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  • 11
    Electronic Resource
    Electronic Resource
    Photofragmentation of ketene to CH2(~X3B1) + CO. 1. Barrier height and dissociation rate constant (1990)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 94 (1990), S. 263-269 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100364a043
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  • 12
    Electronic Resource
    Electronic Resource
    Vibrational photochemistry of porphine imbedded in a hexane-d14 Shpol'skii matrix (1990)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 94 (1990), S. 7847-7851 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100383a021
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  • 13
    Electronic Resource
    Electronic Resource
    Bond-breaking without barriers. II. Vibrationally excited products (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 1961-1969 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ketene is photolyzed in a supersonic jet, and the vibrationally excited singlet methylene CH2 (a˜ 1A1), produced is detected by laser-induced fluorescence. The appearance thresholds and yield curves of individual methylene rovibrational states are obtained by scanning the photolysis laser wavelength. As observed previously by probing the (0,0,0) state at lower photolysis energies, there are no barriers to dissociation and nuclear spin is conserved. Sharp steps are observed just above the energetic threshold in each of these photofragment excitation (PHOFEX) curves. This suggests that the rotational state distributions are given by phase space theory (PST). The quantum yield of the (0,1,0)101 rovibrational state is measured and the quantum yield for (0,1,0) inferred. These values are larger than predicted by PST, and are close to values predicted by variational Rice–Ramsberger–Kassel–Marcus (RRKM) theory and by the separate statistical ensembles (SSE) method. This indicates that near the (0,1,0) energy threshold the (0,0,0) yield is constrained, as by a tight transition state. The appearance of steps spaced by the energies of a free CO rotor in the PHOFEX curves close to the thresholds of each vibrational state probed indicates that the near threshold flux of vibrationally excited products is controlled by a loose "transition state'' on a vibrationally adiabatic surface. These observations are consistent with the variational RRKM theory for dissociations without barriers in which each product vibrational state evolves on its own vibrationally adiabatic potential surface and has its own transition state. As the energy increases above the threshold for a vibrational state, its transition state moves in along the reaction coordinate and tightens. Thus total rates increase less rapidly with energy than in PST and vibrational distributions are skewed towards higher levels than in PST.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459918
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  • 14
    Electronic Resource
    Electronic Resource
    A transition state theory-based statistical distribution of unimolecular decay rates with application to unimolecular decomposition of formaldehyde (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 5657-5666 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A statistical distribution of state-specific unimolecular decay rates is derived (within the framework of random matrix theory) that is determined completely by the transition state properties of the potential energy surface. It includes the standard χ-square distributions as a special case. Model calculations are presented to show the extent to which it can differ from the χ-square distribution, and specific application is made to the state-specific unimolecular decay rate data for D2CO→D2+CO.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459636
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  • 15
    Electronic Resource
    Electronic Resource
    Photodissociation dynamics of formaldehyde: H2 rotational distributions and product quantum state correlations (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 92 (1990), S. 377-393 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The alignment and rotational state populations of ortho and para H2 (v=1) and H2 (v=3) produced from formaldehyde photolysis near the threshold for dissociation are measured with Doppler-resolved laser-induced fluorescence spectroscopy in the vacuum ultraviolet (VUV). A single rovibronic level of the first excited singlet state of H2 CO in a molecular jet is excited by a pulsed UV laser and the H2 is probed under collisionless conditions with VUV generated by third harmonic generation in Kr or Xe. The rotation of the excited H2 CO during the 10−7–10−8 s before dissociation does not completely wash out the alignment: e.g., photolysis on the 2141 rR0(0)e H2 CO line produces H2 (v=3,J=2) with A(2)0=−0.31±0.06. The ortho and para H2 rotational distributions lie on a single smooth curve peaked at J≈3. The H2 CO(S1 ) vibrational state has little influence on the JH2 distributions, but there is evidence that photolysis of higher rotational states of H2 CO(S1 ) results in warmer JH2 distributions. The Doppler profiles enable the measurement of the H2 (v,J) average velocities. Conservation of energy and linear momentum permit the corresponding CO J state to be calculated. H2 formed in high vibrational states is correlated with CO formed in low rotational states. A three-parameter, semiclassical model which combines an impulsive force with the momenta of zero-point vibrations of the parent near or at the transition state geometry reproduces all of the general features of the H2 (v,J) and CO(v,J) distributions from H2 CO photolysis near the barrier to dissociation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458440
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  • 16
    Electronic Resource
    Electronic Resource
    State-specific unimolecular reaction dynamics of HFCO. I. Dissociation rates (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 1010-1021 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Rovibrationally resolved unimolecular reaction rates of highly vibrationally excited HFCO (S0) are measured and found to be strongly state specific in the energy range of 14 500 cm−1–23 000 cm−1. HFCO molecules are excited to single rovibrational levels in the tunneling region by stimulated emission pumping (SEP) and the dissociation rate of each level is measured by monitoring the temporal evolution of its population by laser-induced fluorescence. The dissociation rates increase by a factor of 10 to 100 or more for small increases in rotational quantum number from J=0 up to J=4 and Ka=2. The dependence on Ka is the strongest. At higher energies, where dissociation lifetimes are shorter than the laser pulse duration, dissociation rates are estimated from the linewidths of well-resolved transition lines measured by high-resolution SEP spectroscopy. In this energy region, dissociation rates are also dependent upon rotational state but much less strongly than in the tunneling region. Vibrational mode specificity in the dissociation rates is observed. For states with approximately the same total energy those with higher excitation in the out-of-plane bending mode (ν6) dissociate more slowly than others. For rotationless levels in the tunneling region, the A' states decay much more rapidly than the A‘ states indicating symmetry-induced mode specificity.The dissociation rates in HFCO almost surely exhibit mode specificity because the ν6 mode is weakly coupled to the reaction coordinate. The enhancement of this coupling by Coriolis forces appears to produce the unprecedentedly large rotational level dependence of the rates. However, the spectroscopically observed coupling of ν6 to the background of vibrationally mixed levels is not significantly increased by rotation. Thus states with nearly all of the excitation energy in the reaction coordinate appear not to be strongly mixed into the background states. The large effect of rotation on the rates thus seems to result from Coriolis coupling of extreme motion in the reaction coordinate to the background levels or directly to ν6. By applying the Rice–Ramsperger–Kassel–Marcus theory to the measured dissociation rates for high rotational states, the barrier height for the molecular dissociation of HFCO to HF+CO is estimated to be 49±4 kcal/mole.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463280
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  • 17
    Electronic Resource
    Electronic Resource
    Kinetics of intramolecular carbon atom exchange in ketene (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 95 (1991), S. 4081-4093 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Intramolecular carbon atom exchange in highly vibrationally excited ketene was studied by monitoring the carbon monoxide fragments (12CO and 13CO ) from the photodissociation of 12CH213CO and 13CH212CO. Two experimental techniques were employed. In one set of experiments the IR transient absorptions of 12CO and 13CO were measured following pulsed excimer excitation of ketene 13C isotopomers, giving carbon atom exchange yields at 351 and 308 nm in a low pressure gas cell. In the other set of experiments, jet-cooled ketene 13C isotopomers were excited with tunable near-UV radiation, and the CO products were detected by monitoring their VUV laser-induced fluorescence. Carbon atom exchange yields were measured for energies extending from below the triplet decomposition threshold (CH2CO→CH2(X 3B1)+CO(X 1Σ+)) to about 4000 cm−1 above the singlet threshold (CH2CO→CH2(a 1A1)+CO(X 1Σ+)).The exchange yields range from 4 to 19%, and the energy dependence of the yield exhibits pronounced structure, with maxima at the triplet and singlet decomposition thresholds. Kinetic measurements of the appearance of the CO products were also performed. The time constant for the appearance of the exchanged CO (e.g., 13CO from 13CH212CO ) is significantly longer than that for the direct CO fragment (e.g., 12CO from 13CH212CO ). All the experimental observations are consistent with a simple reaction mechanism involving ketene isomerization, 13CH212CO(arrow-right-and-left)12CH213CO, and dissociation, 13CH212CO→13CH2+12CO and 12CH213CO→12CH2+13CO. The isomerization rate constant was determined by analyzing the CO kinetics and the carbon atom exchange yields in terms of the simple isomerization mechanism. A fit of the energy dependence of the isomerization rate constant to the results of tunneling-corrected Rice–Ramsberger–Kassel–Marcus (RRKM) calculations gave the threshold (28360±60 cm−1 ) for the isomerization process.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460764
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  • 18
    Electronic Resource
    Electronic Resource
    Quasistable extreme motion vibrational states of HFCO above its dissociation threshold (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 5414-5425 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The anharmonic mixing of highly excited vibrational states of HFCO above its dissociation threshold is studied by stimulated emission pumping spectroscopy. At 0.05-cm−1 resolution, individual molecular eigenstates were resolved and state mixings observed by the distribution of oscillator strength to nearby dark states of the background. Most of the zero-order vibrational levels observed in the energy range between 13 000 and 23 000 cm−1 are assigned to long Franck–Condon-allowed progressions of extremely high overtones of the C–H out-of-plane bending mode (ν6) in combination with the C(large-closed-square)O stretching mode (ν2). The extent of state mixing of highly excited vibrational states is strongly mode dependent. For vibrational states with almost the same total vibrational energy, states with the most quanta in ν6 show the least coupling with other states. More strikingly, as the total energy is increased by adding ν6 quanta, the state mixing becomes weaker. For the vibrational states observed above 18 000 cm−1, those that have all of the energy in ν6 or at most one quantum of excitation in ν2 are extremely stable against state mixing. These appear to be the regular states or quasiperiodic trajectories predicted in some theoretical studies of two-dimensional systems. Extreme motion of the C–H out-of-plane bending mode seems to localize and prevent coupling with other modes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.460503
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  • 19
    Electronic Resource
    Electronic Resource
    Determination of the singlet/triplet branching ratio in the photodissociation of ketene (1991)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 94 (1991), S. 1954-1960 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The rotational distributions of CO products from the dissociation of ketene at photolysis energies 10 cm−1 below, 56, 110, 200, 325, 425, 1107, 1435, 1720, and 2500 cm−1 above the singlet threshold (30 116.2 cm−1 ), are measured in a supersonic free jet of ketene. The CO(v‘=0) rotational distributions at 56, 110, 200, 325, and 425 cm−1 are bimodal. The peaks at low J's, which are due to CO from the singlet channel, show that the product rotational distribution of CO product from ketene dissociation on the singlet surface is well described by phase space theory (PST). For CO(v‘=0) rotational distributions at higher excess energies (1107, 1435, 1720, and 2500 cm−1 ), the singlet and triplet contributions are not clearly resolved, and the singlet/triplet branching ratios are estimated by assuming that PST accurately predicts the CO rotational distribution from the singlet channel and that the distribution from the triplet channel changes little from that at 10 cm−1 below the singlet threshold. The singlet yield shows a rapid increase in the low excess energy region (0–300 cm−1 ), and a slower increase above. The singlet and triplet rate constants are derived from the directly measured total rate constants using the singlet yields. The triplet rate constant increases monotonically with increasing photolysis energy through the singlet threshold region. The singlet rate constant is accurately established in the threshold region and found to increase much less rapidly than predicted by phase space theory. At 2500 cm−1 excess energy, the CO(v‘=1) rotational distribution is obtained, and the ratio of CO(v‘=1) to CO(v‘=0) products for the singlet channel is measured to be 0.045±0.017. This ratio is close to the variational Rice–Ramsberger–Kassel–Marcus (RRKM) calculation 0.038, and the separate statistical ensembles (SSE) prediction 0.041, but much greater than the PST prediction, 0.016.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459917
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  • 20
    Electronic Resource
    Electronic Resource
    Photodissociation dynamics of formaldehyde: H2 (v,J) vector correlations (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 3907-3918 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Several bipolar moments of the H2 (v,J) correlated angular momentum and velocity distribution produced from the photodissociation of formaldehyde near the threshold for dissociation have been measured by analysis of Doppler-resolved LIF line shapes. It is determined that the fragment H2 〈v⋅J〉 correlation is not at the limit of v⊥J, but is closer to the limit of v⊥J than to v(parallel)J. The rotation of the excited H2CO during the 10−7–10−8 s before dissociation does not completely wash out the lab-frame vector correlations. Anisotropy parameters as large as 0.85 and as small as −0.41 have been measured; these are outside the limits imposed by classical models of parent rotation. A quantum mechanical model for parent rotation is introduced that accounts for the large magnitude of the measured anisotropy parameters. Photolysis on the rR0(0) line of the 43 band produces fragments with β〈0 while photolysis on the same rotational transition of the 2141 band produces fragments with β〉0. It is not known if the different anisotropies are caused by differences in the parent transition dipole moment or by differences in the dissociation dynamics. The simple impulsive model that reproduces the fragment rotational distributions and product quantum-state correlations does not adequately describe the measured H2 (v,J) vector correlations.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.458777
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