Building on our earlier study, we examine the kinetic barriers to decomposition of alane, AlH$_3$, on the Si(001) surface, using the nudged elastic band (NEB) approach within DFT. We find that the initial decomposition to AlH with two H atoms on the surface proceeds without a significant barrier. There are several pathways available to lose the final hydrogen, though these present barriers of up to 1 eV. Incorporation is more challenging, with the initial structures less stable in several cases than the starting structures, just as was found for phosphorus. We identify a stable route for Al incorporation following selective surface hydrogen desorption (e.g. by STM tip). The overall process parallels PH$_3$, and indicates that atomically precise acceptor doping should be possible.

Vibrational state resolved measurements of methane's dissociation on Ni(111) show a strong surface temperature dependence near the translational energy threshold for reaction. The reactivity of molecules excited to nu = 1 of the nu(3) C-H stretching vibration and incident on the surface with a translational energy of 40 kJ mol(-1) increased 8-fold as the surface temperature increased from 90 to 475 K. This enhancement is much larger than that reported for earlier studies at higher incident energies. These results support recent calculations that predict an important role for lattice deformation in transition state access. At higher surface temperatures, surface phonon excitation allows substrate atoms to sample lattice geometries with more favorable transition state energetics. We have also measured the coverage-dependent reactivity of these molecules at both surface temperatures and report a simple model that quantitatively predicts the observed coverage-dependent reactivity.

Author Institution: Department of Chemistry, Tufts University, Medford, MA, 02155; Department of Chemistry, Leiden University

Author Institution: Department of Chemistry, Tufts University, Medford, MA, 02155; Department of Chemistry, Leiden University

State-resolved measurements on clean Ni(100) and Ni(111) surfaces quantify the reactivity of CH4 excited to v = 3 of the nu4 bend vibration. A comparison with prior data reveals that 3nu4 is significantly less effective than the nu3 C-H stretch at promoting dissociative chemisorption, even though 3nu4 contains 30% more energy. These results contradict statistical theories of gas-surface reactivity, provide clear evidence for vibrational mode specificity in a gas-surface reaction, and point to a central role for C-H stretching motion along the reaction path to dissociative chemisorption.

State-resolved gas-surface reactivity measurements revealed that vibrational excitation of ν 3 (the antisymmetric C-H stretch) activates methane dissociation more efficiently than does translational energy. Methane molecules in the vibrational ground state require 45 kilojoules per mole (kJ/mol) of translational energy to attain the same reactivity enhancement provided by 36 kJ/mol of ν 3 excitation. This result contradicts a key assumption underlying statistical theories of gas-surface reactivity and provides direct experimental evidence of the central role that vibrational energy can play in activating gas-surface reactions.

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.

We have measured the sticking probability of methane excited to v = 1 of the v3 antisymmetric C-H stretching vibration on a clean Ni(100) surface as a function of rotational state (J = 0, 1, 2 and 3) and have investigated the effect of Coriolis-mixing on reactivity. The data span a wide range of kinetic energies (9-49 kJ mol-1) and indicate that rotational excitation does not alter reactivity by more than a factor of two, even at low molecular speeds that allow for considerable rotation of the molecule during the interaction with the surface. In addition, rotation-induced Coriolis-splitting of the v3 mode into F+, F0 and F- states does not significantly affect the reactivity for J = 1 at 49 kJ mol-1 translational energy, even though the nuclear motions of these states differ. The lack of a pronounced rotational energy effect in methane dissociation on Ni(100) suggests that our previous results for (v = 1, v3, J = 2) are representative of all rovibrational sublevels of this vibrational mode. These experiments shed light on the relative importance of rotational hindering and dynamical steering mechanisms in the dissociative chemisorption on Ni(100) and guide future attempts to accurately model methane dissociation on nickel surfaces.

A new experimental technique uses state-resolved infrared laser excitation to probe a polyatomic molecule's dissociative chemisorption dynamics with previously unattainable detail. Methane molecules excited to v = 1 of the v(3) C-W stretching vibration are up to 1600 times more reactive on a clean Ni(100) surface than are molecules in the ground vibrational state. Over a translational energy range of 27 to 54 kJ/mol, their absolute reaction probability increases from 3 x 10(-1) to 6 x 10(-3), which indicates that v(3) is responsible only in part for the vibrational activation reported in previous studies.