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  • 1
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Recent state-selected photodissociation experiments on isocyanic acid, HNCO, have provided a wealth of data on its photochemistry and dissociation dynamics. The excited state potential energy surface on which the dissociation occurs is central to these observations but is relatively uncharacterized. We construct a two-dimensional analytical model for the excited state potential that is consistent with experimental observations, including the ultraviolet absorption spectrum and the dynamics of the C–N and N–H bond dissociations. We then test this surface by running classical trajectories on it, using Morse oscillator vibrational wave functions from the ground electronic state to determine the probability distributions of initial conditions. The trajectory calculation reproduces the experimentally observed variation in the photochemical branching with photolysis wavelength. It also reproduces the bond selectivity in the photodissociation of HNCO molecules containing three quanta of N–H stretching excitation (3ν1) that we observed experimentally. Although the model for the surface is very simple and includes only two degrees of freedom, it captures the essential features that determine the photochemical branching in a direct dissociation. © 1996 American Institute of Physics.
    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 111 (1999), S. 5829-5837 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Reaction of well-characterized vibrational states prepared in the region of three quanta of N–H stretching excitation explores how vibrations with different components along the reaction coordinate influence the bimolecular reaction of Cl atoms with isocyanic acid (HNCO) to form HCl and NCO. Near prolate symmetric top states corresponding to different amounts of a-axis rotation are well separated in energy, and perturbations by background states make each of the eigenstates a different mixture of zero-order states. Molecules in the essentially unperturbed K=1 and 4 states, which are nearly pure N–H stretching excitation, react efficiently, but those in the perturbed states, K=0, 2, and 3, which are a mixture of N–H stretching and lower frequency vibrations react only half as well. Detailed analysis of resolved, perturbed eigenstates for J=6 and 7 of K=3 reveals the relative reactivity of the two interacting zero-order states. The less reactive zero-order state, which most likely contains only two quanta of N–H stretch and several quanta of other vibrations, reacts only 10% as well as the pure N–H stretch zero-order state. Ab initio calculations suggest that bending excitation alters the interaction potential to reduce the fraction of reactive collisions. © 1999 American Institute of Physics.
    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 110 (1999), S. 10843-10849 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Photolysis of dimethyl sulfoxide in a molecular beam with 210 and 222 nm photons reveals the decomposition mechanism and energy disposal in the products. Using vacuum ultraviolet light and a time-of-flight spectrometer, we identify CH3 and CH3SO as primary fragments and CH3 and SO as secondary fragments. From CH3 quantum yield measurements, we find that secondary decomposition is minor for 222 nm photolysis, occurring in only about 10% of the fragments, but it increases to about 30% in the 210 nm photolysis. Laser-induced fluorescence measurements on the B3Σ−←X3Σ− transition of SO in the 235 to 280 nm region determine the internal energy of that photoproduct. We compare our results to a simple statistical model that captures the essential features of the decomposition, predicting both the extent of secondary decomposition and the recoil energy of the primary and secondary methyl fragments. © 1999 American Institute of Physics.
    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 115 (2001), S. 7086-7093 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Wavelength dependent, transient, electronic absorption spectroscopy of methylene iodide (CH2I2) in CCl4, CDCl3, C6D6, and (CD3)2CO following excitation of the fundamental C–H stretching vibration reveals the time scales of intramolecular vibrational energy redistribution and energy transfer to the solvent. In contrast to the case for overtone excitation, state-specific relaxation to one or a few states that are coupled by low order interactions with the initially prepared state dominates the intramolecular vibrational energy redistribution. This mechanism is consistent with previous infrared pump–probe measurements of CH2I2 fundamental relaxation as are the measured relaxation time scales. We also find a previously unobserved relaxation pathway through weakly-coupled states that have several quanta of excitation in the Franck–Condon active modes, primarily C–I stretch and bend. Although this statistical component is a minor channel in the relaxation, it is the only contribution to the signal at the longest probe wavelengths in CCl4 and CDCl3. Time scales for both intramolecular energy redistribution and intermolecular relaxation to the solvent depend strongly on the strength of interaction with the solvent. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
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