Far-infrared radiation can be used to direct a surface chemical reaction among competing pathways toward a selected outcome, according to a study published in Physical Review Letters (2015, DOI: 10.1103/physrevlett.115.036103). The finding suggests ways to use far-IR radiation, also known as terahertz light (1 THz = 1012 Hz), to enhance chemical selectivity in catalytic surface reactions and to probe chemical reaction dynamics. Jerry L. LaRue and Hirohito Ogasawara of SLAC National Accelerator Laboratory and colleagues exposed a ruthenium crystal to CO and oxygen and studied the way terahertz pulses affect CO oxidation. The reaction is an archetypal metal surface reaction and is central to the catalytic chemistry of automobile emissions cleanup. The team found that terahertz light selectively activates oxygen atoms, driving them to react with CO to form CO2, but does not cause CO desorption. In contrast, earlier studies showed that laser light in the ultraviolet-visible-IR range stimulates CO desorption and CO oxidation but favors desorption. And conventional surface heating drives CO to desorb from the surface without forming CO2.