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7 Oct 2006

Volume 125, Issue 13, Articles (13xxxx)

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back to top Surfaces and Clusters

Applied reaction dynamics: Efficient synthesis gas production via single collision partial oxidation of methane to CO on Rh(111)

K. D. Gibson, M. Viste, and S. J. Sibener

J. Chem. Phys. 125, 133401 (2006); http://dx.doi.org/10.1063/1.2336221 (4 pages) | Cited 1 time

Online Publication Date: 2 October 2006

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Supersonic molecular beams have been used to determine the yield of CO from the partial oxidation of CH4 on a Rh(111) catalytic substrate, CH4+(1/2)O2→CO+2H2, as a function of beam kinetic energy. These experiments were done under ultrahigh vacuum conditions with concurrent molecular beams of O2 and CH4, ensuring that there was only a single collision for the CH4 to react with the surface. The fraction of CH4 converted is strongly dependent on the normal component of the incident beam’s translational energy, and approaches unity for energies greater than ∼ 1.3 eV. Comparison with a simplified model of the methane-Rh(111) reactive potential gives insight into the barrier for methane dissociation. These results demonstrate the efficient conversion of methane to synthesis gas, CO+2H2, are of interest in hydrogen generation, and have the optimal stoichiometry for subsequent utilization in synthetic fuel production (Fischer-Tropsch or methanol synthesis). Moreover, under the reaction conditions explored, no CO2 was detected, i.e., the reaction proceeded with the production of very little, if any, unwanted greenhouse gas by-products. These findings demonstrate the efficacy of overcoming the limitations of purely thermal reaction mechanisms by coupling nonthermal mechanistic steps, leading to efficient C–H bond activation with subsequent thermal heterogeneous reactions.
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82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.20.Bc State selected dynamics and product distribution
34.50.Lf Chemical reactions

The collimation angle shift of desorbing product N2 in a steady-state N2O+CO reaction on Rh(110)

Tatsuo Matsushima, Osamu Nakagoe, Kosuke Shobatake, and Anton Kokalj

J. Chem. Phys. 125, 133402 (2006); http://dx.doi.org/10.1063/1.2352744 (10 pages) | Cited 4 times

Online Publication Date: 3 October 2006

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The angular distribution of desorbing product N2 was studied in N2O decompositions on Rh(110) in the temperature range of 60–700 K. The N2 desorption collimates along 62°–68° off normal toward either the [001] or [00math] direction in a transient N2O decomposition below ca. 470 K or in the steady-state N2O+CO reaction above 540 K. In the steady-state reaction at the temperature from ca. 470 to 540 K, however, the collimation angle shifts from 62° to 45° with decreasing surface temperature. This angle shift is ascribed to the steric hindrance by coadsorbed CO because the N2 collimation in transient N2O decomposition at around 65° is recovered in the range of 380–500 K by an abrupt CO pressure drop followed by the decrease in CO coverage. N2O is oriented along the [001] direction before dissociation. A scattering model of the nascent N2 by adsorbed CO is proposed, yielding smaller collimation angles.
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82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
82.30.Cf Atom and radical reactions; chain reactions; molecule-molecule reactions
68.43.Mn Adsorption kinetics
82.20.Hf Product distribution
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)

Infrared spectroscopy of large ammonia clusters as a function of size

Christof Steinbach, Udo Buck, and Titus A. Beu

J. Chem. Phys. 125, 133403 (2006); http://dx.doi.org/10.1063/1.2345057 (8 pages) | Cited 9 times

Online Publication Date: 3 October 2006

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We have measured the vibrational spectra of large ammonia (NH3)n clusters by photofragment spectroscopy in the spectral range from 3150 to 3450 cm−1 for the average sizes 〈n〉 = 29, 80, 212, 447, and 989 and by depletion spectroscopy for 〈n〉 = 8. The spectra are dominated by peaks around 3385 cm−1 which are attributed to the asymmetric ν3 NH-stretch mode. Two further peaks between 3200 and 3260 cm−1 have about equal intensity for 〈n〉 = 8 and 29, but only about 0.40 of the intensity of the ν3 peak for the larger sizes. The spectra for the smallest and largest size agree with those obtained by Fourier transform infrared spectroscopy in slit jet expansion and collision cells, respectively. By accompanying calculation we demonstrate that the energetic order of the spectral features originating from the bending overtone 2ν4 and the symmetric NH-stretch ν1 in the range from 3150 to 3450 cm−1 is changed between n = 10 and 100, while the asymmetric NH-stretch ν3 only exhibits a moderate redshift. The reason is the coupling of the ground state modes to the overtones.
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36.40.Mr Spectroscopy and geometrical structure of clusters
33.20.Ea Infrared spectra
33.15.Mt Rotation, vibration, and vibration-rotation constants
33.20.Tp Vibrational analysis
33.70.Jg Line and band widths, shapes, and shifts

How many metal atoms are needed to dehydrogenate an ethylene molecule on metal clusters?: Correlation between reactivity and electronic structures of Fen+, Con+, and Nin+

Masahiko Ichihashi, Tetsu Hanmura, and Tamotsu Kondow

J. Chem. Phys. 125, 133404 (2006); http://dx.doi.org/10.1063/1.2236117 (6 pages) | Cited 3 times

Online Publication Date: 3 October 2006

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The absolute cross section for dehydrogenation of an ethylene molecule on Mn+ [Fen+ (n = 2–28), Con+ (n = 8–29), and Nin+ (n = 3–30)] was measured as a function of the cluster size n in a gas-beam geometry at a collision energy of 0.4 eV in the center-of-mass frame in an apparatus equipped with a tandem-type mass spectrometer. It is found that (1) the dehydrogenation cross section increases rapidly above a cluster size of ≈ 18 on Fen+, ≈ 13 and ≈ 18 on Con+, and ≈ 10 on Nin+ and (2) the rapid increase of the cross section for Mn+ occurs at a cluster size where the 3d electrons start to contribute to the highest occupied levels of Mn+. These findings lead us to conclude that the 3d electrons of Mn+ play a central role in the dehydrogenation on Mn+.
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82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
36.40.Jn Reactivity of clusters
34.50.Lf Chemical reactions

Time-resolved study of solvent-induced recombination in photodissociated IBr−(CO2)n clusters

Vladimir Dribinski, Jack Barbera, Joshua P. Martin, Annette Svendsen, Matthew A. Thompson, Robert Parson, and W. Carl Lineberger

J. Chem. Phys. 125, 133405 (2006); http://dx.doi.org/10.1063/1.2217741 (7 pages) | Cited 7 times

Online Publication Date: 3 October 2006

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We report the time-resolved recombination of photodissociated IBr−(CO2)n (n = 5–10) clusters following excitation to the dissociative IBr−A′ math state of the chromophore via a 180 fs, 795 nm laser pulse. Dissociation from the A′ state of the bare anion results in I− and Br products. Upon solvation with CO2, the IBr− chromophore regains near-IR absorption only after recombination and vibrational relaxation on the ground electronic state. The recombination time was determined by using a delayed femtosecond probe laser, at the same wavelength as the pump, and detecting ionic photoproducts of the recombined IBr− cluster ions. In sharp contrast to previous studies involving solvated I2−, the observed recombination times for IBr−(CO2)n increase dramatically with increasing cluster size, from 12 ps for n = 5 to 900 ps for n = 8,10. The nanosecond recombination times are especially surprising in that the overall recombination probability for these cluster ions is unity. Over the range of 5–10 solvent molecules, calculations show that the solvent is very asymmetrically distributed, localized around the Br end of the IBr− chromophore. It is proposed that this asymmetric solvation delays the recombination of the dissociating IBr−, in part through a solvent-induced well in the A′ state that (for n = 8,10) traps the evolving complex. Extensive electronic structure calculations and nonadiabatic molecular dynamics simulations provide a framework to understand this unexpected behavior.
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78.47.-p Spectroscopy of solid state dynamics
82.80.Dx Analytical methods involving electronic spectroscopy
82.50.Bc Processes caused by infrared radiation
82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
82.30.Nr Association, addition, insertion, cluster formation
78.30.Hv Other nonmetallic inorganics

Photodissociation of polycrystalline and amorphous water ice films at 157 and 193 nm

Akihiro Yabushita, Daichi Kanda, Noboru Kawanaka, Masahiro Kawasaki, and Michael N. R. Ashfold

J. Chem. Phys. 125, 133406 (2006); http://dx.doi.org/10.1063/1.2335840 (7 pages) | Cited 29 times

Online Publication Date: 3 October 2006

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The photodissociation dynamics of amorphous solid water (ASW) films and polycrystalline ice (PCI) films at a substrate temperature of 100 K have been investigated by analyzing the time-of-flight (TOF) mass spectra of photofragment hydrogen atoms at 157 and 193 nm. For PCI films, the TOF spectrum recorded at 157 nm could be characterized by a combination of three different (fast, medium, and slow) Maxwell-Boltzmann energy distributions, while that measured at 193 nm can be fitted in terms of solely a fast component. For ASW films, the TOF spectra measured at 157 and 193 nm were both dominated by the slow component, indicating that the photofragment H atoms are accommodated to the substrate temperature by collisions. H atom formation at 193 nm is attributed to the photodissociation of water species on the ice surface, while at 157 nm it is ascribable to a mixture of surface and bulk photodissociations. Atmospheric implications in the high latitude mesopause region of the Earth are discussed.
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33.15.Ta Mass spectra
33.80.Gj Diffuse spectra; predissociation, photodissociation
82.33.Tb Atmospheric chemistry

Molecular dynamics of haloalkane corral formation and surface halogenation at Si(111)-7×7

S. Dobrin, K. R. Harikumar, R. V. Jones, I. R. McNab, J. C. Polanyi, Z. Waqar, and J. (S. Y.) Yang

J. Chem. Phys. 125, 133407 (2006); http://dx.doi.org/10.1063/1.2352745 (9 pages) | Cited 8 times

Online Publication Date: 4 October 2006

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Long-chain organic molecules, 1-halododecane, RX (X = Cl,Br), adsorbed on Si(111)-7×7 were shown to form stable dimeric corrals; type I around corner holes and type II around corner adatoms S. Dobrin et al. [Surf. Sci. Lett. 600, L43 (2006)]. Here we examine the molecular dynamics of corral formation, in which mobile physisorbed adsorbates spontaneously convert to immobile. At high coverage the mechanism gives evidence of involving collisions between mobile vertical monomers, giving types I and II immobile horizontal dimers, vD+vD→h2 (I, II). At low coverage mobile vertical monomers collide with immobile horizontal ones to form largely type-II corrals, vD+h→h2 (II). Thermal reaction of corrals with X = Br brominates the surface by two distinct molecular pathways, thought to have more general applicability: “daughter-mediated” reaction of vertical vA with a low activation energy (here Ea ∼ 5 kcal mol−1) and “parent-mediated” reaction of horizontal h or h2 with high activation energy (here Ea = 29 kcal mol−1).
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68.43.-h Chemisorption/physisorption: adsorbates on surfaces
82.65.+r Surface and interface chemistry; heterogeneous catalysis at surfaces
82.20.-w Chemical kinetics and dynamics

Ultrafast vectorial and scalar dynamics of ionic clusters: Azobenzene solvated by oxygen

D. Hern Paik, J. Spencer Baskin, Nam Joon Kim, and Ahmed H. Zewail

J. Chem. Phys. 125, 133408 (2006); http://dx.doi.org/10.1063/1.2205855 (10 pages) | Cited 4 times

Online Publication Date: 5 October 2006

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The ultrafast dynamics of clusters of trans-azobenzene anion (A−) solvated by oxygen molecules was investigated using femtosecond time-resolved photoelectron spectroscopy. The time scale for stripping off all oxygen molecules from A− was determined by monitoring in real time the transient of the A− rise, following an 800 nm excitation of A− (O2)n, where n = 1–4. A careful analysis of the time-dependent photoelectron spectra strongly suggests that for n>1 a quasi-O4 core is formed and that the dissociation occurs by a bond cleavage between A− and conglomerated (O2)n rather than a stepwise evaporation of O2. With time and energy resolutions, we were able to capture the photoelectron signatures of transient species which instantaneously rise (<100 fs) then decay. The transient species are assigned as charge-transfer complexes: A∙O2− for A−O2 and A∙O4−∙(O2)n−2 for A−(O2)n, where n = 2–4. Subsequent to an ultrafast electron recombination, A− rises with two distinct time scales: a subpicosecond component reflecting a direct bond rupture of the A−-(O2)n nuclear coordinate and a slower component (1.6–36 ps, increasing with n) attributed to an indirect channel exhibiting a quasistatistical behavior. The photodetachment transients exhibit a change in the transition dipole direction as a function of time delay. Rotational dephasing occurs on a time scale of 2–3 ps, with a change in the sign of the transient anisotropy between A−O2 and the larger clusters. This behavior is a key indicator of an evolving cluster structure and is successfully modeled by calculations based on the structures and inertial motion of the parent clusters.
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36.40.Wa Charged clusters
36.40.Jn Reactivity of clusters
82.30.Lp Decomposition reactions (pyrolysis, dissociation, and fragmentation)
82.30.Fi Ion-molecule, ion-ion, and charge-transfer reactions
36.40.Mr Spectroscopy and geometrical structure of clusters
33.60.+q Photoelectron spectra

A mass and time-of-flight spectroscopy study of the formation of clusters in free-jet expansions of normal D2

Y. Ekinci, E. L. Knuth, and J. P. Toennies

J. Chem. Phys. 125, 133409 (2006); http://dx.doi.org/10.1063/1.2217942 (12 pages) | Cited 9 times

Online Publication Date: 5 October 2006

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The mass spectra in the range of 2(D+)–38(D19+) amu of clusters formed in a supersonic free-jet expansion of normal D2 are investigated as functions of source temperature in the range of 95–220 K and of source pressure in the range of 10–120 bars. For some of the small ion fragments, time-of-flight distributions are also measured. For large clusters (n>200) the intensities of the odd-numbered ion fragments exhibit magic numbers at D9+ and D15+ in accordance with previous experiments and calculations. The even-numbered ion fragments have much smaller intensities and exhibit new magic numbers at D10+ and D14+. For source conditions such that large clusters are formed, the intensities of the various different ion fragments are observed to saturate beyond a certain source pressure. At lower source pressures, where only small clusters are formed, the terminal mole fractions of the neutral dimers are analyzed in the light of available theories which take into account both the thermodynamics and the kinetics of the expansion. At higher source pressures and lower temperatures, where larger clusters are formed, the sizes of the neutral clusters are estimated using scaling laws and are found to be consistent with the mass spectra and measured time-of-flight distributions. By using a variety of techniques it has been possible to obtain reliable conclusions about the neutral cluster sizes for the present free-jet expansion conditions.
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36.40.Mr Spectroscopy and geometrical structure of clusters
82.30.Nr Association, addition, insertion, cluster formation
82.80.Ms Mass spectrometry (including SIMS, multiphoton ionization and resonance ionization mass spectrometry, MALDI)
82.20.-w Chemical kinetics and dynamics
33.70.Fd Absolute and relative line and band intensities
33.15.Ta Mass spectra
36.40.Qv Stability and fragmentation of clusters
82.60.-s Chemical thermodynamics
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