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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 6560-6569 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The recombination O+O2+M→O3+M in the bath gases M=He, Ar, and N2 was studied over the temperature range 90–370 K and the pressure range 1–1000 bar. The temperature and pressure dependences of the reaction rates show an anomalous behavior which is attributed to superpositions of mechanisms involving energy transfer, complex formation and participation of weakly bound electronically excited O3 states. The results also show an analogy to oxygen isotope enhancements observed in ozone recombination and dissociation. Experiments in compressed liquid N2 were also made showing a transition to diffusion control.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 7030-7039 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Modified neglect of differential overlap-Configuration interaction (MNDO-CI) calculations of the low-lying electronically excited states and the electronic ground state of stilbene have been performed. The dependences of the potential energy on the angle of rotation around the central ethylene bond, on the length of this bond, and on the twist angle of the two phenyl groups are explored. There is no evidence for a nonadiabatic participation of the doubly excited "phantom state'' in the photoisomerization dynamics. Instead, the calculated properties of the singly excited 1 1B state support a mechanism with adiabatic rotation around the central ethylene bond to the perpendicular conformation, followed by internal conversion. There appears to be a small "CI-induced'' energy barrier along this pathway which will be overcome by the combined motion in several coordinates. Therefore, a complicated activated complex structure arises.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 3867-3874 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Collisional energy transfer between vibrationally highly excited CS2 molecules and 22 different collision partners has been measured directly by time resolved UV absorption spectroscopy after laser excitation. Experiments at temperatures up to 1200 K were performed in a reactor heated by cw CO2 laser radiation. Experiments at higher temperatures (up to 2400 K) with the collision partner Ar were done with pulsed laser excitation behind reflected shock waves. At all temperatures a marked dependence of the average energy 〈ΔE〉 transferred per collision on the excitation energy is noted. Also the molecular size of the deactivating molecule has a large influence on 〈ΔE〉. The temperature coefficients of 〈ΔE〉∝Tn are small in all cases. They vary between n=−0.6 for n-heptane and n=+1.0 for Ne.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 6171-6182 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Detailed and simplified statistical adiabatic channel calculations of specific rate constants k(E,J) and product quantum state distributions for the simple bond fission reaction HOOH→2 OH are compared with recent measurements of state-resolved dissociation rates, product state distributions, and thermally averaged rate coefficients. A simple modification of phase space theory based on the statistical adiabatic channel model successfully predicts product state distributions and rate constants as well. Because of the amount of experimental data and theoretical analysis available, the dissociation of hydrogen peroxide is becoming a model case for simple unimolecular bond fission processes.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 6594-6598 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Correlation diagrams for adiabatic channels between free atoms and linear molecules (both in nondegenerate electronic states) and the corresponding reaction complexes are constructed. The accurate calculation of adiabatic channel potential curves is discussed. The magnitude of Coriolis coupling terms is illustrated.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 3564-3570 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Repetitive absorption of UV (or visible) photons followed by fast internal conversions allows to deposit large amounts of vibrational energy in polyatomic molecules. Intra- and intermolecular processes with relatively weak energy dependences can be studied by this excitation over broad energy ranges. The technique is illustrated for the unimolecular isomerization azulene → naphthalene and collisional energy transfer of vibrationally excited azulene. With ns excimer laser pulses, up to three photons at λ=308 nm were deposited in the molecule, and collisional quenching competing with isomerization was studied. The results can be modeled conveniently by a multistep mechanism.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 1755-1760 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The HO2 self-reaction HO2+HO2→H2O2+O2 was studied in shock waves between 750 and 1120 K. Two different HO2 sources were used, the thermal dissociation of CH3O2CH3 in the presence of excess O2 and the thermal dissociation of H2O2. The reaction is characterized by a curved Arrhenius plot of the rate constant k. The combination with low temperature data shows a deep minimum of k near 700 K. The rate constant, at 1 bar gas pressure, between 300 and 1100 K can be represented as k/cm3 mol−1 s−1=4.2×1014 exp(−6030 K/T) +1.3×1011 exp(+820 K/T). The first term in the present work was derived from measured ratios of k/ε and was evaluated with ε (HO2, 230 nm, 1000 K)=670 1 mol−1 cm−1. Furthermore, a value of the rate constant of 3×1012 cm3 mol−1 s−1 was derived near 1100 K for the reaction HO+H2O2→HO2+H2O; for the reaction HO+HO2→H2O+O2 a value of 3×1013 cm3 mol−1 s−1 was obtained.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 2393-2404 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The pressure and temperature dependence of the photoisomerization rate coefficient of trans-stilbene in the S1 state have been measured in the solvents C2H6, C3H8, C4H10, Xe, Co2, SF6, and CHF3. At constant temperature, the pressure dependences up to 6 kbar can be well represented by the Kramers–Smoluchowski model. The comparison of results in different solvents clearly indicates the importance of reactant–solvent cluster formation modifying the height and imaginary frequency of the barrier. The change of the temperature dependence with pressure points towards a multidimensional barrier of nonseparable character. Multidimensional barrier effects manifest themselves most clearly via the temperature dependence of the rate coefficient in the Kramers–Smoluchowski limit.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 4805-4816 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photoisomerization of diphenylbutadiene was studied by picosecond absorption spectroscopy over wide pressure and temperature ranges in liquid and supercritical alkanes, CO2, SF6, and He. The reaction shows typical features of a thermal unimolecular reaction on the S1 potential energy surface. The rate can be expressed by a combination of standard unimolecular rate theory and Kramers–Smoluchowski theory. However, multidimensional behavior manifests itself in the transition to the gas phase low pressure range as well as to the high density Kramers–Smoluchowski range: in the former case, the low pressure limit of a unimolecular reaction of the polyatomic molecule is approached; in the latter case, the effective imaginary barrier frequency shows a marked apparent temperature dependence. The experiments also suggest contributions of reactant–solvent cluster interactions, which modify the barrier height even in nonpolar solvents.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 890-899 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Transient UV absorption spectra of CF3I were recorded during the IR multiphoton excitation of this molecule. Short laser pulses (20–50 ns) without longer tails were employed. Pressure effects were analyzed in the 10–100 mTorr range. Unimolecular dissociation rates (in the 106–107 s−1 range) after the pulse were identified unambiguously by time-resolved low-pressure observations and collisional quenching experiments. Marked laser intensity effects on the rates are attributed to intensity-dependent changes of rotational state distributions of the reacting molecules. Excited state distributions have been resolved in direct observations. Bimodal distributions were identified.
    Type of Medium: Electronic Resource
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