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  • 1
    Electronic Resource
    Electronic Resource
    Photodissociation processes in the CH molecule (1987)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 86 (1987), S. 196-214 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photodissociation processes in the CH molecule have been investigated using ab initio self-consistent-field with configuration-interaction (CI) methods. Potential energy curves of all states which dissociate into the lowest four atomic limits have been calculated, as well as those of several higher-lying states with Rydberg character. Transition dipole moments connecting the excited states with the lowest states have been obtained. Direct photodissociation of CH may occur by absorption into the continua of the B 2∑− and 2 2∑+ states, and cross sections for absorption from the X 2Π v‘=0 state into these states are reported. Indirect photodissociation of CH may take place by absorption into bound states, which are subsequently dissociated through interactions with continuum states. The A 2Δ vibrational levels couple with the continuum of the X 2Π state through the nuclear rotational operator. The nuclear rotational coupling elements have been obtained from the CI wave functions, and predissociation rates are presented. The C 2∑+ v'=0 state is shown to be predissociated efficiently by spin-orbit coupling with the B 2∑− state. The 2, 3, and 4 2Π states of CH interact strongly through the radial nuclear kinetic energy operator. The nuclear radial coupling elements between these states have been computed by numerical differentiation of the CI wave functions. The coupled equations for the nuclear wave functions have been solved in a diabatic formulation. The calculated cross section forabsorption into the coupled states is similar to that found recently for absorption into the 2 2Π and 3 2Π states of the OH molecule: it consists of a series of resonances with asymmetric Beutler–Fano profiles superimposed on a strong continuous background. The 3(F)2∑+ state, as well as the higher-lying (Rydberg) states, may interact with the 2Π states through rotational coupling or with the 2 4∑− state through spin-orbit coupling. Due to uncertainties in the coupling elements, the efficiencies with which these states are dissociated are not well determined, but they are probably small for most states. The photodissociation rate of CH by the unattenuated interstellar radiation field has been computed. The total rate is 9.5×10−10 s−1. The 2 2∑+ and coupled 2Π channels are most effective in the photodissociation of interstellar CH.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.452610
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  • 2
    Electronic Resource
    Electronic Resource
    Photodissociation of CH2. V. Three-dimensional adiabatic potential energy surfaces and transition dipole moments (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 8930-8941 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Full three-dimensional adiabatic potential energy surfaces are presented for the lowest five 3A‘ and five 3A' states of CH2. Both the 1 3A' and 2 3A‘ states are dissociative with respect to the C–H coordinates, consistent with our earlier two-dimensional results. All higher lying states are found to be bound for this coordinate, although the barrier toward dissociation is small for some states. In terms of angle dependence, the 1 3A' state shows a flat behavior, but tends towards larger angles as dissociation proceeds. Most excited 3A' states are somewhat bent with only a small barrier to linearity. Transition dipole moments connecting the ground state with the excited triplet states are presented as well. The 1 3A' state is the only state of that symmetry with a large transition dipole moment in the Franck–Condon region. Other 3A' states exhibit large values only if one bond is stretched compared with the ground state equilibrium geometry. The 1 3A‘, 3 3A‘, and 4 3A‘ states are also slightly bent with a small barrier to linearity. However, the 2 3A‘ state has an absolute minimum at very small angles (less than 60°), and shows a considerable local minimum (∼1.5 eV) for the linear configuration. The 5 3A‘ state prefers the linear shape. The 3 3A‘ state has the largest transition dipole moment function in the Franck–Condon region, but the transition moments to other 3A‘ states can exhibit large values outside this region. The 2 3A‘ and 3 3A‘ states undergo an avoided crossing in the Franck–Condon region, so that a coupled states treatment is necessary for a correct description of the photodissociation dynamics. In order to provide the corresponding transition dipole moments in an appropriate form, a transformation to the principal axes of inertia was performed. The adopted transformations are discussed in detail. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468947
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  • 3
    Electronic Resource
    Electronic Resource
    Photodissociation of CH2. III. Two-dimensional dynamics of the dissociation of CH2, CD2, and CHD through the first excited triplet state (1994)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 100 (1994), S. 1113-1127 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We present quantitative results on photodissociation of CH2 (X˜ 3B1) and its isotopomers CHD and CD2 through the first excited triplet state (1 3A1). A two-dimensional wave packet method employing the light–heavy–light approximation was used to perform the dynamics. The potential energy surfaces and the transition dipole moment function used were all taken from ab initio calculations. The peak positions in the calculated CH2 and CD2 spectra nearly coincide with the positions of unassigned peaks in experimental CH2 and CD2 3+1 resonance enhanced multiphoton ionization spectra, provided that the experimental peaks are interpreted as two-photon transitions. Comparing the photodissociation of CH2 and its isotopomers to photodissociation of water in the first absorption band, we find these processes to be very similar in all aspects discussed in this work. These aspects include the origin of the diffuse structure and the overall shape of the total absorption spectra of vibrationless and vibrationally excited CH2 , trends seen in the fragment vibrational level distribution of the different isotopomers, and selectivity of photodissociation of both vibrationless and vibrationally excited CHD. In particular, we find that the CD/CH branching ratio exceeds two for all wavelengths in photodissociation of vibrationless CHD.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.466643
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  • 4
    Electronic Resource
    Electronic Resource
    Photodissociation of the HCO+ ion. I. Two-dimensional calculations through the I 1Π state (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 103 (1995), S. 7006-7015 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The HCO+ ion plays an important role in the chemistry of interstellar space and in combustion flames. The 1 1Π state is the only dissociative state below the hydrogen ionization potential, to which transitions are dipole allowed. Ab initio MRD-CI calculations have been performed for the two-dimensional potential energy surfaces of the ground state and the 1 1Π state of HCO+ as functions of the C–H and C–O bond distances, keeping the ion in the linear configuration. The 1 1Π state is interesting because of an avoided crossing with the 2 1Π state. The potential energy surfaces and geometry dependent dipole transition moments have been employed in two-dimensional photodissociation dynamics calculations. Total and vibrationally resolved partial cross sections are calculated as functions of the excitation energy. The potential barrier arising from the avoided crossing leads to vibrational resonances in the cross sections, which correspond to levels of the v3 (C–O) stretching motion in the excited state. The structure of the cross sections reveals the competition between resonant and direct photodissociation. The difference between the total cross section derived from the Fourier transform of the autocorrelation function and from the sum of the partial cross sections of the CO++H dissociation channel indicates the existence of a second photodissociation channel leading to CH++O. The total and partial cross sections of this channel have been computed in an independent calculation; its contribution to the integrated total cross section amounts to about 4%. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.470327
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  • 5
    Electronic Resource
    Electronic Resource
    ISO SPECTROSCOPY OF GAS AND DUST: From Molecular Clouds to Protoplanetary Disks (2004)
    Palo Alto, Calif. : Annual Reviews
    Annual Review of Astronomy and Astrophysics 42 (2004), S. 119-167 
    Details
    ISSN: 0066-4146
    Source: Annual Reviews Electronic Back Volume Collection 1932-2001ff
    Topics: Physics
    Notes: Observations of interstellar gas-phase and solid-state species in the 2.4-200 mum range obtained with the spectrometers on board the Infrared Space Observatory (ISO) are reviewed. Lines and bands caused by ices, polycyclic aromatic hydrocarbons, silicates, and gas-phase atoms and molecules (in particular H2, CO, H2O, OH, and CO2) are summarized and their diagnostic capabilities illustrated. The results are discussed in the context of the physical and chemical evolution of star-forming regions, including photon-dominated regions, shocks, protostellar envelopes, and disks around young stars.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1146/annurev.astro.42.053102.134010
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  • 6
    Electronic Resource
    Electronic Resource
    CHEMICAL EVOLUTION OF STAR-FORMING REGIONS (1998)
    Palo Alto, Calif. : Annual Reviews
    Annual Review of Astronomy and Astrophysics 36 (1998), S. 317-368 
    Details
    ISSN: 0066-4146
    Source: Annual Reviews Electronic Back Volume Collection 1932-2001ff
    Topics: Physics
    Notes: Abstract Recent advances in the understanding of the chemical processes that occur during all stages of the formation of stars, from the collapse of molecular clouds to the assemblage of icy planetesimals in protoplanetary accretion disks, are reviewed. Observational studies of the circumstellar material within 100-10,000 AU of the young star with (sub)millimeter single-dish telescopes, millimeter interferometers, and ground-based as well as space-borne infrared observatories have only become possible within the past few years. Results are compared with detailed chemical models that emphasize the coupling of gas-phase and grain-surface chemistry. Molecules that are particularly sensitive to different routes of formation and that may be useful in distinguishing between a variety of environments and histories are outlined. In the cold, low-density prestellar cores, radicals and long unsaturated carbon chains are enhanced. During the cold collapse phase, most species freeze out onto the grains in the high-density inner region. Once young stars ignite, their surroundings are heated through radiation and/or shocks, whereupon new chemical characteristics appear. Evaporation of ices drives a "hot core" chemistry rich in organic molecules, whereas shocks propagating through the dense envelope release both refractory and volatile grain material, resulting in prominent SiO, OH, and H2O emission. The role of future instrumentation in further developing these chemical and temporal diagnostics is discussed.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1146/annurev.astro.36.1.317
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  • 7
    Electronic Resource
    Electronic Resource
    Photodissociation of CH2. I. Potential energy surfaces of the dissociation into CH and H (1992)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 97 (1992), S. 8240-8249 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The photodissociation processes of CH2 into CH and H have been studied using ab initio multireference configuration-interaction methods. Two-dimensional potential energy surfaces of the ten lowest triplet states correlating with the seven lowest states of CH have been calculated as functions of bond angle and one C–H bond distance, keeping the other C–H distance fixed at the equilibrium CH2 value. Transition dipole moments connecting the excited states with the ground state have been obtained as well. It is shown that efficient photodissociation of CH2 into CH (X 2Π)+H can occur by absorption from the ground X˜ 3B1 (1 3A‘) state into the 1 3A1 (1 3A') state at about 6.3 eV. Photodissociation into excited CH (a 4Σ−)+H can take place through the 1 3A2 (2 3A‘) and 2 3B1 (3 3A‘) states, although in a more complex manner since several avoided crossings occur along the reaction path. The 1 3A2 state is a so-called low-angle state, which has an equilibrium bond angle of less than 60° and correlates directly with C(3P)+H2. At 180°, when the molecule has D∞h or C∞v symmetry, interesting crossings between the ground and low-lying surfaces are found. Altogether, these crossings and correlations are predicted to lead to complicated dissociation dynamics for most of the states. The higher-lying states of CH2 can photodissociate either directly into excited states of CH, or they can be predissociated by the repulsive 1 5A2 (1 5A‘) state, which correlates with CH (a 4Σ−)+H.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.463395
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  • 8
    Electronic Resource
    Electronic Resource
    Photodissociation of CH2. II. Three-dimensional wave packet calculations on dissociation through the first excited triplet state (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 228-236 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Quantitative results on photodissociation of CH2(X˜ 3B1) through the first excited (1 3A1) triplet state, producing CH (X 2Π)+H(2S), are presented. A three-dimensional time dependent quantum mechanical method was adopted to perform the dynamics using ab initio potential energy surfaces and an ab initio transition dipole moment function. The calculations were performed for J=0, where J is the angular momentum associated with the overall rotation of the nuclei. Comparison with calculations in which the bending angle was kept fixed at its ground state equilibrium value shows that a two-dimensional treatment suffices for obtaining the absorption spectrum. On the other hand, a three-dimensional calculation is necessary for correctly predicting the final rotational state distribution of the CH fragment.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.465800
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  • 9
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    Photodissociation of CH2. III. Two‐dimensional dynamics of the dissociation of CH2, CD2, and CHD through the first excited triplet state (1994)
    American Institute of Physics
    Details
    Publication Date: 1994-01-15
    Print ISSN: 0021-9606
    Electronic ISSN: 1089-7690
    Topics: Chemistry and Pharmacology , Physics
    Published by American Institute of Physics
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  • 10
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    Photodissociation of CH2. II. Three‐dimensional wave packet calculations on dissociation through the first excited triplet state (1993)
    American Institute of Physics
    Details
    Publication Date: 1993-07-01
    Print ISSN: 0021-9606
    Electronic ISSN: 1089-7690
    Topics: Chemistry and Pharmacology , Physics
    Published by American Institute of Physics
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