes some general concepts in photochemistry, with
two aims: to provide an overview of phenomena and empirical rules that will be discussed on theoretical grounds in the next chapters, and to present a language and some
physical laws concerning light and its interaction with matter. We shall introduce the
main differences between thermal chemistry that takes place in the ground electronic state, and photochemistry, that involves optical excitation. The first overview
of elementary photoinduced events will highlight the different ways the excitation
energy can be disposed of. We shall distinguish primary and secondary processes
and define their quantum yields. Excitation rates will be introduced in connection
with the Lambert and Beer law. Some elementary examples of photochemical kinetics will be discussed. Much more extended introductions to photochemistry can be
found in well-renowned textbooks by Wayne (Principles and Applications of Photochemistry. Oxford University Press, Oxford, 1988 [1]), Wardle (Principles and Applications of Photochemistry. Wiley, Chichester, 2009 [2]), Turro (Modern Molecular
Photochemistry of Organic Molecules. University Science Books, Sausalito, 2010
[3]), Balzani (Photochemistry and Photophysics: Concepts Research, Applications.
Wiley, Chichester, 2014 [4]), Rohatgi-Muckerjee (Fundamentals of Photochemistry.
New Age International, New Delhi, 2017 [5]), and others.
Keywords Light · Photon · Primary and Secondary processes · Quantum yeild
Photochemical kinetics
1.1 What Is Photochemistry?
Photochemistry deals with the chemical reactions and other physicochemical
phenomena induced by the absorption of light. This definition stems from a basic
rule formulated by Grotthuss in 1917 and independently by Draper in 1842. The
Grotthuss–Draper law states that only the light absorbed by a material sample can
induce chemical transformations. In fact, if the sample is transparent light goes
through it without being attenuated, i.e., no light is absorbed (see Sect. 1.6.1). Then,
the sample can be temporarily affected; for instance, it can experience periodic oscillations of its charge distribution in response to the electric field of light, but no
© Springer International Publishing AG, part of Springer Nature 2018
M. Persico and G. Granucci, Photochemistry, Theoretical Chemistry
and Computational Modelling, https://doi.org/10.1007/978-3-319-89972-5_1
1