Femtochemistry is the branch of chemistry in which chemical reactions are detected as they occur and the reaction times are measured. Femtochemistry gets its name from the fact that these times are measured in femtoseconds. One femtosecond is one-millionth of one-billionth of a second. This number is written out as a decimal point followed by 14 zeros and a 1. Scientists believe that no chemical reaction can take place more rapidly than this.
There are many kinds of chemical reactions. For example, a molecule may break apart, or the atoms in the molecule may change their positions. In a reaction involving two molecules, one or more atoms may move from one molecule to the other, creating a new molecule. But all reactions involve changes in chemical bonds (attractions between the atoms that make up the molecules). In some reactions, bonds break, while in others, they form. In still other reactions—as when an atom jumps from one molecule to another—bonds both break and form. Furthermore, many reactions occur in stages. For instance, when a complex molecule breaks apart, bonds may break one at a time.
In a simple femtochemistry experiment, a laser shoots two extremely short pulses of laser light, one after the other, into a group of molecules. The pulses are precisely timed so that the second pulse reaches the molecules a certain number of femtoseconds after the first pulse.
The first pulse, known as the pump pulse, causes a chemical reaction—or a stage of a reaction—to occur in the molecules. The pulse acts by breaking bonds. Light is a form of energy, and if a molecule absorbs a certain amount of energy, a bond will break.
The second pulse, called the probe pulse, also puts energy into the molecules. The molecules then react by emitting (sending out) light energy. The intensity and color of the emission depend on the structure of the molecules when the probe pulse arrived. Light sensors measure the emissions, and a computer analyzes the results.
Ahmed H. Zewail, an Egyptian-born American chemist, began to develop the fundamental techniques of femtochemistry in the mid-1980’s. Zewail won the 1999 Nobel Prize for chemistry for his pioneering work in femtochemistry.
