Freezing Atoms in Motion: Principles of Femtochemistry and Demonstration by Laser Stroboscopy

The motion of atoms in molecules and in the course of reactions occurs on infinitesimal length and time scales, down to the angstrom (0.1 nm or 10^-8 cm) length and femtosecond (10^-15 s) time scales. To observe the behavior of atoms in motion, which is fundamental to chemical and biological transformations of molecular structures, femtosecond "stop-motion photography" with lasers is the methodology employed. This contribution highlights the basic concepts involved in the methodology and in the development of the field of femtochemistry. The concepts are described with the aid of a laser demonstration designed at Caltech and used elsewhere as an exhibit.