Micha Asscher

The dissociative chemisorption dynamics of N2 on catalytic metal surfaces: A quantum‐mechanical tunneling mechanism

A tunneling mechanism is suggested for explaining the dissociative chemisorption of nitrogen molecules on metal surfaces. The time‐dependent Schrodinger equation was numerically solved for the transition dynamics from the N2–metal to the N–metal potential‐energy surfaces for two degrees of freedom. The dynamics was found to be sensitive to the topology at the crossing region between the two nonadiabatic potential‐energy surfaces (PES). The resulting rapid increase of the dissociation probability (S0) with incident kinetic energy, its saturation at high energies and vibrational enhancement are in good agreement with recent experiments. A substantial isotope effect is predicted by the calculations. Recombinative desorption experiments of 14N2 and 15N2 from Re(0001) are in excellent agreement with the tunneling model.A tunneling mechanism is suggested for explaining the dissociative chemisorption of nitrogen molecules on metal surfaces. The time‐dependent Schrodinger equation was numerically solved for the transition dynamics from the N2–metal to the N–metal potential‐energy surfaces for two degrees of freedom. The dynamics was found to be sensitive to the topology at the crossing region between the two nonadiabatic potential‐energy surfaces (PES). The resulting rapid increase of the dissociation probability (S0) with incident kinetic energy, its saturation at high energies and vibrational enhancement are in good agreement with recent experiments. A substantial isotope effect is predicted by the calculations. Recombinative desorption experiments of 14N2 and 15N2 from Re(0001) are in excellent agreement with the tunneling model.

The quenching mechanism of electronically excited Rydberg states of nitric oxide

Two photon excited fluorescence (TPEF) is used to measure the quenching cross sections of some NO Rydberg states by a number of molecules. The cross sections obtained are quite large, particularly for the C 2Π and D 2Σ states, often significantly exceeding the gas kinetic value. It is suggested that a mechanism common to many collision partners involves an ion pair intermediate. Using this model, the quenching efficiency is related to the electron affinity and to the weakest bond energy of the quencher.

A quantum mechanical mechanism for the dissociative chemisorption of N2 on metal surfaces

The time dependent Schrlidinger equation was numerically solved for the transition dynamics from the N,-metal to the N-metal to the N-metal potential energy surfaces. The resulting rapid increase of the dissociation probability (Sc) with incident kinetic energy. its saturation at high energies and vibrational enhancement are in good agreement with recent experiments. A novel explanation for the small value of S, is based on the high energy value of the crossing region between the two potentials, predicting that the dissociation occurs via a tunneling mechanism. Recombinative desorption experiments of 14Nz and “N, from Re(OOO1) are in excellent agreement with the tunneling model.

Adsorption and dissociation of N2 on rhenium single crystal surfaces

The interaction of N 2 with a rhenium polycrystalline foil, Re(0001) and Re(11¯20) single crystal surfaces was studied under ultrahigh vacuum conditions at the temperature range of 80–1800 K. Extrinsic precursor adsorption kinetics at 80 K on the two crystals imply that the binding energy of the second layer nitrogen molecules is only sligthly affected by the substrates structure. Molecular nitrogen was found to adsorb on a single physisorption site on Re(0001) but at least two different binding energies were identified on Re(11¯20) and the polycrystalline foil. The sticking coefficient into the molecular adsorption sites is 0.90±0.05 on the two single crystal surfaces at 80 K. Dissociation of nitrogen occurs at all three surfaces the initial dissociative sticking probabilities ( S 0 ) measured at 218 K are (4±1)×10 −4 and (9±2)×10 −6 for the Re(11¯20) and Re(0001), respectively. The polycrystalline foil was found to be most effective for the dissociation of nitrogen with a probability of 0.25 at low coverage which decreases to 0.025 at saturation coverage. Study of the dissociative sticking probability as a function of crystal temperature implies the existence of a barrier for dissociative chemisorption of 6±1 kJ/mol and 14±2 kJ/mol on the Re(11¯20) and the Re(0001) single crystal surfaces, respectively. The reactivity for nitrogen dissociation of these surfaces is in agreement with high pressure ammonia synthesis study over the same surfaces.

Two-photon excitation of nitric oxide to levels near and above the dissociation limit

Abstract The A2σ+ (υ = 0, 1, 2, 3), C2Π (υ = 0, 1) and D2Σ+ (υ = 0) states of NO have been populated by two-photon absorption. Low resolution emission spectra from all these states were recorded. The two-photon absorption technique ensures operation in the optically thin regime even for high NO pressures. We report self-quenching rate constants for all the levels studied

The work function of adsorbed alkalis on metals revisited : a coverage-dependent polarizability approach

A model is presented to explain the change in work function as a function of alkali metal coverage on transition metals based on dipole-dipole depolarization assuming a coverage-dependent polarizability of the adsorbate-substrate complex. The fit of the model is a great improvement over fixed-polarizability depolarization models, and the resultant prediction for the variation of the polarizability with coverage is qualitatively similar for different alkalis on the same surface, and quantitatively similar for the same alkali on different surfaces.

CF3NO photodissociation dynamics

The predissociation of CF3NO from 600–680 nm has been studied by monitoring the nascent NO product in real time using a two‐photon laser excited fluorescence (TPEF) technique. The observation that the rate of production of NO is equal to the rate of decay of excited CF3NO indicates that no long‐lived intermediate is involved in the dissociation. Detection of NO(v″=3) following 600 nm dissociation sets an upper limit of 33 kcal/mole on the C–N bond dissociation energy. The nascent vibrational and rotational distributions estimated from the observed TPEF spectra establish that greater than 95% of the NO product is formed in its ground vibrational state and that all vibrational states are rotationally excited. The rotational excitation is discussed in terms of the dynamics of the dissociation. Possible predissociation mechanisms are considered.

Collision induced desorption of N2 from Ru(001)

The dynamics of collision-induced desorption (CID) of N2 from Ru(001) exposed to hyperthermal rare gas colliders generated in a supersonic atomic beam source have been studied. Low coverage of 0.01 ML 15N2 at crystal temperature of 96 K was chosen to represent a CID process of a practically isolated molecule, neglecting the effect of lateral N2–N2 interactions. The cross sections for CID of nitrogen molecules, σdes(Ei,θi), as a function of the kinetic energy and angle of incidence of Ar and Kr colliders have been measured. It was found that σdes(Ei,θi=0°) changes monotonically in the range 0–25 A2 for beam energy in the range of 0.5–5.5 eV and is insensitive to the type of collider (Ar, Kr) as well as to the adsorbate isotope (14N2, 15N2). The threshold energy for desorption has been determined to be 0.50±0.10 eV, which is twice the binding energy of N2 to Ru(001). The cross section for CID at a fixed collider’s energy rises approximately four times as the incidence angle θi increases from 0° to 70° relat...

Laser induced thermal desorption: A time resolved study

Real time monitoring of laser induced thermal desorption of NH3 from Re(0001) at ns time scale was performed utilizing optical SHG. Coverage dependent desorption kinetic parameters determined under equilibrium conditions were found to correctly describe the 8 orders of magnitude faster desorption rates. The role of strong repulsive interactions is discussed.

Interaction of NH3 with Re(0001): An optical second harmonic generation study

Abstract The adsorption desorption kinetics of ammonia on Re(0001) single crystal surface was studied in the crystal temperature range of 80–300 K., utilizing optical second harmonic generation (SHG) and temperature programmed desorption (TPD) measurements. Four different binding states, denoted as α 1 , α 2 , β and γ were detected by TPD. The α 1 and α 2 it states represent a direct intera NH 3 with the metal while β and γ are the second and multilayer adsorption states, respectively. A coverage independent sticking probability of 0.75 ± 0.15 is measured at an adsorption temperature of 80 K. It does not decrease during the population of the second layer (β state). Isothermal desorption and equilibrium measurements were performed by employing the SHG ability to directly probe, nondestructively the surface coverage as a function of crystal temperature and ambient NH 3 pressure. Very strong coverage dependence was found for both the activation energy for desorption ( E d )it and for the pre-exponential factor ( A ). E d decreases from 21 ± 1 to 8.5 ± 1 kcal mol and A changes from 10 17 ± 1.5 to 10 12 ± 1.0 as coverage increases from 0.2 of the saturation coverage (θ s ) to 0.9. E d and A are found to be coverage independent in the range θ/θ s = 0.4–0.8. A comparison is made betwe kinetic parameters obtained from TPD lineshape analysis, isothermal desorption and the equilibrium measurements. It is found that while TPD and isothermal desorption are preferred methods for the low coverage, equilibrium measurements provide more reliable kinetic parameters and their coverage dependence at the high coverage regime.