Patterned formation of enolate functional groups on the graphene basal plane.

Chemical functionalization of graphene is one method pursued to engineer new properties into a graphene sheet. Graphene oxide is the most commonly used chemical derivative of graphene. Here we present experimental evidence for the formation of enolate moieties when oxygen atoms are added to the graphene basal plane. The exotic functional groups are stabilized by simultaneous bond formation between the graphene sheet and the underlying Ir(111) substrate. Scanning tunneling microscopy images demonstrate the patterned nature of C-O bond formation and X-ray photoelectron spectroscopy and high-resolution electron energy loss spectroscopy are used to characterize the enolate moiety. The results present a new mechanism for the formation of patterned graphene oxide and provide evidence of a functional group rarely considered for graphene oxide materials.

Exciting H2 Molecules for Graphene Functionalization.

Hydrogen functionalization of graphene by exposure to vibrationally excited H2 molecules is investigated by combined scanning tunneling microscopy, high-resolution electron energy loss spectroscopy, X-ray photoelectron spectroscopy measurements, and density functional theory calculations. The measurements reveal that vibrationally excited H2 molecules dissociatively adsorb on graphene on Ir(111) resulting in nanopatterned hydrogen functionalization structures. Calculations demonstrate that the presence of the Ir surface below the graphene lowers the H2 dissociative adsorption barrier and allows for the adsorption reaction at energies well below the dissociation threshold of the H-H bond. The first reacting H2 molecule must contain considerable vibrational energy to overcome the dissociative adsorption barrier. However, this initial adsorption further activates the surface resulting in reduced barriers for dissociative adsorption of subsequent H2 molecules. This enables functionalization by H2 molecules with lower vibrational energy, yielding an avalanche effect for the hydrogenation reaction. These results provide an example of a catalytically active graphene-coated surface and additionally set the stage for a re-interpretation of previous experimental work involving elevated H2 background gas pressures in the presence of hot filaments.

Quantum state-resolved gas/surface reaction dynamics probed by reflection absorption infrared spectroscopy.

We report the design and characterization of a new molecular-beam/surface-science apparatus for quantum state-resolved studies of gas/surface reaction dynamics combining optical state-specific reactant preparation in a molecular beam by rapid adiabatic passage with detection of surface-bound reaction products by reflection absorption infrared spectroscopy (RAIRS). RAIRS is a non-invasive infrared spectroscopic detection technique that enables online monitoring of the buildup of reaction products on the target surface during reactant deposition by a molecular beam. The product uptake rate obtained by calibrated RAIRS detection yields the coverage dependent state-resolved reaction probability S(θ). Furthermore, the infrared absorption spectra of the adsorbed products obtained by the RAIRS technique provide structural information, which help to identify nascent reaction products, investigate reaction pathways, and determine branching ratios for different pathways of a chemisorption reaction. Measurements of the dissociative chemisorption of methane on Pt(111) with this new apparatus are presented to illustrate the utility of RAIRS detection for highly detailed studies of chemical reactions at the gas/surface interface.

Vibrationally bond-selected chemisorption of methane isotopologues on Pt(111) studied by reflection absorption infrared spectroscopy.

Reflection absorption infrared spectroscopy (RAIRS) was used to probe for vibrational bond-selectivity in the dissociative chemisorption of three partially deuterated methane isotopologues on a Pt(111) surface. While a combination of incident translational energy and thermal vibrational excitation produces a nearly statistical distribution of C-H and C-D bond cleavage products, we observe that laser excitation of an infrared active C-H stretch normal mode leads to highly selective dissociation of a C-H bond for CHD3, CH2D2, and CH3D. Our results show that vibrational energy redistribution between C-H and C-D stretch modes due to methane/surface interactions is negligible during the sub-picosecond collision time which indicates that vibrational bond-selectivity may be the rule rather than the exception in heterogeneous reactions of small polyatomic molecules.

The sticking probability of D2O-water on ice: isotope effects and the influence of vibrational excitation.

The present study measures the sticking probability of heavy water (D(2)O) on H(2)O- and on D(2)O-ice and probes the influence of selective OD-stretch excitation on D(2)O sticking on these ices. Molecular beam techniques are combined with infrared laser excitation to allow for precise control of incident angle, translational energy, and vibrational state of the incident molecules. For a translational energy of 69 kJ/mol and large incident angles (θ ≥ 45°), the sticking probability of D(2)O on H(2)O-ice was found to be 1% lower than on D(2)O-ice. OD-stretch excitation by IR laser pumping of the incident D(2)O molecules produces no detectable change of the D(2)O sticking probability (<10(-3)). The results are compared with other gas/surface systems for which the effect of vibrational excitation on trapping has been probed experimentally.

Alignment dependent chemisorption of vibrationally excited CH4(ν3) on Ni(100), Ni(110), and Ni(111).

We present a stereodynamics study of the dissociative chemisorption of vibrationally excited methane on the (100), (110), and (111) planes of a nickel single crystal surface. Using linearly polarized infrared excitation of the antisymmetric C-H stretch normal mode vibration (ν(3)), we aligned the angular momentum and C-H stretch amplitude of CH(4)(ν(3)) in the laboratory frame and measured the alignment dependence of state-resolved reactivity of CH(4) for the ν(3) = 1, J = 0-3 quantum states over a range of incident translational energies. For all three surfaces studied, in-plane alignment of the C-H stretch results in the highest dissociation probability and alignment along the surface normal in the lowest reactivity. The largest alignment contrast between the maximum and minimum reactivity is observed for Ni(110), which has its surface atoms arranged in close-packed rows separated by one layer deep troughs. For Ni(110), we also probed for alignment effects relative to the direction of the Ni rows. In-plane C-H stretch alignment perpendicular to the surface rows results in higher reactivity than parallel to the surface rows. The alignment effects on Ni(110) and Ni(100) are independent of incident translational energy between 10 and 50 kJ/mol. Quantum state-resolved reaction probabilities are reported for CH(4)(ν(3)) on Ni(110) for translational energies between 10 and 50 kJ/mol.

Steric effects in the chemisorption of vibrationally excited methane on Ni(100).

Newly available, powerful infrared laser sources enable the preparation of intense molecular beams of quantum-state prepared and aligned molecules for gas/surface reaction dynamics experiments. We present a stereodynamics study of the chemisorption of vibrationally excited methane on the (100) surface of nickel. Using linearly polarized infrared excitation of the C-H stretch modes of two methane isotopologues [CH4(nu3) and CD3H(nu1)], we aligned methane's angular momentum and vibrational transition dipole moment in the laboratory frame. An increase in methane reactivity of as much as 60% is observed when the laser polarization is parallel rather than normal to the surface. The dependence of the alignment effect on the rotational branch used for excitation indicates that alignment of the vibrational transition dipole moment of methane is responsible for the steric effect. Potential explanations for the steric effect in terms of an alignment-dependent reaction barrier height or electronically nonadiabatic effects are discussed.

State-resolved reactivity of CH4 on Pt(110)-(1 x 2): the role of surface orientation and impact site.

The reactivity of methane (CH(4)) on Pt(110)-(1 x 2) has been studied by quantum state-resolved surface reactivity measurements. Ground state reaction probabilities, S(0)(v=0) congruent with S(0)(laser-off), as well as state-resolved reaction probabilities S(0)(2nu(3)), for CH(4) excited to the first overtone of the antisymmetric C-H stretch (2nu(3)) have been measured at incident translational energies in the range of 4-64 kJ/mol. We observe S(0)(2nu(3)) to be up to three orders of magnitude higher than S(0)(v=0), demonstrating significant vibrational activation of CH(4) dissociation on Pt(110)-(1 x 2) by 2nu(3) excitation. Furthermore, we explored the azimuthal and polar incident angle dependence of S(0)(2nu(3)) and S(0)(v=0) for a fixed incident translational energy E(t)=32 kJ/mol. For incidence perpendicular to the missing row direction on Pt(110)-(1 x 2) and polar angles theta>40 degrees, shadowing effects prevent the incident CH(4) molecules from impinging into the trough sites. Comparison of this polar angle dependence with reactivity data for incidence parallel to the missing rows yields state-resolved site specific reactivity information consistent with a Pt(110)-(1 x 2) reactivity that is dominated by top layer Pt atoms located at the ridge sites. A comparison of S(0)(v=0) measured on Pt(110)-(1 x 2) and Pt(111) yields a lower average barrier for Pt(110)-(1 x 2) by 13.7+/-2.0 kJ/mol.

Vibrational activation in direct and precursor-mediated chemisorption of SiH(4) on Si(100).

The quantum state-resolved reactivity S(0) of SiH(4) on Si(100)-2x1 has been measured for the first time for two vibrationally excited Si-H stretch local mode states (mid R:2000 and mid R:1100) as well the ground state S(0) as a function of translational energy E(n) and surface temperature T(s). We observe evidence for both direct and precursor-mediated chemisorption pathways. As expected, increasing E(n) (or T(s)) decreases S(0) for the precursor-mediated reaction and increases S(0) for the direct chemisorption. However, vibrational excitation of the incident SiH(4) increases S(0) for both the direct and the precursor-mediated pathway with a higher S(0) for the mid R:2000 state than for the mid R:1100 state, indicating a nonstatistical reaction mechanism.