© Ultrafast Systems LLC /Vernier Software & Technology
25
6.3
The Basics of Photochemistry
Light, as is true for all types of electromagnetic radiation, is a form of energy. A beam of light can
be thought of as a train of energetic particles that we call photons. The light to which our eyes are
sensitive is in the spectral range of about 350 nm (violet) to 800 nm (red). The amount of energy
contained by a photon is in inverse proportion to the wavelength. The energy content of the shorter
wavelengths (200–500 nm) is sufficient to break chemical bonds and even the longer wavelengths can
have chemical consequences.
Useful relationships are provided in the following equations, where
E
is the photon energy:
1
( )
( ) ( )
E J
h Js
s
Equation
14
1
( )
( ) (
)/ ( )
E J
h Js c ms
m
Equation
15
When a photon comes within close approach of a molecule, if its energy (see Equation 14) is
equivalent to that of another state of the molecule, the whole of the photon energy can be given to the
molecule with annihilation of the photon. We say that the molecule has absorbed the light, meaning the
energy of the photon has become transferred to the molecule. In doing so, the molecule changes its
nature—its electronic configuration is altered from that in the ground state-an
excited
electronic state
has
been generated. This electronic change is usually accompanied by changes to the motions of the nuclei,
resulting in vibrational (and torsional) excitations. Since it is the electronic configuration of a molecule that
governs its chemical properties, excited electronic states show different chemical properties from their
parent ground states. It is the study of the chemical nature of molecules in excited states that is the
essence of p
hotochemistry
.
Molecules that have been promoted to excited states by absorption of photons are intrinsically
unstable. They seek to reduce their energy content and reach a lower energy condition. One of the ways
to do this is to lose the energy in a radiative process such as fluorescence or phosphorescence. Often,
these emissions can be observed, and in such cases, the observation provides us with a means to
measure the rates of decay of the excited species. Another way to reduce energy is by chemical
transformations in which chemical bonds are broken or formed. These new species may also be unstable
and react further to produce another generation of products. The intermediates in the sequence and the
eventual final products are likely to absorb light themselves in spectral regions that are different from the
parent compound. We can follow the process of the chemical sequence by the use of spectrophotometric
methods. Since the life times of the intermediates are often very short (ms, µs, ns, and less) and their
reactions are fast, it is necessary for the methods to be capable of high time resolution. The experiments
with the Vernier Flash Photolysis Spectrometer are designed to demonstrate the use of time-resolved
techniques in a straightforward and cost-effective way.
