Spectroscopy ultimately is based upon light. Historically, the first source of artificial light was probably fire and the
flame that accompanied it. Here, let us consider the spectroscopy of fire and flame.
Figure 1: A fire in a fireplace can be a cozy companion on a cold winter evening. This picture, however, only demonstrates
two of three aspects of a fire: the presence of a flame and the emission of light. Fire is also accompanied by the emission
of heat.
The impact of fire on human development cannot be underestimated. Fire provided light and heat to ward off cold, and lent
itself as a mechanism to cook food so that it could be eaten more easily. It could be used as a tool to help shape metals,
decompose ores, and boil water for steam that could be used to power turbines for electricity or cylinders for locomotion.
Fire can be devastating when uncontrolled, as in a forest or house fire, or emotionally satisfying, as in a nice fire in the
fireplace on a cold winter's night (my own personal favorite; Figure 1).
What is fire? What is flame? How do they apply to spectroscopy? To answer the last question first, there are some simple and
complex spectroscopic methods in which a flame has a central role.
Fire
Fire is a rapid oxidation–reduction (or, paradoxically, "redox") reaction that is characterized by the presence of a flame
and the emission of light and heat. Fire requires the presence of a fuel, which is oxidized, and an oxidizer, which is reduced.
When a simple match burns, the wood or heavy paper of the match is the fuel (along with the chemicals on the head of the match,
which initiate the combustion process), while the oxygen in the air is the oxidizer. If one were to try to make a fire on
the surface of the Saturnine moon Titan, one would issue a stream of oxidizer such as oxygen into the atmosphere and burn
it with the 1% or so of hydrocarbons in Titan's atmosphere (as discussed in Arthur C. Clarke's science fiction novel Imperial Earth). When sodium metal and chlorine gas deflagrate (which is a sort of subsonic combustion), sodium is the fuel and chlorine
is the oxidizer; the product is the salt sodium chloride.
Figure 2: Bunsen burner flames with different amounts of air premixed with the fuel. As the flames progress from 1 to 4, more
and more air is premixed with the fuel. Note that not only the color, but also the construction of the flame varies.
Fuels and oxidizers can be premixed, or a fire can depend upon diffusion of one or the other component. For example, a laboratory
Bunsen burner has an opening in the bottom to allow for entry of air, which will mix with the fuel before reaching the fire
at the top of the burner. If this opening is closed, the fire must depend upon diffusion of oxygen from the surrounding air
to sustain combustion. The characteristics of the fires are very different, as shown in Figure 2.
