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Experiments were conducted using a low-temperature scanning tunneling microscope (Omicron) in an ultrahigh vacuum chamber with base pressure <3.0 × 10−11 mbar. The Cu(110) surface was prepared by repeated cycles of sputtering by Ar+ (0.6 keV) and annealing (800 K) until a clean surface was observed by STM. To generate CF3 on the surface, trifluoroiodomethane (CF3I; SynQuest; purity, 99%) was dosed onto the copper surface via a capillary tube. The sample temperature rose up to ~14 K during the dose. The CF3I was found to dissociate upon adsorption to give chemisorbed CF3 and I atoms on the surface.

All STM images were taken at 4.6 K using constant current mode. The bias reported refers to the sample bias. The STM images were obtained at a bias of −0.05 V and a current of 0.05 nA. The C–F bond breaking of CF3 was induced by (i) placing the tip over the adsorbate, (ii) adjusting the tip height, (iii) turning off the feedback loop, and (iv) ramping up the sample bias to Vpulse. A single discontinuity observed in the tunneling current recorded against time indicated that an event had occurred underneath the tip. The reaction was confirmed by the imaging of the reaction products in a subsequent scan. The distance and direction of the product fragments were analyzed using the WSxM software (25).

To establish the number of electrons required in the dissociation of CF3, the reaction rate was measured as a function of tunneling current following a previously established method (26, 27). For a chosen tunneling current, the time prior to the discontinuity in the current-versus-time curve was plotted as a histogram whose bin size was set by Doane’s formula (28). By fitting a single-parameter exponential function (e-Rt) to the normalized histogram, the decay constant (R), which was the average reaction rate, and its fitting error were obtained. Only cases with a single discontinuity were considered.

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