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  • 1995-1999  (44)
  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 10395-10398 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The reaction between ground state carbon atoms, C(3Pj), and acetylene, C2H2(1∑+g), was studied at an average collision energy of (8.4±0.3) kJ mol−1 using the crossed molecular beam technique. The product angular distribution and time-of-flight spectra of m/z=37, i.e., C3H, were recorded. Only m/z=37 was detected, but no signal from the thermodynamically accessible C3(1∑+g)+H2(1∑+g) channel. Forward-convolution fitting of the results yielded a center-of-mass angular flux-distribution forward scattered in respect to the carbon beam, whereas the translational energy flux distribution peaked at only (5.4±1.2) kJ mol−1, suggesting a simple C–H-bond-rupture to H+C3H. The reaction likely proceeds on the triplet surface with an entrance barrier to the C3H2–PES of 〈(8.4±0.3) kJ mol−1 via addition of the carbon atom to two bonding π-orbitals located both at C1 or at C1 and C2 of the acetylene molecule. The explicit identification of C3H product under single collision conditions strongly demands incorporation of atom-neutral reactions in reaction networks simulating chemistry in the interstellar medium, in interstellar shock waves, and in outflows of carbon stars. © 1995 American Institute of Physics.
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  • 2
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 6290-6292 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Photodissociation of 1,4 diazine (pyrazine) following 248 nm (S2←S0) and 308 nm (S1←S0) excitation has been studied by photofragment translational spectroscopy. While the latter wavelength was inefficient at generating reactive products, the former wavelength afforded a rich variety of fragments, the most translationally energetic being HCN carrying up to 36 kcal/mol. The implications of this reactive mechanism toward interpretation of vibrational energy transfer experiments from hot ground state pyrazine are considered. The absolute quantum yields φ(248 nm, 27-A)=0.40±0.10, φ(248 nm, 27-B)=0.08±.02, φ(308 nm, 27-B′)=0.003±.001 and the two corresponding translational energy distributions are presented. © 1995 American Institute of Physics.
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  • 3
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 6067-6077 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photodissociation of ozone has been studied at 193 nm using high resolution photofragment translational spectroscopy. The results show six distinct peaks in the time-of-flight spectra for the O2 product and its momentum-matched O atom counterpart. The translational energy distributions determined from the time-of-flight spectra reveal the production of a range of electronic states of the photofragments. The product electronic states were identified based on the translational energy distributions, with the aid of state-resolved imaging experiments by Houston and co-workers. The results reveal the production of a substantial yield of highly excited triplet states of O2, recently suggested to play an important role in the stratospheric ozone balance. In addition, peaks corresponding to O2(a 1Δg) and O2(b 1Σg+) were observed, the latter confirming a previous report [A. A. Turnipseed et al., J. Chem. Phys. 95, 3244 (1991)]. Evidence was seen for a small contribution from the triple dissociation O3→3O(3P), and insight into the dissociation dynamics for this process was inferred from the translational energy distributions. Branching fractions and angular distributions were measured for all channels. The latter were found in general to yield negative β parameters, in contrast to what is seen at longer wavelengths. © 1995 American Institute of Physics.
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 5279-5279 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: © 1999 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 100-107 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photodissociation dynamics of furan at 193 nm have been studied using photofragment translational spectroscopy with tunable vacuum ultraviolet (VUV) probe provided by synchrotron radiation on the Chemical Dynamics Beamline at the Advanced Light Source. Three primary channels are observed: HCO+C3H3, CO+C3H4, and H2CCO+C2H2. The evidence suggests that the two closed-shell channels occur on the ground-state potential energy surface (PES) following internal conversion, while the radical channel likely takes place on an excited PES. All channels exhibit a barrier for dissociation with the acetylene+ketene channel having the largest value at about 25 kcal/mol. Angular distribution measurements show anisotropy only for the radical channel. These findings are consistent with a rapid excited state dissociation for the radical channel and slow dissociation for the other two pathways. The two ground-state dissociation channels—propyne+CO and acetylene+ketene—should be important in the thermal decomposition of furan as was found in pyrolytic studies [A. Lifshitz, M. Bidani, and S. Bidani, J. Phys. Chem. 90, 5373 (1986)] and theoretical investigations [R. Liu, X. Zhou, and L. Zhai, J. Comput. Chem. 19, 240 (1998)]. © 1999 American Institute of Physics.
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  • 6
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 4363-4368 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photodissociation dynamics of propyne and allene are investigated in two molecular beam/photodissociation instruments, one using electron impact ionization and the other using tunable vacuum ultraviolet (VUV) light to photoionize the photoproducts. The primary dissociation channels for both reactants are C3H3+H and C3H2+H2. Measurement of the photoionization efficiency curves on the VUV instrument shows that the C3H3 product from propyne is the propynyl (CH3CC) radical, whereas the C3H3 product from allene is the propargyl (CH2CCH) radical. The dominant C3H2 product from both reactants is the propadienylidene (H2CCC) radical. We also observe a small amount of secondary C3H2 product from photodissociation of the C3H3 radicals in both cases. © 1999 American Institute of Physics.
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  • 7
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 7202-7208 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Photofragment translational spectroscopy was used to examine the infrared multiphoton dissociation of octafluoro-1-butene and octafluoro-2-butene. The predominant unimolecular reaction in octafluoro-1-butene at moderate laser fluences is cleavage of a carbon–carbon single bond to give the products CF3 and C3F5. The two other reactions that take place are CF2 elimination and the formation of equal weight fragments with the chemical composition C2F4; both reactions take place via a diradical intermediate. Dissociation of octafluoro-1-butene to the resonance stabilized perfluoroallyl radical is suggested to account for the favoring of simple bond rupture. These three reaction pathways were also observed in octafluoro-2-butene dissociation, however, the branching fraction is different than from octafluoro-1-butene. In octafluoro-2-butene all three channels occur with roughly equal probability. The reactions involving CF2 loss and C2F4 formation in octafluoro-2-butene are thought to proceed through the same diradical intermediate as in octafluoro-1-butene, necessitating a 1,2-fluorine migration. © 1997 American Institute of Physics.
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  • 8
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 5372-5382 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Photodissociation of the allyl radical, CH2CHCH2, has been studied using the method of molecular beam photofragment translational spectroscopy following excitation to the C˜(2 2B1) and A˜(1 2B1) states by 248 and 351 nm photons. Two different primary channels have been detected following 248 nm excitation: H-atom loss (84%) and CH3 elimination (16%). From the product translational energy distribution and polarization dependence studies, dissociation processes from the ground-state C3H5 potential energy surface are inferred for both wavelengths. At 248 nm there may also be a contribution to the H-atom loss channel from predissociation by a higher electronically excited state. Rice–Ramsperger–Kassel–Marcus (RRKM) calculations show that the formation of cyclopropene is not important, while formation of allene and methylacetylene from dissociation of 1- and 2-propenyl radicals are important reaction pathways at both wavelengths. Translational energy distributions peaking well away from zero provide evidence for CH3 elimination directly from an allylic structure through a four-member cyclic transition state. © 1998 American Institute of Physics.
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  • 9
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 109 (1998), S. 5338-5343 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present a crossed molecular beam study of the dynamics of the metathesis reaction Cl+n-C5H12→HCl+C5H11 at a collision energy of 16.8 kcal/mol. The experiments were conducted on the Chemical Dynamics Beamline at the Advanced Light Source, using tunable undulator radiation to effect soft ionization of the pentyl radical product. Laboratory angular distributions and time-of-flight spectra at many laboratory angles were used to obtain the center-of-mass translational energy and angular distributions. These distributions were found to be strongly coupled, with the forward scattered pentyl radical formed extremely cold, while the backscattered radicals were formed leaving nearly 15 kcal/mol in internal energy in the products. These results are contrasted with recent studies on the analogous reaction with propane, suggesting in this case direct involvement of the carbon skeleton in the collision process. © 1998 American Institute of Physics.
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  • 10
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 7617-7624 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photodissociation of oxalyl chloride, (ClCO)2, has been studied near 235 nm using the photofragment imaging technique. Observed products include both ground state Cl (2P3/2) and spin-orbit excited Cl*(2P1/2) chlorine atoms and ground electronic state CO molecules. The rotational distribution obtained for the CO v=0 product is peaked at about J=30 and extends beyond J=50. Photofragment images were recorded for both chlorine atom fine structure components as well as many rotational levels of the CO v=0, yielding state-resolved angular and translational energy distributions. The recoil speed distribution for the Cl* exhibits a dominant fast component, with a translational energy distribution peaking at about 48 kJ/mol. The ground state chlorine atom showed two components in its speed distribution, with the slow component dominant. The corresponding translational energy distribution peaked at 10 kJ/mol but extended to 80 kJ/mol. The total average translational energy release into the Cl product is 34 kJ/mol. Similarly, the low rotational levels of the CO showed only a slow component, the intermediate rotational levels showed a bimodal speed distribution, and the highest rotational levels showed only the fast component. The fast components of both chlorine atom product and the higher rotational levels of the CO show an anisotropic angular distribution, while all slow fragments show a nearly isotropic angular distribution. These observations suggest a novel dissociation mechanism in which the first step is an impulsive three-body dissociation yielding predominantly Cl*, rotationally excited CO and chloroformyl radical ClCO, with only modest momentum transfer to the latter species. Most of the remaining ClCO undergoes subsequent dissociation yielding low rotational levels of CO and little translational energy release. © 1997 American Institute of Physics.
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