Hans-Robert Volpp

The effect of reagent translation on the reaction dynamics and the absolute reaction cross section of H+H2O→OH+H2

With H atoms from ultraviolet laser photolysis of H2S and HI, the influence of the translational excitation of the reagents on the reaction dynamics and the absolute value of the reaction cross section of H+H2O→OH+H2 has been studied in the center of mass (c.m.) energy range from the reaction threshold up to 2.2 eV. To determine the OH product rotational fine‐structure distributions, the nascent OH radicals were detected with quantum state resolution by laser‐induced fluorescence (LIF). It was found that at all c.m. collision energies, the OH radicals are produced exclusively in the vibrational ground state. The measured OH(v=0) rotational fine‐structure distributions can be described by Boltzmann distributions, with rotational temperatures which increase only slightly with increasing collision energy. Near the threshold, the OH fine structure rotational temperatures are almost equal; at higher collision energies, the rotational temperature of the OH(A’) fine structure distribution is about a factor of 1....

Absolute rate constants and reactive cross sections for the reactions of O(1D) with molecular hydrogen and deuterium

Abstract The dynamics of the reaction of O( 1 D) with molecular hydrogen and deuterium has been investigated using “superthermal” O( 1 D) atoms generated by laser photolysis of N 2 O at 193 nm. H/D atom products have been detected under single-collision conditions by vacuum ultraviolet laser-induced fluorescence at the Lyman-α transition. With a calibration method using HCl photolysis as a source of well-defined H atom concentrations, the following absolute rate constants k and absolute reactive cross sections σ R have been determined: k = (2.7±0.6) × 10 −10 cm 3 s −1 molecule −1 and σ R (0.12 eV) =7.6±1.5 A 2 for the reaction O( 1 D)+H 2 →OH+H, and k =(2.3±0.5)×10 −10 cm 3 s −1 molecule −1 and σ R (0.18 eV)=7.0±1.4 A 2 for the reaction O( 1 D)+D 2 →OD+D.

Absolute primary H atom quantum yield measurements in the 193.3 and 121.6 nm photodissociation of acetylene

Absolute quantum yields for primary H atom formation (UH) were measured under collision-free conditions for the room-temperature gas-phase dissociation of acetylene (C2H2) after photoexcitation at 193.3 nm and at the H-atom Lyman-a wavelength (121.6 nm) by a pulsed laser photolysis (LP)–laser-induced fluorescence (LIF) ‘pump-and-probe’ technique. Using HCl and CH4 photolysis at 193.3 and 121.6 nm, respectively, as a reference, values of UH ð193: 3n mÞ¼0:94� 0:12 and UH ð121: 6n mÞ¼1:04� 0:16 were obtained which demonstrate that for both photolysis wavelengths the Hþ C2H product channel dominates the primary C2H2 photochemistry. 2002 Published by Elsevier Science B.V.

Absolute rate constants, reactive cross-sections and isotopic branching ratio for the reaction of O(1D) with HD

Abstract Using the laser photolysis vacuum-UV laser-induced fluorescence ‘pump-and-probe’ technique, Doppler profiles of H and D atoms from the reaction O( 1 D) + HD were measured under single-collision conditions, O( 1 D) atoms were generated by laser photolysis of N 2 O at 193 nm. With a calibration method using HCl photolysis as a source of well defined H atom concentrations the following absolute rate constants k and absolute reactive cross-sections σ R have been determined: k = (1.3 ± 0.3) × 10 −10 cm 3 s −1 molec −1 and σ R (0.14 eV) = 4.0 ± 0.9 A 2 for the reaction channel O( 1 D) + HD → OD + H, and k = (1.0 ± 0.3) × 10 −10 cm 3 s −1 molec −1 and σ R (0.14 eV) = (3.0 ± 0.7) A 2 for the reaction channel O( 1 D) + HD → OH + D. The isotopic branching ratio for the reaction O( 1 D) + HD was measured directly to be Γ H/D = 1.35 ± 0.20. In addition, from the measured H and D atom Doppler profiles the fraction ƒ T of the available energy released as translational energy was determined for the individual product channels to be ƒ T (OD + H) = 0.41 ± 0.07 and ƒ T (OH + D) = 0.32 ± 0.05.

Absolute reactive cross sections for the reaction H+O2→OH+O

Abstract Using translationally excited H atoms generated by 193 nm photolysis of HCl and HBr, we have investigated the reaction H+O 2 at center-of-mass collision energies of E 1 =1.86±0.26 eV, E 2 =2.14±0.18 eV and E 3 =2.57±0.19 eV. The nascent OH vibrational and rotational distributions were probed by laser-induced fluorescence. With a calibration method using H 2 O 2 photolysis as a well-defined source of OH radicals, absolute reactive cross sections at single-collision energies could be determined: σ R (1.86 eV) =0.37±0.20 A 2 , σ R (2.14 eV)=0.25±0.14A 2 and σ R (2.57 eV)=0.10±0.05 A 2 .

PHOTODISSOCIATION OF CBRCL3 AT 234 AND 265 NM : EVIDENCE OF THE CURVE CROSSING

The photodissociation dynamics of CBrCl3 was studied near 234 and 265 nm using a two-dimensional photofragment ion imaging technique. Bromine fragments monitored in this study were produced via direct dissociation of CBrCl3, represented by CBrCl3→CCl3+Br(2P1/2)/Br(2P3/2). The branching ratio of Br(2P1/2) (denoted Br*)/Br(2P3/2) (denoted Br) showed strong excitation energy dependence. The product quantum yields at two different excitation wavelengths were Φ 234 nm(Br*)=0.31±0.01 and Φ 265 nm(Br*)=0.68±0.02, respectively. The speed and angular distributions of Br* and Br fragments were determined. Similar values of β(234 nm)=−0.44 and β(265 nm)=−0.47 for Br were observed, while β values for Br* were found to be markedly different, β(234 nm)=−0.34 and β(265 nm)=1.43. The strong curve crossing, 1Q1→3Q0, and the angular distribution of Br* suggesting a typical perpendicular transition, were observed at 234 nm photodissociation.

Elementary Reaction Kinetics of the CO ∕ CO2 ∕ Ni ∕ YSZ Electrode

Results of combined experimental and theoretical investigations of elementary chemical reaction processes of COA¢Â�Â�CO2 gas mixtures at nickel/yttria-stabilized zirconia (Ni/YSZ) solid oxide fuel cell (SOFC) model anode systems are presented. Temperature-programmed desorption and reaction measurements were performed in order to determine adsorption/desorption kinetic data as well as thermodynamic parameters for the CO/CO2/Ni and CO/CO2/yttria-stabilized zirconia (YSZ) heterogeneous reaction systems. From these data, an elementary kinetic reaction mechanism of the electrochemical CO oxidation at Ni/YSZ anodes was developed. Numerical simulations were performed for three different spillover mechanisms. Steady-state polarization curves and electrochemical impedance spectra were calculated, allowing for a direct comparison with experiments performed by Lauvstad et al. [J. Electrochem. Soc., 149, E506 (2002)]. Best agreement with the experimental data was obtained when assuming two consecutive charge-transfer steps from YSZA¢Â�Â�O2A¢Â�Â� via YSZA¢Â�Â�OA¢Â�Â� to NiA¢Â�Â�O, the second step being accompanied by oxygen spillover over the three-phase boundary.

Reaction dynamics, OH energy partitioning, and OH product angular momentum alignment studies: H+H2O→OH+H2 versus H+CO2→OH+CO

Using translationally excited H atoms generated by laser photolysis of H2S at 193 nm, the reaction dynamics of H+H2O→OH+H2 and H+CO2→OH+CO was investigated at collision energies of 2.2 and 2.3 eV, respectively. Nascent OH rotational and vibrational quantum-state distributions and OH product translational energies were measured by means of the laser photolysis/laser-induced fluorescence pump-probe technique. A markedly non-statistical distribution of the available energy was found: H+H2O→OH+H2: frot=0.04±0.01, fvib<4×10−3, ftrans=0.65±0.26; H+CO2→OH+CO: frot=0.14±0.02, fvib=0.11 ±0.04, ftrans=0.59±0.16. In addition, velocity-aligned H atoms generated via polarized H2S photodissociation were used to investigate vector correlations for both reactions.

Quantum yield for H atom formation in the methane dissociation after photoexcitation at the Lyman-α (121.6 nm) wavelength

Abstract The gas-phase photodissociation dynamics of methane after excitation at the Lyman-α wavelength (121.6 nm) was investigated under collision-free conditions at room temperature. Narrow-band tunable Lyman-α laser radiation was generated by the resonant third-order sum-difference frequency conversion of pulsed-dye laser radiation and used both to photodissociate the parent molecules and to detect the photolytically produced hydrogen atoms via (2p 2 P → 1s 2 S) laser-induced fluorescence. H atom Doppler profiels were recorded and the absolute quantum yield for H atom formation, Φ H = (0.47 ± 0.11), was determined by means of photolytic calibration method where the Lyman-α photolysis of H 2 O was used as a reference source of well-defined H atom concentrations. This value will be used in combination with previous results from H 2 and CH yield measurements to estimate the relative importance of the different product pathways in the CH 4 photodissociation.

Absorption cross-sections for some atmospherically important molecules at the H atom Lyman-α wavelength (121.567 nm)

Abstract Absolute values of gas-phase absorption cross-sections for some atmospherically important molecules (HCl, H2O, H2S, NH3, H2O2, HNCO, CH4, CH3Cl, CH 2 Cl 2 , CHCl 3 , CHF 2 Cl , CHBr 3 , CH3CF2Cl and CH3CFCl2) have been measured at the H atom Lyman-α wavelength (121.567 nm) employing narrow band (Δλ=0.0006 nm) laser radiation generated by resonant third-order sum-difference frequency conversion. The present values can be used to access the uncertainty associated with Lyman-α absorption cross-section values derived from available literature absorption spectra which usually have been measured with quite different and sometimes with quite low spectral resolutions.