Probing Femtosecond Molecular Dynamics

With a new, fast, and high-energy laser spectroscopy method, researchers can now probe the dynamics of highly excited molecular species on the femtosecond timescale with angstrom-level spatial resolution (Science 2007, 317, 1374). Developed by Etienne Gagnon, Arvinder S. Sandhu, and coworkers at the University of Colorado, Boulder, and elsewhere, the procedure provides a way to probe atmospheric chemical processes stimulated by high-energy photons as well as interactions between ionizing radiation and various forms of matter. Using an optical method, the researchers converted an intense beam of infrared light into an ultrashort burst of soft X-rays and then trained the high-energy light onto a cold jet of nitrogen. That process ionized the N₂ molecules by ejecting electrons from their valence shells. Then, by using additional pulses of IR light, the team probed the molecular dynamics on the femtosecond timescale and found two main pathways leading to N₂ fragmentation—dissociation following photoejection of an electron and photoejection coupled with excitation of a second electron.