Chapter 4
Photolysis of Free Molecules
Abstract The chapter deals with radiative electronic transitions (allowed and forbidden as electric dipole ones), intramolecular perturbations in electronically-excited
states of di- and polyatomic molecules. The definitions of primary and secondary
processes of gas-phase photolysis and quantum yields of various photoprocesses are
introduced in the beginning. Then radiative transitions in absorption and emission,
Franck-Condon principle for bound–bound and bound-free transitions are discussed.
Intramolecular perturbation in di- and polyatomic molecules, including electronic
predissociation, and dissociation, are analyzed in detail.
4.1 Primary and Secondary Processes of Gas-Phase
Photolysis. Quantum Yields
What are the primary and secondary processes of the photolysis of molecules in the
gas phase? However, these concepts are different in gas-phase photochemistry and
photochemistry of condensed systems. Besides, the terminology of this field of
science, at least in gas-phase photochemistry, does not settle yet. Definitions for
basic terms are either absent or not implicit. The situation may lead to terminological confusion and mistakes. The author gives some definitions that will be used
further.
Let us first consider the processes occurring during the photolysis of free
(isolated) molecules by photons with energy hm provided that the photon energy is
less than the molecule ionization potential. By free molecules, the author means
molecules in conditions allowing the free removal of their fragments without a
so-called ‘cage effect’ [1], i.e., the molecules in gas-phase at not-to-high pressures,
less than 10 bars.
AB þ hv ! AB
Ã
v; J
ð Þ ! A 1 þ B 1
ð4:1:1Þ
! A 2 þ B 2
ð4:1:2Þ
© Springer Nature Switzerland AG 2021
A. Pravilov, Gas-Phase Photoprocesses, Springer Series in Chemical Physics 123,
https://doi.org/10.1007/978-3-030-65570-9_4
79
Photolysis of Free Molecules
Abstract The chapter deals with radiative electronic transitions (allowed and forbidden as electric dipole ones), intramolecular perturbations in electronically-excited
states of di- and polyatomic molecules. The definitions of primary and secondary
processes of gas-phase photolysis and quantum yields of various photoprocesses are
introduced in the beginning. Then radiative transitions in absorption and emission,
Franck-Condon principle for bound–bound and bound-free transitions are discussed.
Intramolecular perturbation in di- and polyatomic molecules, including electronic
predissociation, and dissociation, are analyzed in detail.
4.1 Primary and Secondary Processes of Gas-Phase
Photolysis. Quantum Yields
What are the primary and secondary processes of the photolysis of molecules in the
gas phase? However, these concepts are different in gas-phase photochemistry and
photochemistry of condensed systems. Besides, the terminology of this field of
science, at least in gas-phase photochemistry, does not settle yet. Definitions for
basic terms are either absent or not implicit. The situation may lead to terminological confusion and mistakes. The author gives some definitions that will be used
further.
Let us first consider the processes occurring during the photolysis of free
(isolated) molecules by photons with energy hm provided that the photon energy is
less than the molecule ionization potential. By free molecules, the author means
molecules in conditions allowing the free removal of their fragments without a
so-called ‘cage effect’ [1], i.e., the molecules in gas-phase at not-to-high pressures,
less than 10 bars.
AB þ hv ! AB
Ã
v; J
ð Þ ! A 1 þ B 1
ð4:1:1Þ
! A 2 þ B 2
ð4:1:2Þ
© Springer Nature Switzerland AG 2021
A. Pravilov, Gas-Phase Photoprocesses, Springer Series in Chemical Physics 123,
https://doi.org/10.1007/978-3-030-65570-9_4
79