2
1 Introduction to Photochemistry
chemical reactions occur. If instead a fraction of the impinging radiation is absorbed
by the sample, chemical change can result and the amount of chemically transformed substrate will be proportional to the absorbed radiation. How much radiation
is adsorbed depends on experimental conditions such as the irradiation time, the
radiation intensity, and the concentration of the absorbing compound. As we shall
see, pure proportionality is observed only for simple reactions or when the outcome
of an elementary photochemical event can be singled out in a more complex mechanism. The exact meaning of the proportionality between absorbed radiation and
photochemical change requires the definition of some basic physical quantities, as
we shall see in the next sections.
During the nineteenth century the concept of molecule was progressively precised
and at the beginning of the next century the quantum of light, later called photon, was
introduced. This opened the way to a new formulation of the relationship between
light absorption and photochemical reaction. Stark and Einstein suggested, in close
analogy with the photoelectric effect, that an isolated molecule (in gas phase) can
absorb one photon and then undergo an elementary photoreaction act. Actually,
after photon absorption, a molecule can react in different ways or else undergo
processes that do not involve chemical transformations and are therefore labeled
as “photophysical.” A given photochemical or photophysical elementary event will
follow photon absorption with a predictable probability (the “quantum yield,” see
Sect. 1.5), which is in the first place determined by quantum dynamics. Exciting a
molecule is then very close to “playing dice,” quite at variance with the good habits
of Einstein’s God. We can therefore reformulate the Stark and Einstein law as: “every
photochemical or photophysical process is triggered by the absorption of one photon
by one molecule.” Much later, with the introduction of lasers, it was demonstrated
that two or more photons can be absorbed at once by one molecule when the radiation
intensity is high enough. However, in usual conditions with solar light or laboratory
lamps, the Stark–Einstein law remains valid.
1.2 Light and Photons
The previous section makes clear that we need to characterize and quantify the
“amount of light” that is absorbed by molecules in a medium. In this book we shall
use both the classical description of radiation in terms of oscillating electric and
magnetic fields, and the quantum one, based on the concept of photon, depending
on which allows for the simpler explanation of photophysical phenomena. For a
comprehensive treatment of this subject see, for instance, Cohen-Tannoudji et al [6].
1 Introduction to Photochemistry
chemical reactions occur. If instead a fraction of the impinging radiation is absorbed
by the sample, chemical change can result and the amount of chemically transformed substrate will be proportional to the absorbed radiation. How much radiation
is adsorbed depends on experimental conditions such as the irradiation time, the
radiation intensity, and the concentration of the absorbing compound. As we shall
see, pure proportionality is observed only for simple reactions or when the outcome
of an elementary photochemical event can be singled out in a more complex mechanism. The exact meaning of the proportionality between absorbed radiation and
photochemical change requires the definition of some basic physical quantities, as
we shall see in the next sections.
During the nineteenth century the concept of molecule was progressively precised
and at the beginning of the next century the quantum of light, later called photon, was
introduced. This opened the way to a new formulation of the relationship between
light absorption and photochemical reaction. Stark and Einstein suggested, in close
analogy with the photoelectric effect, that an isolated molecule (in gas phase) can
absorb one photon and then undergo an elementary photoreaction act. Actually,
after photon absorption, a molecule can react in different ways or else undergo
processes that do not involve chemical transformations and are therefore labeled
as “photophysical.” A given photochemical or photophysical elementary event will
follow photon absorption with a predictable probability (the “quantum yield,” see
Sect. 1.5), which is in the first place determined by quantum dynamics. Exciting a
molecule is then very close to “playing dice,” quite at variance with the good habits
of Einstein’s God. We can therefore reformulate the Stark and Einstein law as: “every
photochemical or photophysical process is triggered by the absorption of one photon
by one molecule.” Much later, with the introduction of lasers, it was demonstrated
that two or more photons can be absorbed at once by one molecule when the radiation
intensity is high enough. However, in usual conditions with solar light or laboratory
lamps, the Stark–Einstein law remains valid.
1.2 Light and Photons
The previous section makes clear that we need to characterize and quantify the
“amount of light” that is absorbed by molecules in a medium. In this book we shall
use both the classical description of radiation in terms of oscillating electric and
magnetic fields, and the quantum one, based on the concept of photon, depending
on which allows for the simpler explanation of photophysical phenomena. For a
comprehensive treatment of this subject see, for instance, Cohen-Tannoudji et al [6].