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  • 1
    Electronic Resource
    Electronic Resource
    Femtosecond spectroscopy of the dissociation and geminate recombination of aqueous CS2 (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 703-710 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photolysis of aqueous CS2 has been studied using subpicosecond transient absorption spectroscopy. CS2 was photolyzed at 6.2 eV and the fast formation and decay of the photoproducts were monitored from 6.2 to 3.96 eV. Upon excitation, aqueous CS2 dissociates into CS+S. However, 93%±2% of the fragments geminately recombine on the electronic ground state potential of CS2 within a few picosecond leaving only 7%±2% of the CS+S fragments separated 100 ps after the excitation. In the gas phase, most of the dissociation occurs on a triplet-state potential surface, and the high recombination yield observed in aqueous solution therefore indicates a strong, solvent-assisted coupling between this state and the singlet ground state of CS2. The vibrationally excited CS2 molecule formed by the recombination transfers its high excess energy to the surrounding water molecules in two processes with time constants 8.4±1 and 33±7 ps. The rotational reorientation time of ground state CS2 is 6±1 ps, suggesting a surprisingly strong interaction between the neutral, nonpolar CS2 and the surrounding water molecules. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479364
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  • 2
    Electronic Resource
    Electronic Resource
    Two-photon dissociation and ionization of liquid water studied by femtosecond transient absorption spectroscopy (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 3453-3462 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photodissociation and photoionization of liquid water following two-photon absorption at 266 nm is studied in the spectral range from 213 to 1108 nm with subpicosecond time resolution. Probing in the UV enables the first direct simultaneous observation of the photoproducts eaq−, Haq, and OHaq. This makes it possible to follow the geminate recombination kinetics between the photoproducts and to determine the relative yields of the dissociation and ionization channels. The concentration of hydrated electrons deduced from the visible and near-infrared transient absorption measurements decays by 40%±2% within the first 90 ps due to recombination with OHaq and H3O+. Analyzing our measurements of the hydrated electron concentration using the independent reaction time approximation results in the relative yields of 82%±3% and 18%±3% for recombination with OHaq and with H3O+, respectively. This is in excellent agreement with the relative yield of 82%±10% for recombination with OHaq determined directly from our ultraviolet transient absorption measurements. The contribution of hydrated electrons from direct ionization is insignificant when liquid water is excited below 9.32 eV and the ionization is likely to occur via dissociation and proton transfer. The transient ultraviolet absorption data shows that if ionization exclusively occurs via dissociation, 65% of the produced H(hot) atoms react with the surrounding solvent molecules to produce hydrated electrons. If proton transfer, on the other hand, is the only process responsible for the ionization, our measurements show that the ratio between dissociation and ionization is 55%. Geminate recombination of OHaq and Haq fragments following the photodissociation at 9.32 eV is not observed indicating that the translation energy of at least one of the fragments is sufficient to penetrate the water solvent cage. Finally, we have measured the two-photon absorption spectrum of liquid water from 110 to 160 nm, and the spectrum is in good agreement with our ab initio gas-phase calculations of the two-photon absorption cross sections for the transitions involved. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478212
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  • 3
    Electronic Resource
    Electronic Resource
    Vibrational relaxation of ClO2 in water (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 8461-8471 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Photodissociation of ClO2 in aqueous solution at 400 nm results in the formation of ClO+O and Cl+O2. ClO and O geminately recombine to ClO2 in the electronic ground state (2B1), formed with an initial vibrational energy of (approximate)2.5 eV. In this paper the vibrational relaxation of ClO2(2B1) in aqueous solution is studied by femtosecond transient absorption spectroscopy in the spectral range 234 to 1024 nm. The measured transient absorption of the vibrationally relaxing ClO2 molecules is compared with the transient absorption calculated for relaxation in the asymmetric stretch as well as the symmetric stretch and bending modes. The calculations of the absorption spectra pertaining to the asymmetric stretch are based on a harmonic potential derived from the experimentally determined fundamental vibrational energy, whereas that of the symmetrical vibrations are based on ab initio potentials. An excellent agreement is obtained by assuming that the vibrational relaxation predominantly occurs in the asymmetric stretch with a 9.5 ps relaxation time. A weak spectral feature in the ultraviolet part of the spectrum is assigned to vibrational relaxation in the symmetric stretch and bending modes, indicating a coupling between the asymmetric and symmetric modes. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476274
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  • 4
    Electronic Resource
    Electronic Resource
    Temperature dependent relaxation and recombination dynamics of the hydrated electron (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 1126-1134 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The ultrafast solvation and recombination dynamics of the hydrated electron generated by two-photon ionization of water at 4.65 eV is studied by transient absorption spectroscopy as a function of temperature in the range from 277 K to 355 K. The part of the spectral blue shift which is observed in the absorption spectrum of the hydrated electron after 1 ps is purely continuous and is accurately described by the well known analytical expression for the temperature dependent absorption spectrum of the ground state hydrated electron. This indicates that thermal relaxation or more likely solvation of the hydrated electron predominantly causes the blue shift. The survival probability of the hydrated electron shows a strong temperature dependence, which is satisfactory explained by the temperature dependent mobility and reaction rates of the species involved in the recombination. This implies that the average initial separation between the hydrated electron and the ionization site of 〈r0〉=1.0±0.1 nm does not depend significantly on the bulk water temperature. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481891
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  • 5
    Electronic Resource
    Electronic Resource
    Femtosecond photolysis of aqueous HOCl (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 9361-9369 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: This paper reports an experimental study of the photolysis of aqueous HOCl using femtosecond pulses at 266 nm. The formation of photoproducts is monitored by transient absorption spectroscopy from 230 to 400 nm. The HOCl molecules dissociate with unity quantum yield to form OH+Cl faster than 1 ps, and as a result of the potential along the HO–Cl reaction coordinate, all excess energy is given to the fragments as translational energy. After dissociation, and solvent cage escape, the majority of the Cl and OH fragments recombine after diffusion on a time scale of 50 ps. The diffusion dynamics is studied using a simple model for diffusive recombination and a more extensive molecular dynamics simulation. A minor fraction of the Cl atoms (∼10%) reacts with HOCl in a diffusion limited reaction to form Cl2+OH. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1413964
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  • 6
    Electronic Resource
    Electronic Resource
    Vibrational relaxation of aqueous CS2 (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 4099-4106 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Vibrational relaxation of aqueous CS2 formed by geminate recombination of CS+S after photolysis of CS2(aq) at 200 nm is studied by subpicosecond transient absorption spectroscopy. Comparison of the transient absorption measured from 200 to 313 nm with calculated absorption transients pertaining to vibrationally relaxing CS2 shows that relaxation to the vibrational ground state of CS2 predominantly occurs in the asymmetric stretch on a 5 ps time scale. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1344611
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  • 7
    Electronic Resource
    Electronic Resource
    Third harmonic generation in standard and erbium-doped fibers
    Amsterdam : Elsevier
    Optics Communications 110 (1994), S. 435-444 
    Details
    ISSN: 0030-4018
    Source: Elsevier Journal Backfiles on ScienceDirect 1907 - 2002
    Topics: Physics
    Type of Medium: Electronic Resource
    URL: http://linkinghub.elsevier.com/retrieve/pii/0030-4018(94)90447-2
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  • 8
    Electronic Resource
    Electronic Resource
    Bio-stabilization of drinking water in supply systems as observed in the tropics (1996)
    Springer
    World journal of microbiology and biotechnology 12 (1996), S. 543-547 
    Details
    ISSN: 1573-0972
    Keywords: Biofilm ; bio-stabilization ; drinking water ; growth modelling ; regrowth ; water storage
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Abstract Changes in the bacterial quality of drinking water were examined in the supply system of tropical Dar es Salaam, Tanzania. The water treatment fulfilled WHO standard guidelines. However, the water quality allowed for significant regrowth of bacteria, demonstrating that the WHO guidelines are insufficient to check whether or not the drinking water has an acceptable regrowth potential. An experimental pipe system was examined on site. A growth model was established, based on a zero order bacterial detachment kinetics for the biofilms colonizing the inner surface of the pipes, with a release rate factor R, and a first order growth kinetics in the water flowing in the pipes with a growth rate factor μ. The rate factors were estimated to be R=1.9×108 m-2 h-1 and μ=2.5 h-1. It was concluded that growth in the biofilm played a dominant role in the bio-stabilization processes, while the initial microbial quality of the water works water and the growth in the water phase were of minor importance. The bio-stabilization process is understood herein as the reduction in the concentration of nutrients and assimilable organic carbon in the water, and thus as the reduction of the growth potential of the water. Bacterial growth and recontamination were examined during storage at the user site. The results showed that heterotrophic bacteria grew, while faecal coliforms decayed during storage. It was concluded that the users handled the water hygienically.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00419470
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  • 9
    Electronic Resource
    Electronic Resource
    Chlorine decay and bacterial inactivation kinetics in drinking water in the tropics (1996)
    Springer
    World journal of microbiology and biotechnology 12 (1996), S. 549-556 
    Details
    ISSN: 1573-0972
    Keywords: Bacteria inactivation ; chlorine decay ; combined chlorine ; drinking water ; free chlorine ; kinetics
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology , Process Engineering, Biotechnology, Nutrition Technology
    Notes: Abstract The decay of free chlorine (Cl2) and combined chlorine (mostly monochloramine: NH2Cl) and the inactivation of bacteria was examined in Dar es Salaam, Tanzania. Batch experiments, pilot-scale pipe experiments and full-scale pipe experiments were carried out to establish the kinetics for both decay and inactivation, and to compare the two disinfectants for use under tropical conditions. The decay of both disinfectants closely followed first order kinetics, with respect to the concentration of both disinfectant and disinfectant-consuming substances. Bacterial densities exhibited a kinetic pattern consisting of first order inactivation with respect to the density of the bacteria and the concentration of the disinfectant, and first order growth with respect to the bacterial density. The disinfection kinetic model takes the decaying concentration of the disinfectant into account. The decay rate constant for free chlorine was 114 lg-1h-1, while the decay rate constant for combined chlorine was 1.84 lg-1h-1 (1.6% of the decay rate for free chlorine). The average concentration of disinfectant consuming substances in the water phase was 2.6 mg Cl2/l for free chlorine and 5.6 mg NH2Cl/l for combined chlorine. The decay rate constant and the concentration of disinfectant consuming substances when water was pumped through pipes, depended on whether or not chlorination was continuous. Combined chlorine especially could clean the pipes of disinfectant consuming substances. The inactivation rate constant λ, was estimated at 3.06×104 lg-1h-1. Based on the inactivation rate constant, and a growth rate constant determined in a previous study, the critical concentration of free chlorine was found to be 0.08 mg Cl2/l. The critical concentration is a value below which growth rates dominate over inactivation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1007/BF00419471
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