In studying a chemical reaction on a surface, it's useful to know not only the identities of the desorbed products but also their angular distribution. Velocity-map imaging, a technique developed for gas-phase chemistry, can achieve that goal: The products are ionized and electrically accelerated toward a two-dimensional detector that incorporates a CCD camera. The products arrive at the detector at different times, segregated by their mass-to-charge ratio. But commercial CCDs are too slow to separately image each product in a single experiment. And repeating the experiment to capture different products requires a new, clean surface, which can complicate comparisons of data from different repetitions. Over the past several years, chemists and physicists at Oxford University and the Rutherford Appleton Laboratory have developed a new ultrafast sensor—called PImMS, for pixel imaging mass spectrometry—that overcomes the CCDs' limitation. Now Michael White and colleagues (Brookhaven National Laboratory and Stony Brook University) have used PImMS to study the surface-catalyzed oxidation of 2-butanone on titanium dioxide. Previous studies of that reaction identified signals corresponding to the hydrocarbon fragments C2H3, C2H4, and C2H5. From their proof-of-principle PImMS data, shown in the figure, White and colleagues concluded that C2H3 and C2H4 aren't products of the surface reaction at all. Instead, they're formed in the ionizer, where some C2H5 fragments lose not only an electron but also one or two hydrogen atoms. (M. D. Kershis et al., J. Chem. Phys. 139, 084202, 2013.)—Johanna Miller
