Photochemistry

Illustration of the electromagnetic spectrum. Note the visible spectrum, as well as ultraviolet and infrared regions.

Photochemistry, a sub-discipline of chemistry, is the study of the interactions between atoms, small molecules, and light (or electromagnetic radiation).

Like most scientific disciplines, photochemistry utilizes the SI or metric measurement system. Important units and constants that show up regularly include meters (and variants such as centimeter, millimeter, micrometer, nanometer, etc.), seconds, hertz, joules, moles, the gas constant R, and the Boltzman constant. These units and constants are also integral to the field of physical chemistry.

The first law of photochemistry, known as the Grotthuss-Draper law (for chemists C.J.T. de Grotthuss and John W. Draper), states that light must be absorbed by a chemical substance in order for a photochemical reaction to take place.

The second law of photochemistry, the Stark-Einstein law, states that for each photon of light absorbed by a chemical system, only one molecule is activated for a photochemical reaction. This is also known as the photoequivalence law and was derived by Albert Einstein at the time when the quantum (photon) theory of light was being developed.

Photochemistry may also be introduced to laymen as a reaction that proceeds with the absorption of light. Normally a reaction (not just a photochemical reaction) occurs when a molecule gains necessary energy activation energy to undergo change. A simple example can be the combustion of gasoline (a hydrocarbon) into carbon dioxide and water. This is a chemical reaction where one or more molecules/chemical species are converted into others. For this reaction to take place activation energy should be supplied. The activation energy is provided in the form of heat or a spark. In case of photochemical reactions light provides the activation energy.

The absorption of a photon of light by a reactant molecule may also permit a reaction to occur not just by bringing the molecule to the necessary activation energy, but also by changing the symmetry of the molecule's electronic configuration, enabling an otherwise inaccessible reaction path, as described by the Woodward-Hoffman selection rules. A 2+2 cycloaddition reaction is one example of a pericyclic reaction that can be analyzed using these rules or by the related frontier molecular orbital theory.

Some common terms that may help you learn more about photochemistry are:

Photons, absorption and emission (electronic transitions), the Franck-Condon Principle, energy levels, singlet and triplet states, internal conversion, intersystem crossing, sensitization, etc.

Photochemists use techniques such as:

• Quantum yield - finding what proportion of molecules react when a known flux of light is used to irradiate them.
• Flash photolysis - using a high intensity xenon flash lamp to initiate a reaction which can then be studied by techniques such as absorption spectroscopy.

Electromagnetic Radiation

Regions of the electromagnetic spectrum

The electromagnetic spectrum is broad, however, a photochemist will find themselves working with several key regions. Some of the most widely used sections of the electromagnetic spectrum include:

• Visible Light:

400–700 nm wavelengths

• Ultraviolet:

100–400 nm wavelengths

• Near Infrared:

700–1000 nm wavelengths

• Far Infrared:

1000 nm–1 cm wavelengths

Electric and magnetic fields and interactions with charged particles

This is a light wave frozen in time and shows the two components of light; an electric field and a magnetic field that oscillate perpendicular to each other and to the direction of motion (a transverse wave).

The electric and magnetic fields are perpendicular to the direction of travel and to each other. This picture depicts a very special case, linearly polarized light. See Polarization for a description of the general case and an explanation of linear polarization.

While these relations of the electric and magnetic fields are always true, the subtle difference in the general case is that the direction and amplitude of the magnetic (or electric) field can vary, in one place, with time, or, in one instant, can vary along the direction of propagation.

Wave and particle nature of light and matter

The modern theory that explains the nature of light is wave-particle duality, described by Albert Einstein in the early 1900s, based on his work on the photoelectric effect and Planck's results. Einstein determined that the energy of a photon is proportional to its frequency. More generally, the theory states that everything has both a particle nature and a wave nature, and various experiments can be done to bring out one or the other. The particle nature is more easily discerned if an object has a large mass, so it took until an experiment by Louis de Broglie in 1924 to realise that electrons also exhibited wave-particle duality. Einstein received the Nobel Prize in 1921 for his work with the wave-particle duality on photons, and de Broglie followed in 1929 for his extension to other particles.

Atoms and molecules also exhibit wave motion. Each contain various energy states: vibrational, translational, and rotational energies.

Photoelectric effect

A depiction of the photoelectric effect.

Photons of a beam of light have a characteristic energy given by the wavelength of the light. In the photoemission process, if an electron absorbs the energy of one photon and has more energy than the work function, it is ejected from the material. If the photon energy is too low, however, the electron is unable to escape the surface of the material. Increasing the intensity of the light beam does not change the energy of the constituent photons, only their number, and thus the energy of the emitted electrons does not depend on the intensity of the incoming light.

Electrons can absorb energy from photons when irradiated, but they follow an "all or nothing" principle. All of the energy from one photon must be absorbed and used to liberate one electron from atomic binding, or the energy is re-emitted. If the photon is absorbed, some of the energy is used to liberate it from the atom, and the rest contributes to the electron's kinetic (moving) energy as a free particle. For more information see photoelectric effect.

Compton effect

In quantum mechanics, Compton scattering or Compton effect is the increase in wavelength (decrease in energy) which occurs when X-ray (or gamma ray) photons interact with electrons in a material. The amount the wavelength increases by is called the Compton shift. Compton's experiment became the ultimate observation that convinced all physicists that light can behave as a stream of particles whose energy is proportional to the frequency.

Because the photons have such high energy, the interaction results in the electron being given enough energy to be completely ejected from its atom, and a photon containing the remaining energy being emitted in a different direction from the original, so that the overall momentum of the system is conserved. (If the photon still has enough energy, the process may be repeated.) Because of the overall reduction in energy of the photon, there is a corresponding increase in its wavelength. Thus overall there is a slight 'reddening' and scattering of the photons as they pass through the material. This scattering is known as Compton Scattering.

In a material where there are free electrons, this effect will occur at all photon energies and hence all wavelengths. In other materials, it is observed only with high-energy photons; photons of visible light, for example, do not have sufficient energy to eject the bound electrons.

The effect is important in scientific terms because it demonstrates that light cannot be explained purely as a wave phenomenon. Thomson scattering, the classical theory of charged particles scattered by an electromagnetic wave, cannot explain any shift in wavelength. Light must behave as if it consists of particles in order to explain the Compton scattering. For more information, see compton scattering

Optics

• Reflection
• Refraction (index of refraction n)
• Diffraction (ex. double slit diffraction)
• polarization

Scattering and polarizability

• Rayleigh scattering
• Mie Scattering
• Raman

• Stokes/anti-Stokes

• Absorption and emission line widths and broadening