For centuries scientists have controlled the fate of chemical reactions using macroscopic variables such as temperature, pressure, concentration, and pH. Since the advent of lasers, they have also increasingly exploited a microscopic variable—the internal structure of the reactants themselves. In that approach, a coherent electromagnetic field directly manipulates the wavefunctions of the reactants to steer the system’s dynamics toward some desired outcome.
With its high intensity and broad bandwidth, an ultrashort laser pulse is an ideal tool for the job: The multiple frequency components in its Fourier spectrum can simultaneously excite electronic and vibrational degrees of freedom of molecules. The pulses can also be precisely shaped by adjusting the phase, amplitude, and polarization of each spectral component to produce the quantum interference needed to constructively enhance one outcome of the reaction while destructively suppressing the alternatives. (See the article by Ian Walmsley and Herschel Rabitz, Physics Today, August 2003,...
