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  • 1
    Electronic Resource
    Electronic Resource
    The determination of the vector correlation between photofragment rotational and translational motions from the analysis of Doppler-broadened spectral line profiles (1986)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 85 (1986), S. 1866-1879 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A theoretical framework is presented to describe the possible correlation between the translational and rotational motion of a molecular photofragment, and to relate this to experimental observables. The correlation is defined by the values of a number of bipolar moments of the translational and rotational angular distributions which reflect the dynamics of the dissociation process. Detailed equations are presented for the polarization dependence of the profiles of recoil Doppler–broadened spectral lines of the photoproduct for two distinct classes of system: (i) molecular photodissociation, followed by spontaneous fluorescence by a molecular product with dispersion of its emission spectrum, and (ii) LIF excitation of a photoproduct, with or without dispersion of the resulting emission. The application of the theory is illustrated by calculating the line profiles for two model systems, each with several excitation–detection geometries. It is pointed out that there may still be detectable correlation between the product motions even for weakly predissociated parent molecule transitions in which the memory of the initial excitation is lost through extensive rotation before dissociation.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.451131
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  • 2
    Electronic Resource
    Electronic Resource
    State selective photodissociation dynamics of A˜ state ammonia. II (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 2901-2911 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photodissociation dynamics of A˜ state ammonia molecules (both NH3 and ND3) has been further investigated using the technique of H(D) atom photofragment translational spectroscopy. The resulting NH2(ND2) fragments are observed to carry high levels of internal excitation, the precise disposition of which is sensitively dependent upon the parent v'2 level excited. Dissociation from the v2=0 level of the A˜ state yields ground state NH2(ND2) fragments, primarily in their zero-point level, but with high levels of rotational excitation specifically concentrated about the a-inertial axis; the population distribution over the energetically accessible product rotational levels with N(approximately-equal-to)Ka appears near to statistical. In contrast, dissociation from the parent v'2=1 level yields a markedly inverted fragment internal energy distribution. These different energy disposals have been rationalized via classical trajectory calculations employing the best available ab initio potential energy surfaces for the A˜ and X˜ states of the ammonia molecule. The energy disposal following excitation to the parent v2=2 and 3 levels is found to mimic that observed for, respectively, the v'2=0 and 1 levels. These results provide clear evidence for the importance of anharmonic coupling (whereby an even number of bending quanta are redistributed into stretching motions) in promoting the fragmentation of parent levels with v2≥2. The threshold energy for producing electronically excited NH2(A˜2A1) fragments is 6.02 eV [∼6.16 eV for ND2(A˜)]. The present studies of NH3 photolysis suggest that this fragmentation channel opens at threshold and clearly indicate that branching into this channel occurs with much higher quantum yield than hitherto believed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457644
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  • 3
    Electronic Resource
    Electronic Resource
    State-selected photodissociation dynamics of HONO(A˜ 1A‘): Characterization of the NO fragment (1989)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 91 (1989), S. 2308-2320 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Trans-HONO has been photodissociated through the 220 band of the A˜1A‘–X˜ 1A' system at 355 nm. The energy disposal in the NO fragment, and vector correlations between its motions, have been characterized using polarized one-photon excited laser induced fluorescence. The rotational distribution, rotational alignment, degree of electron alignment, spin–orbit branching ratio, approximate vibrational distribution, and energy partitioning are determined. The rotational distributions in each of v(NO)=3,2 and 1 are highly inverted and approximately Gaussian in quantum number. The peak J decreases with increasing v to give a fairly narrow overall internal energy distribution. The disposal of the available energy into all NO and OH motions is 60% and 40%, respectively, and is also 40% into internal motions, mainly of NO, and 60% into translational recoil. The NO rotation shows a positive alignment. All the vector correlations are consistent with planar fragmentation dynamics, but with a finite dissociation lifetime of about 100 fs. The Λ-doublet populations of NO indicate the preference of the unpaired π electron to occupy an orbital lying perpendicular to the plane of rotation, which is opposite to that found earlier for the OH fragment. The population of the NO spin–orbit states is inverted. These results are discussed in terms of dynamical models and the features of a recently published ab initio surface for the HONO A˜ state.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.457038
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  • 4
    Electronic Resource
    Electronic Resource
    State selective photodissociation dynamics of A˜ state ammonia. I (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 88 (1988), S. 3607-3616 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photofragmentation dynamics of ammonia molecules following pulsed laser excitation to the two lowest levels (v'2 =0 and 1) of their A˜ 1A‘2 excited state has been investigated by monitoring the time-of-flight spectra of the nascent H-atom products. These spectra show well resolved structure. Analysis of this structure confirms recent revised estimates of the quantity D00 (H–NH2) (4.645±0.01 eV) and reveals that the majority of the accompanying NH2 (X˜ 2B1) fragments are formed vibrationally unexcited, but with high levels of rotational excitation specifically concentrated about the a-inertial axis. The detailed energy disposal is sensitive to the initially excited parent vibronic (and even rovibronic) level: the NH2 (X˜) fragments resulting from photodissociation via the v'2 =1 level of NH3 (A˜) carry a higher level of excitation of the N=Ka rotational levels, which show an inverted population distribution. We also describe the results of trajectory calculations employing the recently reported [M. I. McCarthy et al., J. Chem. Phys. 86, 6693 (1987)] ab initio potential energy surfaces for the A˜ and X˜ states of ammonia. These provide a detailed rationale for the experimentally observed energy disposal and highlight the massive influence on the eventual fragmentation dynamics of the conical intersection between these surfaces along the H–NH2 dissociation coordinate.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.453910
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  • 5
    Electronic Resource
    Electronic Resource
    The B˜ 1E‘ state of ammonia: Sub-Doppler spectroscopy at vacuum ultraviolet energies (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 1754-1761 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Spectra of the 2n0 progression in the B˜ 1E‘–X˜ 1A'1 transition in NH3 and ND3 have been recorded at an effective resolution down to 0.008 cm−1 at the VUV equivalent energy. Line shape analysis reveals a predissociation lifetime of 0.25(2) ns for ND3 and 6.1(7) ps for NH3 independent of rotational state and v2. Measurement of the magnetic sensitivity of low J lines gave an electronic g value. Rotational analyses of the 2n0 bands with n up to 6 for ND3 and 8 in NH3 are presented and indicate some small vibrational perturbations. The observed difference between the Coriolis coupling constant ζ derived from the rotational structure and the g value indicates a mild Jahn–Teller effect.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455715
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  • 6
    Electronic Resource
    Electronic Resource
    A nomenclature for Λ-doublet levels in rotating linear molecules (1988)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 89 (1988), S. 1749-1753 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: It is proposed that the two Λ-doublet levels of linear molecules with nonzero electronic orbital angular momentum be labeled Λ(A') and Λ(A‘), e.g., Π(A') and Π(A‘) for Π states, etc., according to the following prescription: All series of levels in which the electronic wave function at high J is symmetric with reflection of the spatial coordinates of the electrons in the plane of rotation will be designated Λ(A') for all values of J, and all those for which the electronic wave function is antisymmetric with respect to reflection will be denoted Λ(A‘). It is emphasized that this notation is meant to supplement, and not replace, the accepted spectroscopic e/f labeling and the parity quantum number. The utility of the Λ(A')/Λ(A‘) notation is that it is of most relevance in the mechanistic interpretation of reactive or photodissociative processes involving open-shell molecules.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.455121
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  • 7
    Electronic Resource
    Electronic Resource
    Emission and Absorption Spectra of NCO and NCS (1958)
    [s.l.] : Nature Publishing Group
    Nature 182 (1958), S. 336-337 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] A few years ago, two of us (R. H. and D. W. G. S.) observed a series of bands in the region 4250?4650 A. in the fluorescence from ethyl socyanate (C2H5NCO). The bands were also observed in emission from a high-frequency discharge through this compound at a pressure of about 1 mm. mercury. A ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/182336a0
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  • 8
    Electronic Resource
    Electronic Resource
    Luminescence from the H—O 2 Reaction (1963)
    [s.l.] : Nature Publishing Group
    Nature 197 (1963), S. 1198-1198 
    Details
    ISSN: 1476-4687
    Source: Nature Archives 1869 - 2009
    Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics
    Notes: [Auszug] Hydrogen atoms were produced by the dissociation of commercial hydrogen in two Wood's tubes and pumped through two side-arms on opposite sides of a reaction vessel 3-5 cm in diameter and 35 cm long connected directly to a 150 l./min rotary pump. Commercial oxygen was introduced through a third ...
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1038/1971198a0
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  • 9
    Electronic Resource
    Electronic Resource
    Dissociation dynamics of H2O(D2O) following photoexcitation at the Lyman-α wavelength (121.6 nm) (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 7360-7375 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The technique of H(D) atom photofragment translation spectroscopy has been used to investigate the collision free photodissociation of jet cooled H2O(D2O) molecules following excitation to their B˜(1A1) excited state at 121.6 nm. The resolution of the total kinetic energy release spectrum obtained with this technique, allows assignment of the eigenvalues for the individual rotational quantum states and an estimation of the respective quantum state population distributions for the nascent OH(X 2Π) and OH(A 2Σ+) photofragments (and their deuterated analogs). This provides us the first experimental observations of high angular momentum states of OD(X). Analysis of the quantum state population distribution show both the ground (X 2Π) and electronically excited (A 2Σ+) OH(OD) fragments to be formed with little vibrational excitation but with highly inverted rotational distributions. Spectral simulation enables estimation of relative branching ratios for these two dissociation channels, and for the three-body fragmentation yielding ground state atoms. The observed energy disposal has been rationalized by considering the motion of a wavepacket launched on the B˜ state surface at a geometry corresponding to the ground state equilibrium configuration. Electronically excited OH(OD) fragments result from that fraction of the photoexcited molecules that dissociate on the B˜ state surface; their rotational excitation results from the marked angular anisotropy of the B˜ state surface. Ground state OH(OD) fragments can arise as a result of radiationless transfer to the lower A˜(1B1) or X˜(1A1) surfaces. The wavepacket calculations show that B˜(large-closed-square)X˜ transfer via the conical intersection linking these two surfaces leads to the most highly rotationally excited OH(OD) fragments. These calculations also show that the contribution made by B˜(large-closed-square)A˜ radiationless transfer to the overall rotational distribution in the ground state OH(OD) fragments scales with the amount of a-axis rotational excitation in the photoexcited molecules: The detailed form of the OH(OD) product state population distribution is thus predicted to be temperature dependent.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466880
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  • 10
    Electronic Resource
    Electronic Resource
    Primary product channels in the photodissociation of methane at 121.6 nm (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 2054-2065 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The technique of H(D) atom photofragment translational spectroscopy has been applied to the photodissociation of CH4(CD4) at 121.6 nm. Contrary to the previous consensus view, we find simple C–H bond fission to be the dominant primary process following excitation at this wavelength. The resulting CH3 fragments are formed with very high levels of internal excitation: Some (∼25%) possess so much internal energy that they must undergo subsequent unimolecular decay. The present experiments do not provide a unique determination of the products of this secondary decay process, but statistical arguments presented herein suggest that they will be predominantly CH and H2 fragments. Similar considerations point to a significant role for the direct three body process yielding the same products H+H2+CH. This overall pattern of energy disposal can be rationalized by assuming that most of the initially prepared CH4(A˜ 1T2) molecules undergo rapid internal conversion (promoted by the Jahn–Teller distortion of this excited state) to high vibrational levels of the ground state prior to fragmentation. The realization that CH4 photodissociation at 121.6 nm yields CH3 (and CH) fragments, rather than methylene radicals, will necessitate some revision of current models of the hydrocarbon photochemistry prevailing in the atmospheres of the outer planets and some of their moons, notably Titan.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.464237
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