For just over 20 years, a laser-based tool called sum frequency generation (SFG) vibrational spectroscopy has allowed chemists to probe the composition, structure, and orientation of molecules at surfaces and interfaces. The technique, which is closely related to the method of second harmonic generation, is sufficiently sensitive to detect less than a monolayer of molecules and readily does so on surfaces that are exposed to high gas pressures, to liquids, and to other conditions that limit the usefulness of classic surface analysis methods.
As a result of spectroscopy selection rules, SFG signals cannot originate from the bulk of a solid or liquid, according to University of California, Berkeley, physics professor Yuen Ron Shen, one of the field's pioneers. At an interface, such as the one between a solid surface and a gas or liquid, however, that symmetry is broken. Simultaneously directing visible and infrared laser beams at such an interface causes the beams to combine and produce light with a frequency equal to the sum of the two inputs. Scanning the IR beam while measuring the intensity of the summed output beam generates a vibrational spectrum of molecules at the interface.
The technique has been applied broadly. For example, in one of the field's older studies, Shen and fellow UC Berkeley colleague Gabor A. Somorjai used SFG to determine the mechanism of olefin hydrogenation on platinum catalysts. The team ruled out various pathways and deduced that the reaction proceeds from π-bonded propylene (one of two possible adsorbate configurations) to propane by way of a 2-propyl intermediate. Other groups employed the method in its early days to analyze semiconductor and electrode surfaces.
More recently, SFG has been used to analyze systems of greater chemical complexity. The University of Utah's John C. Conboy, for example, observed lipids flip-flopping across a bilayer, a process central to cellular membrane chemistry (J. Am. Chem. Soc. 2004, 126, 8376). And at Texas A&M University, Paul S. Cremer's research group capitalized on SFG's knack for pinpointing absolute molecular orientation at interfaces to test proposed mechanisms of protein denaturation by urea. That reaction was discovered more than 100 years ago. But whether the mechanism is driven by hydrogen bonding or by hydrophobic interactions had remained unknown, Cremer says. "By using SFG, we showed that urea flips orientation based on net protein charge, thereby disproving the hydrogen-bonding mechanism" (J. Am. Chem. Soc. 2007, 129, 15104).