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  • 1
    Electronic Resource
    Electronic Resource
    Iodine-fluorine bond strength in IIF, ClIF, and HIF (1976)
    s.l. : American Chemical Society
    Journal of the American Chemical Society 98 (1976), S. 853-854 
    Details
    ISSN: 1520-5126
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/ja00419a045
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  • 2
    Electronic Resource
    Electronic Resource
    State-to-state dynamics of H+HX collisions. I. The H+HX→H2+X (X=Cl,Br,I) abstraction reactions at 1.6 eV collision energy (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 4795-4808 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The rotational and vibrational state distributions of the H2 product from the reactions of translationally excited H atoms with HCl, HBr, and HI at 1.6 eV are probed by coherent anti-Stokes Raman scattering spectroscopy after only one collision of the fast H atom. Despite the high collision energy, only the very exoergic (ΔH=−1.4 eV) hydrogen atom abstraction involving HI leads to appreciable H2 product vibrational excitation. For this reaction the H2 vibrational distribution is strongly inverted and peaks in v'=1, with 25% of the total available energy partitioned to vibration. For the mildy exoergic (ΔH=−0.72 eV) reaction with HBr and the nearly thermoneutral (ΔH=−0.05 eV) reaction with HCl, very little energy appears in H2 vibration, 9% and 2%, respectively, and the vibrational state distributions peak at v'=0. However, in all three reactions a significant fraction, 18% to 21%, of the total energy available appears as H2 rotation. All three reactions show a strong propensity to conserve the translational energy, that is the translational energy of the H2+X products is very nearly the same as that of the H+HX reactants. For the reactions with HCl, HBr, and HI the average translational energy of the products is 1.3, 1.7, and 1.7 eV, respectively, and the width of the translational energy distribution is only about 0.5 eV full width at half maximum. The energy disposal in all three reactions is quite specific, despite the fact that this high collision energy is well above the barrier to reaction in all three systems and a large number of product quantum states are energetically accessible. Only a few of these energetically allowed final states are appreciably populated. Although detailed theoretical calculations will be required to account completely for the state specifity, quite simple models of the reaction dynamics can explain much of the dynamical bias that we observe.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456574
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  • 3
    Electronic Resource
    Electronic Resource
    State-to-state dynamics of H+HX collisions. II. The H+HX→HX°+H (X=Cl,Br,I) reactive exchange and inelastic collisions at 1.6 eV collision energy (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 4809-4818 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report measurement of product state distributions for the rotationally and/or vibrationally excited HX formed in collisions of translationally hot H atoms with HX (X=Cl, Br, and I) at 1.6 eV collision energy. The product state distributions are probed after only one collision of the fast H atom, using coherent anti-Stokes Raman scattering spectroscopy. Whether proceeding by inelastic collisions or reactive exchange, the transfer of translational energy to vibrational and rotational energy is quite inefficient in H+HX collisions at 1.6 eV. For all three hydrogen halides only 2–3% of the initial translational energy appears as HX vibration. For H+HCl only 6% of the initial energy is converted to HCl rotational energy, while for H+HBr and H+HI, this percentage is twice as large, 11–12%, but still small. The indistinguishability of the two H atoms involved makes it impossible to distinguish reactive exchange from inelastic energy transfer in these H+HX collisions. However, the difference in rotational energy partitioning for H+HBr and H+HI as compared with H+HCl, suggests that reactive exchange is dominant in the former and inelastic energy transfer dominates in the latter. The total cross sections for the combined energy transfer/reactive exchange do not change much with the identity of X, being 13±3, 11±2, and 11±2 A(ring)2, for H+HCl, H+HBr, and H+HI, respectively.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456693
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  • 4
    Electronic Resource
    Electronic Resource
    The effect of parent internal motion on photofragment rotational distributions: Vector correlation of angular momenta and C2v symmetry breaking in dissociation of AB2 molecules (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 2594-2610 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A semiclassical calculation is presented that fully accounts for the angular momentum disposal in photodissociation of triatomic molecules. Rotational distributions are calculated for O2(3Σ−g) produced by the visible photolysis of ozone, O2(1Δg) by the UV photolysis of ozone, and OH by the 157 nm dissociation of water, to illustrate the effects of parent internal motion on fragment rotational distributions in the dissociation of C2v geometry molecules. A simple, but realistic, impulsive model of the energy release is used to describe the dissociation dynamics. The calculations are carried out for parent molecules at room temperature, as well as at the low temperatures characteristic of molecular beams. The contributions to the diatomic fragment rotational distribution from both parent triatom rotation and zero-point bending vibration are computed. Comparison of the calculated distributions with experimentally measured distributions indicates that the spread in rotational and bending vibrational angular momenta of the parent molecule can account for all or nearly all of the spread in final J of the diatomic photofragment. However, the rotational distributions of the diatomic photofragment reveal a strong vector correlation between the diatom angular momentum produced by the dissociative energy release, and the angular momentum associated with the in-plane rotation. The correlation is such that only half of all the photofragment states allowed by energy and angular momentum conservation are actually produced with appreciable probability. Of two energy degenerate photofragment states, corresponding to breaking of one or the other nominally equivalent bonds in the AB2 molecule, the one with the smaller orbital angular momentum/recoil linear momentum is strongly favored. This is explained by larger Franck–Condon overlap in the photoexcitation for the state of lower recoil angular momentum. The correlation involves selection of which of the two nominally equivalent bonds will break in the photodissociation of an AB2 triatom of C2v geometry, and thus represents a symmetry breaking mechanism in such a photodissociation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453098
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  • 5
    Electronic Resource
    Electronic Resource
    Direct observation of preferential bond fission by excitation of a vibrational fundamental: Photodissociation of HOD (0,0,1) (1990)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 93 (1990), S. 2146-2148 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The 193 nm photodissociation of individual rotational levels of HOD molecules excited with one quantum of O–H stretching vibrational energy is described. Stimulated Raman excitation and coherent anti-Stokes Raman scattering are used to prepare and detect, respectively, the (0,0,1) vibrationally excited HOD. The OD and OH fragments are detected by laser induced fluorescence. The photodissociation of the HOD (0,0,1) molecules yields at least three times more OD than OH.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.459040
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  • 6
    Electronic Resource
    Electronic Resource
    CARS spectroscopy of O2(1Δg) from the Hartley band photodissociation of O3: Dynamics of the dissociation (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 6745-6756 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Rotationally and vibrationally resolved CARS spectra of the O2(1Δg) photofragment produced by the photodissociation of O3 at 17 wavelengths between 230 and 311 nm are reported. The spectra are taken under collision-free conditions, therefore, they reveal the nascent rotational and vibrational state distributions of the O2(1Δg) photofragment. At all photolysis wavelengths studied the vibrational distribution peaks very sharply at v=0, although all energetically allowed vibrational states are observed. The rotational state distributions are narrow, and peak typically at high J. The rotational distribution shifts to lower J as the photolysis wavelength increases. These observations imply vibrationally adiabatic, rotationally impulsive energy release in the dissociation. The shape and width of the rotational distributions can be completely accounted for by the spread in the O3 thermal rotation and zero-point vibration contributions to the O2(1Δg) photofragment angular momentum. The most striking observation about the O2(1Δg) photofragment quantum state distribution is an apparent propensity for even-J states. Experiments with 18O enriched ozone indicate that this propensity is observed only for 16O16O, not for 18O16O, and by implication not for 17O16O. We show that this is the consequence of a selective depletion of only odd-J rotational states of 16O16O(1Δg) by a curve crossing to O2(3Σg), but an equal depletion of both even-J and odd-J rotational states of 18O16O and 17O16O(1Δg) by the curve crossing. The odd-J selectivity for 16O16O is a consequence of the restriction of 3Σg to only odd-J states, due to the requirement of even nuclear exchange symmetry for this homonuclear species with spin-zero nuclei. As a result of the different curve crossing behavior, the quantum yield for 3Σg is twice as great for 18O16O and 17O16O as it is for 16O16O, and this imposes a mass-independentisotopic fractionation in the photodissociation: the O2(1Δg) fragments are depleted of 17O and 18O, while the O2(3Σg) fragments are enriched in these isotopes.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452374
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  • 7
    Electronic Resource
    Electronic Resource
    Mass-independent isotopic fractionation in nonadiabatic molecular collisions (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 6757-6765 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Symmetry and parity constraints associated with nonadiabatic collisions are shown to result in selection rules that can produce mass-independent isotopic fractionation between molecular electronic states coupled by a nonadiabatic process. Nonadiabatic transitions from Π or Δ molecular electronic states to ∑ electronic states can result in isotopic fractionation for atoms occurring in equivalent positions in the molecule if the common isotope of those atoms is a spin-zero nucleus. The Π or Δ state becomes depleted of the rare isotopes of those atoms while the ∑ state is enriched in the rare isotopes. Chemical processes that distinguish between the Π or Δ and ∑ states can convert this isotopic fractionation between electronic states of the same chemical species to a fractionation between different chemical species. Such nonadiabatic-collision-induced isotopic fractionation might explain observations of mass-independent isotopic fractionation in electrical discharges, as well as in meteoritic samples. Nonadiabatic-collision-induced isotopic fractionation also may provide an explanation for observations of isotopic enrichments in the earth's atmosphere.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452375
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  • 8
    Electronic Resource
    Electronic Resource
    Ultraviolet photodissociation dynamics of H2S and D2S (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 90 (1989), S. 1403-1414 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Nascent SH(X 2Πi, v‘=0,1) and SD(X 2Πi, v‘=0,1) rotational state population distributions, spin–orbit state population ratios, and Λ-doublet state population ratios have been measured following the UV excimer laser photodissociation of H2S (λ=193, 222, and 248 nm) and D2S (λ=193 and 222 nm), respectively. Nascent SH(X 2Πi, v‘=0) rotational state distributions following 193 nm photodissociation of cold H2S in a free jet expansion vs 300 K H2S in a flowing gas cell were essentially the same, indicating that photofragment angular momentum must be originating predominantly in the dissociation event, and not from rotational energy contained in the parent triatom. Laser excitation spectra of SH(X 2Πi, v‘=1) and SD(X 2Πi, v‘=1) have been recorded for the first time. Rotational state distributions for SH(X 2Πi, v‘) and SD(X 2Πi, v‘) are independent of v‘. Λ-doublet population ratios of the nascent photofragments are essentially unity for all the cases measured. The nascent rotational state distributions are consistent with an impact parameter model for the dissociating triatomic molecule.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.456082
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  • 9
    Electronic Resource
    Electronic Resource
    Ozone visible photodissociation dynamics (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 87 (1987), S. 2583-2593 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photolysis of ozone at 300 K in the Chappuis band has been investigated through collision-free coherent anti-Stokes Raman scattering (CARS) spectroscopy of the molecular oxygen photofragment. We have obtained the nascent electronic, vibrational, rotational, and translational energy distributions for nine photolysis wavelengths over the range 560–638 nm. The O2 photofragments are always formed in the ground (3Σ−g) state, there is no evidence for production of the excited (1Δg) state. Only a narrow range of high rotational levels are populated. The rotational distribution shifts to higher rotational states as the available energy increases. The vibrational distribution, however, is independent of photolysis wavelength. Only states v=0 to v=4 are populated, and there is a population inversion between v=2 and v=3. The average partitioning of energy among the vibrational, rotational, and translational degrees of freedom is 10%, 24%, and 66%, respectively. These results are interpreted to imply vibrationally adiabatic but rotationally impulsive dissociation dynamics. The O2 photofragment vibrational distribution is explained by Franck–Condon vibrational overlaps between O2 and the ground 1A1 electronic state of O3. Detailed consideration of the angular momentum disposal in the photodissociation indicates a strong correlation between the direction of the O2 angular momentum produced by the dissociative energy release, and the direction of the angular momentum of the O3 in-plane rotation. The correlation is such that of two energy degenerate final states, the one of higher orbital angular momentum is not produced.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453097
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  • 10
    Electronic Resource
    Electronic Resource
    A method for the deconvolution of incompletely resolved CARS spectra in chemical dynamics experiments (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 1719-1725 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We describe a method for deconvoluting incompletely resolved CARS spectra to obtain quantum state population distributions. No particular form for the rotational and vibrational state distribution is assumed, the population of each quantum state is treated as an independent quantity. This method of analysis differs from previously developed approaches for the deconvolution of CARS spectra, all of which assume that the population distribution is Boltzmann, and thus are limited to the analysis of CARS spectra taken under conditions of thermal equilibrium. The method of analysis reported here has been developed to deconvolute CARS spectra of photofragments and chemical reaction products obtained in chemical dynamics experiments under nonequilibrium conditions. The deconvolution procedure has been incorporated into a computer code. The application of that code to the deconvolution of CARS spectra obtained for samples at thermal equilibrium and not at thermal equilibrium is reported. The method is accurate and computationally efficient.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451172
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