L. Schröter

Rotational state distribution of recombinatively desorbing hydrogen from clean and S-covered Pd(100)

Abstract The rotational state distribution of H2 and D2 molecules desorbing from Pd(100) following atomic recombination has been determined by tunable vacuum ultraviolet laser ionisation spectroscopy. The surface temperature of the permeation source was varied between 325 and 740 K. A significant rotational cooling effect has been observed which leads to rotational temperatures of about 350 ± 25 K at high Ts. No isotope effect is apparent. Preadsorption of S atoms changes the rotational temperature of the desorbing hydrogen molecules. The rotational cooling effect is discussed in the context of current theoretical models.

State specific velocity distribution of hydrogen isotopes desorbing from Pd(100)

Abstract Velocity distributions of recombinatively desorbing hydrogen molecules and their isotopic variants HD and D 2 have been determined with internal state selection using resonantly enhanced (VUV + UV) two-photon ionization spectroscopy. In the experiments the surface temperature of the permeation source is kept constant at various temperatures between 440 and 770 K. The velocity distributions of molecules desorbed from a clean Pd(100) surface are found to be Maxwell-Boltzmann like, but an isotope effect of the average kinetic energy is observed. The kinetic energy of hydrogen molecules agrees with that expected for molecules in thermal equilibrium with the surface. For deuterium molecules the average kinetic energy 〈 E kin 〉 is about 10–30 meV higher than expected for molecules in a thermal equilibrium at T s . Within the experimental error bars no significant dependence of the average kinetic energies on the rovibrational states is detected. Preadsorption of sulfur leads to a non-Maxwell-Boltzmann velocity distribution with a significantly enhanced average kinetic energy.

Vibrational activation in associative desorption of hydrogen from Pd(100)

Abstract Using resonantly enhanced two-photon ionization, vibrationally excited hydrogen molecules are observed with rotational state selection in associative desorption from a Pd(100) surface. The population in the excited vibrational states of all three stable isotopes is significantly higher than expected for molecules in thermal equilibrium with the surface. Upon changing the surface temperature Ts from 325 to 740 K, achieved by employing a permeation source for the hydrogen supply, the vibrational population increases exponentially with Ts. A quantum-mechanical approach based on the concept of a reaction-path calculation is applied to this problem. Implications of this model and distinctions from thermal models are discussed.

Laser spectroscopy of hydrogen desorption from Pd(100)

The recombinative desorption flux of hydrogen molecules from a Pd{100} surface is investigated with rovibrational state selectivity by resonantly enhanced [vacuum ultraviolet (VUV)+ ultraviolet (UV)] two photon ionization. Using a permeation source for the hydrogen supply the surface temperature can be changed from 325 to 740 K. The population in the rotational states is significantly lower than expected for molecules in thermal equilibrium at the respective surface temperatures, whereas the vibrational population is much higher. This vibrational population increases exponentially with Ts. When interpreted as desorption rates into (v‘=1) and plotted versus T−1s, activation energies Ea for vibrational excitation can be derived. For the three isotopes D2, HD, and H2 values for Ea of 234, 282, and 428 meV, respectively, are obtained, being in all three cases lower than the corresponding gas phase energies. The rotational cooling and the vibrational excitation are discussed in the context of current theoretic...

Adsorption and decomposition of hydrazine on Pd(100)

Abstract The adsorption and decomposition of N 2 H 4 on Pd(100) has been studied by measuring the sticking coefficient and by thermal desorption spectroscopy. Well-defined molecular beam dosing has been employed to limit the interaction of hydrazine to the palladium surface. Desorption of ammonia and hydrazine occurred at peak temperatures between 380 and 420 K. From the adsorption isosteres binding energies of (360 ± 50) meV and (190 ± 40) meV could be derived for NH 3 and N 2 H 4 , respectively. Low pre-exponential factors of ν 0 (NH 3 ) = 10 (1.1 ± 0.5) s −1 and ν 0 (N 2 H 4 ) = 10 (2.8 ± 0.4) s −1 were found. desorption was not detected which allows identification of the relevant surface reactions leading to hydrazine decomposition.

Internal energy distribution of recombinatively desorbing D2 from sulfur covered poly-Pd

Rotational and vibrational population distributions have been determined for D2 molecules recombinatively desorbing from polycrystalline Pd surfaces by tunable vacuum-ultraviolet laser-induced fluorescence. In the temperature range 550 K≦Ts≦1050 K studied in this work a rotational temperature ofTrot∼ 450 K was found, nearly independent of the surface temperature. Similarly, the vibrational temperature could be described by a value ofTvib∼1100 K, being always higher than the surface temperature.

Internal state selected velocity and population distribution of D2 desorbing from clean Pd(1OO)

Abstract Rotational state distributions have been determined for D 2 , molecules recombinatively desorbing from clean Pd(100) surfaces. Resonantly enhanced two-photon ionization was employed using tunable vacuum ultraviolet and ultraviolet lasers. In the temperature range 340 K ⩽ T s ⩽ 740 K studied in this work, the rotational temperatures T rot of the desorbing D 2 , molecules were found to be always lower than T s . At high T s a limiting temperature of T rot ~ 350 K seem to evolve, thus a distinct rotational cooling effect is observed. With a time-of-flight detection technique also internal state selective velocity distributions of desorbing D 2 molecules have been measured.

State-selective studies of the associative desorption of hydrogen from Pd(100) and Cu(100)

Vibrational-state populations and velocity distributions of H2, HD, and D2 desorbing from Pd(100) are measured with rotational state selectivity over a wide temperature range from Ts= 325 to 825 K. At all surface temperatures the vibrational populations, increasing exponentially with Ts, are found to be significantly higher than those expected for thermally equilibrated molecules. The slopes of Boltzmann plots are considerably lower than expected for a thermal excitation mechanism of the vibrational states at the corresponding gas-phase energies. They also show a non-trivial isotope effect. The velocity distributions from clean Pd(100) are found to be Maxwell–Boltzmann-like. The translational energies of H2 molecules are accommodated at the surface temperatures, whereas those of D2 are higher than kTs by ca. 10–30 meV. Both the vibrational excitation and the isotope effect in translation can be understood with a quantum-mechanical model calculation on a two-dimensional potential-energy surface. The vibrational-state selective angular distributions for desorption from Cu(100) display a different behaviour for ground and excited states. The distributions for the vibrationally excited states are broader than that of ground-state molecules. They also show an isotope effect.

Adsorbate resonance enhancement in optical surface second-harmonic generation: S on Pd(100)

Abstract Optical second-harmonic generation from a fixed-frequency laser at λ = 1064 nm has been used to monitor the adsorption of sulfur atoms on a Pd(100) single-crystal surface. The signal observed was dominated by the contribution from the adsorbate atoms to the second-order susceptibility due to a resonance enhancement from a nearby electronic state of the S atom. This enhancement rendered an adsorbate sensitivity of 0.01 monolayer possible.

Vibrational activation in recombinative hydrogen desorption from Pd 100

Abstract In recombinative desorption of hydrogen from clean Pd100 vibrationally excited molecules are observed. The population in the excited states is higher than corresponding to molecules in thermal equilibrium with the surface. This population also increases nearly exponentially with the surface temperature, T S . If the population is converted to desorption rates, an activation energy for vibrational excitation can be derived. These activation energies of 234 meV, 282 meV, and 428 meV for D 2 , HD, and H 2 , respectively, are in all three cases lower than the corresponding gas phase energies.