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  • 1
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 9764-9771 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report the first gas-phase Raman spectra of the N–C–O stretching fundamentals in isocyanic acid. Using stimulated Raman excitation to prepare vibrationally excited molecules, we record spectra via two different techniques, photoacoustic Raman spectroscopy and action spectroscopy. The former detects the sound wave generated as the Stokes laser tunes through resonances and deposits heat in the gas sample. The latter detects transitions by photodissociating the vibrationally excited states prepared in the vibrational excitation step and detecting the photofragments by laser induced fluorescence. In analogy with the stretching modes in CO2, the N–C–O symmetric stretch (ν3) Raman fundamental in HNCO is strong while the antisymmetric stretch (ν2) is weak, although neither is symmetry forbidden. Both vibrational states are strongly perturbed. The symmetric stretch interacts with combination states that contain two quanta of bending excitation, and the antisymmetric stretch interacts with several different combination states. Both Raman spectra have strong QQ branch rotational structure in which the band origins for different K sublevels in this near-prolate symmetric top follow no simple pattern. Photodissociation of the vibrationally excited states demonstrates the influence of the initial state preparation on the rotational resonances, photofragment appearance thresholds, and Franck–Condon factors in the transition to a dissociative excited electronic state. © 1997 American Institute of Physics.
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  • 2
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report the first gas-phase Raman spectrum of isocyanic acid. Using stimulated Raman excitation (SRE) to prepare vibrationally excited states, we detect transitions by both photoacoustic Raman spectroscopy (PARS) and action spectroscopy. In this paper we present results on the ν1 N–H stretch fundamental, leaving the spectra of the N–C–O symmetric and antisymmetric stretch modes for a separate publication. The Raman spectrum shows extensive state mixing in the ν1 fundamental, in agreement with previous infrared work. Measurement of the effective b-axis rotational constants for different mixed vibrational states in this near-prolate symmetric top limits the number of candidates for perturbing states and shows which vibrational modes participate. Double resonance photodissociation further probes the vibrational spectroscopy of isocyanic acid. The scheme is first to prepare a vibrationally excited state by SRE, then photodissociate only the molecules prepared in the first step, and finally probe the decomposition products by laser-induced fluorescence (LIF). An action spectrum, obtained by scanning the vibrational excitation laser (Stokes) wavelength with the photolysis laser wavelength fixed and the probe laser tuned to a LIF transition in one of the photofragments, is the key to unraveling the spectroscopy. The intensity differences between PARS and action spectrum transitions reveal the vibrational state mixing and provide the Franck–Condon factors for transitions to the excited electronic state. © 1997 American Institute of Physics.
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  • 3
    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.
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  • 4
    Electronic Resource
    Electronic Resource
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 8440-8447 
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report the bond selected photodissociation of isocyanic acid (HNCO). This molecule dissociates from its first excited singlet state, breaking either the N–H bond to form H+NCO (X 2Π) or the C–N bond to form NH (a 1Δ)+CO (1∑+). The threshold for production of NH lies about 3900 cm−1 above that of NCO, and we detect both of these channels by laser induced fluorescence on either the NH or the NCO fragment. Dissociating the molecule out of a vibrationally excited state on its ground electronic surface containing four quanta of N–H stretch (4ν1) enhances the efficiency of the NCO channel over the NH channel by a factor of at least 20. We reach this conclusion by comparing the results of such a vibrationally mediated photodissociation experiment to those from a conventional single photon dissociation at the same total energy (about 1000 cm−1 above the threshold for the NH channel). Our estimate of the branching ratio in the one photon dissociation at this energy is roughly ΦNCO/ΦNH≈20, and it grows to ΦNCO/ΦNH≥400 in the vibrationally mediated photodissociation. © 1995 American Institute of Physics.
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  • 5
    Electronic Resource
    Electronic Resource
    s.l. : American Chemical Society
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
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  • 6
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
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  • 7
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Stimulated Raman excitation (SRE) efficiently prepares excited vibrational levels in the ground electronic state of isocyanic acid, HNCO. Photofragment yield spectroscopy measures the electronic absorption spectrum out of initially selected states by monitoring laser induced fluorescence (LIF) of either NCO (X 2Π) or NH (a 1Δ) photofragments. Near threshold, the N–H bond fission is predissociative, and there is well-resolved rotational and vibrational structure in the NCO yield spectra that allows assignment of Ka rotational quantum numbers to previously unidentified vibrational and rotational levels in the ν1 N–H stretch and ν3 N–C–O symmetric stretch fundamentals in the ground electronic state of HNCO. The widths of NCO yield resonances depend on the initial vibrational state, illustrating one way in which initial vibrational state selection influences dissociation dynamics. Initial excitation of unperturbed ν1 (N–H stretch) states leads to diffuse NCO yield spectra compared to excitation of mixed vibrational levels. The higher energy dissociation channel that produces NH (a 1Δ) has coarser structure near its threshold, consistent with a more rapid dissociation, but the resonance widths still depend on the initially selected vibrational state. © 1997 American Institute of Physics.
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  • 8
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Initial vibrational excitation of a state containing three quanta of N–H stretch (3ν1) decreases the fractional photolysis yield of NH (a 1Δ) relative to NH (X 3Σ−) by a factor of approximately two compared to the isoenergetic photodissociation of a 300 K thermal sample of HNCO. At a total energy of 43 480 cm−1, NH (a 1Δ) accounts for 24% of the total NH yield in the direct photolysis but only 10% in the photodissociation of 3ν1. At 44 440 cm−1, the NH (a 1Δ) yields are 65% and 32% in the single photon and two-step photodissociations, respectively. The variation in branching ratio may arise from dynamical behavior that is closely related to the preferential production of NCO in the photolysis of vibrationally excited HNCO. The initial vibrational excitation has no influence on the rotational and vibrational distributions of NH (X 3Σ−), but it significantly increases the amount of energy in rotation of NH (a 1Δ). These results, along with several recent experimental and theoretical studies, suggest the participation of at least three different potential energy surfaces in the photodissociation of isocyanic acid. © 1998 American Institute of Physics.
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  • 9
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We measure upper limits for the bond enthalpies of the N–H and C–N bonds in HNCO by observation of photodissociation appearance thresholds for the NCO (X2Π) and NH (a1Δ) fragments from initially selected HNCO vibrational states. The upper limit of the dissociation energy of the H–N bond is D0(H–NCO)≤109.6±0.4 kcal/mol and that of the N–C bond is D0 (HN–CO)≤122.1±0.3 kcal/mol. Observation of unrelaxed fragment quantum state distributions at fixed energies supports the bond enthalpy measurement. The two appearance thresholds, together with known heats of formation of NH, NCO, H, and CO, provide two independent methods of calculating the HNCO heat of formation. Both methods give a value of ΔHf00 (HNCO)≥−27.7±1.1 kcal/mol. The consistency of the two methods for calculating ΔHf00 (HNCO) suggests that the actual bond enthalpies for the N–H and C–N bonds are close to the upper limits from the measurement. © 1996 American Institute of Physics.
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  • 10
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We report the bond selected photodissociation of HNCO from a vibrationally excited state containing three quanta of N–H stretch (3ν1) and demonstrate that initial vibrational state preparation strongly influences the photochemical branching in HNCO, producing either NCO (X 2Π) from cleavage of the N–H bond or NH (a 1Δ) from cleavage of the C–N bond. Initial excitation of the N–H dissociation coordinate by excitation of the N–H stretching overtone enhances the probability for breaking the N–H bond in the electronically excited state. Compared to isoenergetic photolysis of the ground vibrational state, photodissociation of the 3ν1 state alters the NCO quantum yield by roughly a factor of 4 at the largest photolysis energy used in this work, changing the channel that breaks the N–H bond from the minor to the major dissociation pathway. In addition, the experiment measures the quantum yields for production of NCO (ΦNCO) in the one-photon dissociation at three different photolysis wavelengths and provides a correction for the influence of photodissociation from vibrationally and rotationally excited states. © 1996 American Institute of Physics.
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