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  • 1
    Electronic Resource
    Electronic Resource
    Using stretching and bending vibrations to direct the reaction of Cl atoms with isocyanic acid (HNCO) (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 5829-5837 
    Details
    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
    URL: http://dx.doi.org/10.1063/1.480266
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  • 2
    Electronic Resource
    Electronic Resource
    Initial state resolved electronic spectroscopy of HNCO: Stimulated Raman preparation of initial states and laser induced fluorescence detection of photofragments (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 8985-8993 
    Details
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475190
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  • 3
    Electronic Resource
    Electronic Resource
    Raman spectroscopy of the ν1 N–H stretch fundamental in isocyanic acid (HNCO): State mixing probed by photoacoustic spectroscopy and by photodissociation of vibrationally excited states (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 106 (1997), S. 5805-5815 
    Details
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.473246
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  • 4
    Electronic Resource
    Electronic Resource
    Mode- and Bond-Selective Reactions of Chlorine Atoms with Highly Vibrationally Excited H2O and HOD (1995)
    s.l. : American Chemical Society
    The @journal of physical chemistry 〈Washington, DC〉 99 (1995), S. 13748-13754 
    Details
    Source: ACS Legacy Archives
    Topics: Chemistry and Pharmacology , Physics
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1021/j100037a023
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  • 5
    Electronic Resource
    Electronic Resource
    Transient electronic absorption of vibrationally excited CH2I2: Watching energy flow in solution (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 113 (2000), S. 5018-5025 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Transient electronic absorption of methylene iodide (CH2I2) in CCl4, CDCl3, and C6D6 after excitation of two quanta of C–H stretching vibration with a 100 fs laser pulse allows direct observation of the times for intramolecular vibrational relaxation and energy transfer to the solvent. Intramolecular energy redistribution populates vibrational states with larger Franck–Condon factors for the electronic transition, leading to an increased absorption of probe pulses in the wavelength range of 380–440 nm. A model based on the temperature dependence of the electronic absorption coefficient describes the transient absorption well for all wavelengths. In the model, the temperature rises and decays exponentially with time, reflecting the initial redistribution of energy within the excited molecule and the subsequent transfer of energy from the vibrationally excited molecule into the solvent. The intramolecular vibrational relaxation time for CH2I2 is essentially the same in the solvents CCl4 (10.8±1.5 ps) and CDCl3 (11.2±2.0 ps) and is only slightly shorter in C6D6 (8.0±1.5 ps). Energy transfer to the solvent takes longer, occurring with a time constant of 68±10 ps for CCl4, 51±10 ps for CDCl3, and 23±2 ps for C6D6. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1289532
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  • 6
    Electronic Resource
    Electronic Resource
    Raman spectroscopy of the N–C–O symmetric (ν3) and antisymmetric (ν2) stretch fundamentals in HNCO (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 9764-9771 
    Details
    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.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.475274
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  • 7
    Electronic Resource
    Electronic Resource
    Time-resolved vibrationally mediated photodissociation of HNO3: Watching vibrational energy flow (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 661-664 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Ultrafast excitation of an O–H stretching vibrational followed by photodissociation of the energized molecules allows direct observation of the time for intramolecular energy redistribution in isolated nitric acid. We excite the first overtone of the O–H stretch vibration in HNO3 with a 100 fs laser pulse. A second, time-delayed pulse preferentially photodissociates molecules having vibrational excitation in modes orthogonal to the O–H stretch. The photodissociation yield increases as a function of time because energy flows out of the initially excited O–H bond into other more efficiently dissociated vibrations. The single exponential time constant for this intramolecular vibrational relaxation is 12 ps, consistent with moderate coupling of the O–H stretch to states close in energy. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474443
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  • 8
    Electronic Resource
    Electronic Resource
    The electronic origin and vibrational levels of the first excited singlet state of isocyanic acid (HNCO) (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 6678-6688 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The combination of vibrationally mediated photofragment yield spectroscopy, which excites molecules prepared in single vibrational states, and multiphoton fluorescence spectroscopy, which excites molecules cooled in a supersonic expansion, provides detailed information on the energetics and vibrational structure of the first excited singlet state (S1) of isocyanic acid (HNCO). Dissociation of molecules prepared in individual vibrational states by stimulated Raman excitation probes vibrational levels near the origin of the electronically excited state. Detection of fluorescence from dissociation products formed by multiphoton excitation through S1 of molecules cooled in a supersonic expansion reveals the vibrational structure at higher energies. Both types of spectra show long, prominent progressions in the N–C–O bending vibration built on states with different amounts of N–C stretching excitation and H–N–C bending excitation. Analyzing the spectra locates the origin of the S1 state at 32 449±20 cm−1 and determines the harmonic vibrational frequencies of the N–C stretch (ω3=1034±20 cm−1), the H–N–C bend (ω4=1192±19 cm−1), and the N–C–O bend (ω5=599±7 cm−1), values that are consistent with several ab initio calculations. The assigned spectra strongly suggest that the N–C stretching vibration is a promoting mode for internal conversion from S1 to S0. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.481242
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  • 9
    Electronic Resource
    Electronic Resource
    Reactions of O, H, and Cl atoms with highly vibrationally excited HCN: Using product states to determine mechanisms (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 4490-4501 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Oxygen, hydrogen, and chlorine atoms react with vibrationally excited HCN to produce CN and OH, H2, or HCl, respectively. The experiments presented here use direct vibrational overtone excitation to prepare states of HCN having four quanta of C–H stretching excitation [(004) state] or three quanta of C≡N stretching and two quanta of C–H stretching excitation [(302) state] and laser-induced fluorescence to determine the rotational and vibrational states of the CN product. We find that the reaction of HCN with O produces CN having little vibrational and rotational energy, with 85% of the CN in v=0, 12% in v=1, and 3% in v=2. The CN from the reaction of H with HCN is slightly more energetic, with 77% in v=0, 17% in v=1, and 6% in v=2. By contrast, the reaction of Cl with HCN produces CN with a considerable amount of excitation, about 30% is in v=1 and at least 10% is in v=2, depending on the initial vibrational state of the HCN reactant. The enhanced excitation of the CN product of the reaction with Cl reflects the contribution of a different mechanism. We conclude that the O-atom reaction forms CN exclusively by a direct abstraction reaction, the H-atom reaction produces CN primarily by direct reaction at the collisional energies of our experiment, and the Cl-atom reaction forms CN by the dissociation of an intermediate complex in addition to the direct abstraction reaction. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471200
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  • 10
    Electronic Resource
    Electronic Resource
    Diffraction mechanisms in gas-phase laser induced grating spectroscopy of vibrational overtone transitions (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 10484-10491 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We have investigated the grating formation mechanism in laser-induced grating spectroscopy by preparing transient gratings via excitation of O–H vibrational overtones in water vapor. In principle, our experiments are sensitive to three different mechanisms of diffraction: a density-based phase grating originating from local thermalization of the vibrational overtone state, a population-based phase grating caused by differences in ground and excited state polarizability volumes, and a population-based amplitude grating that appears when the probe laser is coincident with an electronic transition of the overtone-excited molecules. For water saturated air at 50–760 Torr, bulk acoustic responses originating from both collisional thermalization and from electrostriction dominate the diffraction efficiency. Measurement of the probe-wavelength dependence of the diffraction efficiency at pressures of 18 Torr and below shows that the dominant signal originates from a phase grating and that contributions from the amplitude grating are relatively unimportant at all pressures. Analysis of the temporal evolution of the diffraction efficiency suggests that the thermalization mechanism dominates at 18 Torr, despite there being only one hard-sphere collision on the time scale of the measurement. At 10 Torr, the temporal evolution of the diffraction signal begins to show evidence of a population based phase grating contribution as well. These conclusions are consistent with our measured thermalization rate constants for the vibrational overtone states and also with calculations of the effects of vibrational excitation on the molecular polarizability volume. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469898
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