A chemical reaction in which two molecular beams are crossed; one beam is regarded as the incident beam of gas and the other as the target gas. This technique enables a great deal of information to be gained about the chemical reaction since the states of both the target and projectile molecules can be controlled. The incident beam is characterized by its incident beam flux, I, which is the number of particles per unit area per unit time. The scattered molecules can be detected first by ionizing them and then detecting the ions electrically or using spectroscopy if changes in the vibrational or rotational states of molecules in a reaction are of interest.