Controlling surface chemistry with light: Spatially resolved deposition of rovibrational-state-selected molecules

We report the spatially resolved deposition of infrared laser excited methane molecules onto a Ni(100) substrate. A narrow bandwidth infrared laser excites methane molecules in a supersonic molecular beam to nu = 1 of the nu(3) C-H stretching vibration. Tuning the laser to the center of the Doppler-broadened absorption profile selectively excites only those molecules whose transverse velocity is nearly zero. The molecular beam impinges on a Ni(100) substrate where laser-excited molecules dissociate with up to 1600 times the probability of molecules that do not absorb infrared light. Despite the fact that the entire Ni(100) surface is exposed to the molecular beam, only a narrow region of laser-enhanced carbon deposition appears on the substrate. We can control excitation conditions to deposit a single stripe, or a set of equally spaced parallel stripes of carbon on the surface. A model of the deposition process based on optical broadening mechanisms in our experiment quantitatively predicts deposition area dimensions without any adjustable parameters. Extension of the model to other feasible experimental conditions points to the possibility of achieving submicrometer resolution. These results demonstrate a new means of exerting spatial control over deposition processes and highlight an important experimental consideration for future eigenstate-resolved gas-surface reactivity studies employing narrow-bandwidth optical pumping.