Cold free-radical molecules in the laboratory frame

A special class of molecules that are important to many subfields in molecular dynamics and chemical physics, namely free-radical molecules, now enjoy a significant degree of center-of-mass motion control in the laboratory frame. The example reported in this paper concerns the hydroxyl radical (OH), which, after the internal degrees of freedom are cooled in a supersonic expansion, has been bunched, accelerated, and slowed using time-varying inhomogeneous electric fields. In situ observations of laser-induced fluorescence along the beam propagation path allows for detailed characterization of the longitudinal phase-space manipulation of OH molecules by the electric fields. The creation of a pulse containing 10³--10⁶ molecules possessing a longitudinal velocity spread from 2 to 800.3em0exm∕s around a mean laboratory velocity variable from 5500.3em0exm∕s to rest with only a few mm spatial extent represents an exciting and useful experimental capability for exploring free-radical dynamics.