• Hans-Robert Volpp

      Articles written in Journal of Chemical Sciences

    • Absolute quantum yield measurements for the formation of oxygen atoms after UV laser excitation of SO2 at 222-4 nm

      Mohammed Abu-Bajeh Melanie Cameron Kyung-Hoon Jung Christoph Kappel Almuth Läuter Kyoung-Seok Lee Harip Upadhyaya Rajesh K Vatsa Hans-Robert Volpp

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      The dynamics of formation of oxygen atoms after UV photoexcitation of SO2 in the gas-phase was studied by pulsed laser photolysis-laser-inducedfluorescence ‘pump-and-probe’ technique in a flow reactor. SO2 at room-temperature was excited at the KrCl excimer laser wavelength (222.4 nm) and O(3Pj) photofragments were detected under collision-free conditions by vacuum ultraviolet laser-induced fluorescence. The use of narrow-band probe laser radiation, generated viaresonant third-order sum-difference frequency conversion of dye laser radiation in Krypton, allowed the measurement of the nascent O(3Pj=2,1,0) fine-structure state distribution:nj=2/nj=1/nj=0 = (0.88 ± 0.02)/(0.10 ± 0.01)/(0.02 ± 0.01). Employing NO2photolysis as a reference, a value of Φ0(3P) = 0.13 ± 0.05 for the absolute O(3P) atom quantum yield was determined. The measured O(3P) quantum yield is compared with the results of earlier fluorescence quantum yield measurements. A suitable mechanism is suggested in which the dissociation proceeds via internal conversion from high rotational states of the initially excited SO2(~C1B2 (1, 2, 2) vibronic level to nearby continuum states of the electronic ground state.

    • Lyman-𝛼 photodissociation of CH3CFCl2 (HCFC-141b): Quantum yield and translational energy of hydrogen atoms

      Almuth Laeuter Hans-Robert Volpp Jai P Mittal Rajesh K Vatsa

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      The collision-free, room temperature gas-phase photodissociation dynamics of CH3CFCl2 (HCFC-141b) was studied using Lyman-𝛼 laser radiation (121.6 nm) by the laser photolysis/laserinduced fluorescence `pump/probe’ technique. Lyman-𝛼 radiation was used both to photodissociate the parent molecule and to detect the nascent H atom products via ($2p^2$P → $1s^2$S) laser-induced fluorescence. Absolute H atom quantum yield, $\phi_H = (0.39 \pm 0.09)$ was determined by calibration method in which CH4 photolysis at 121.6 nm was used as a reference source of well-defined H atom concentrations. The line shapes of the measured H atom Doppler profiles indicate a Gaussian velocity distribution suggesting the presence of indirect H atom formation pathways in the Lyman-𝛼 photodissociation of CH3CFCl2. The average kinetic energy of H atoms calculated from Doppler profiles was found to be $E_{\text{T(lab)}} = (50 \pm 3)$ kJ/mol. The nearly statistical translational energy together with the observed Maxwell-Boltzmann velocity distribution indicates that for CH3CFCl2 the H atom forming dissociation process comes closer to the statistical limit.

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