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  • American Institute of Physics (AIP)  (15)
  • American Association for the Advancement of Science  (1)
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  • 1
    Digitale Medien
    Digitale Medien
    Ab initio calculations of the spin–orbit splitting in the C˜ state of CF4+ including the dynamical Jahn-Teller effect (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 4124-4131 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Detailed ab initio calculations on the lowest vibrational state of the electronic C˜ 2T2 manifold of CF4+ show a small Jahn-Teller effect in all asymmetric modes. This results in only small distortions from tetrahedral symmetry, but due to strong coupling between electronic and nuclear motions (dynamical Jahn-Teller effect), the spin–orbit splitting is reduced by an order of magnitude. This answers the outstanding question on the large differences between the results of electronic structure calculations and measurements on spin-orbit splittings in XY4 molecules or molecular ions [R. N. Dixon and R. P. Tuckett, Chem. Phys. Lett. 140, 553 (1987); J. F. M. Aarts and J. H. Callomon, Mol. Phys. 81, 1383 (1994)]. Complete agreement with experiment is however not reached because the dynamical Jahn-Teller effect is very sensitive to small errors in the potential energy surface. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.480961
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  • 2
    Digitale Medien
    Digitale Medien
    Photodissociation of water. II. Wave packet calculations for the photofragmentation of H2O and D2O in the B˜ band (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 5787-5808 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A complete three-dimensional quantum mechanical description of the photodissociation of water in the B˜ band, starting from its rotational ground state, is presented. In order to include B˜-X˜ vibronic coupling and the B˜-A˜ Renner–Teller coupling, diabatic electronic states have been constructed from adiabatic electronic states and matrix elements of the electronic angular momentum operators, following the procedure developed by A. J. Dobbyn and P. J. Knowles [Mol. Phys. 91, 1107 (1997)], using the ab initio results discussed in the preceding paper. The dynamics is studied using wave packet methods, and the evolution of the time-dependent wave function is discussed in detail. Results for the H2O and D2O absorption spectra, OH(A)/OH(X) and OD(A)/OD(X) branching ratios, and rovibrational distributions of the OH and OD fragments are presented and compared with available experimental data. The present theoretical results agree at least qualitatively with the experiments. The calculations show that the absorption spectrum and the product state distributions are strongly influenced by long-lived resonances on the adiabatic B˜ state. It is also shown that molecular rotation plays an important role in the photofragmentation process, due to both the Renner–Teller B˜-X˜ mixing, and the strong effect of out-of-plane molecular rotations (K〉0) on the dynamics at near linear HOH and HHO geometries. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.481154
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  • 3
    Digitale Medien
    Digitale Medien
    Photodissociation of water. I. Electronic structure calculations for the excited states (2000)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 112 (2000), S. 5777-5786 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Results of ab initio calculations for the four lowest excited states of both A′ and A″ have been discussed. In the multireference configuration interaction calculations, a large Rydberg basis set has been used. Three-dimensional potential energy surfaces, and matrix elements of the transition dipole moment between the excited states and the ground X˜ state, and the electronic angular momentum operator between the A˜ state and the B˜ and X˜ states have been presented. The calculations show that above about 124 nm the photodissociation can be well described by the three lowest electronic states, X˜, A˜, and B˜. The ab initio results of matrix elements of the electronic angular momentum operator allow a realistic nonadiabatic treatment of the photodissociation in the B˜ band. At wavelengths smaller than about 124 nm, the dynamics will be more complicated because of the coupling between various electronic states. © 2000 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.481153
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  • 4
    Digitale Medien
    Digitale Medien
    Photodissociation of CH2. VI. Three-dimensional quantum dynamics of the dissociation through the coupled 2A″ and 3A″ states (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 5757-5770 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: We present quantitative results on photodissociation of CH2(X˜ 3B1) through the coupled2A′′ and 3A′′ states. A three-dimensional, hybrid quantum dynamical method was used, employing hyperspherical coordinates. The diabatic potential energy surfaces (PES's) used in the dynamics were derived from ab initio calculations. A small product fraction (2.7%) was computed for the CH(A 2Δ)+H channel, in agreement with experiment and approximate dynamical calculations. The dissociation proceeds mostly on a A2-like diabatic surface, into CH(a 4Σ−)+H(93.3%) andC(3P)+H2(4.0%). Resonances of widths in the range 0.1–10 meV affect the photodissociation. Pre-exciting a vibrational mode of CH2(X˜ 3B1) prior to photodissociation does not alter the picture, except if the antisymmetric stretch mode is excited: In this case the product fractions for the C(3P)+H2 andCH(A 2Δ)+H channels collapse to values of 1% or lower, and the resonances disappear. Model calculations show that the large product fraction found for CH(a 4Σ−)+H is due to the initial motion on the "bright"B1-like surface, which biases the outcome of the dissociation in favor ofCH(a 4Σ−)+H. © 1997 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.475130
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  • 5
    Digitale Medien
    Digitale Medien
    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
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.468947
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  • 6
    Digitale Medien
    Digitale Medien
    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
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: 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.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.466643
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  • 7
    Digitale Medien
    Digitale Medien
    The CH+H reaction studied with quantum-mechanical and classical trajectory calculations (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 6002-6011 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: The CH+H→C+H2 reaction is studied with quantum-mechanical wave packet calculations and quasiclassical trajectory calculations using a CH2 ground-state potential energy surface. Although quantum tunneling is important for direct hydrogen abstraction, the dominance of the complex formation mechanism ensures the reliability of quasiclassical calculations. Most collisions ((approximate)80%) are nonreactive, because of a too-weak excitation of the CH vibration after a H–CH collision with H approaching CH with HCH angles larger than 60 deg. In this aspect the reaction differs from reactions such as the well-studied O(1D)+H2 reaction, where the H–H vibration in the triatomic complex is strongly excited. Also presented is the rate constant for a temperature range between 50 and 2000 K, obtained from quasiclassical cross-section results for collision energies between 0.0005 and 0.3 eV. The role of the excited triplet and singlet states of CH2 on the reaction dynamics is discussed. © 2002 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1459416
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  • 8
    Digitale Medien
    Digitale Medien
    Photodissociation of water in the A˜ band revisited with new potential energy surfaces (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 9453-9462 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Theoretical calculations on the photodissociation of water in the first absorption band have been used to test the accuracy of three available potential energy surfaces for the first excited state of water: the well-known coupled electron pair approximation potential of Staemmler and Palma [Chem. Phys. 93, 63 (1985)], and two new multireference double excitation configuration interaction surfaces: the Dobbyn–Knowles surface (unpublished), and the Leiden surface [R. van Harrevelt and M. C. van Hemert, J. Chem. Phys. 112, 5777 (2000)]. Exact quantum mechanical calculations, using the wave packet approach, have been performed for J″〉0, where J″ is the initial rotational state of the water molecule. The cross section was found not to depend strongly on the rotational state, so that it is reasonable to compare calculated cross sections for J″=0 with experimental room temperature cross sections. Small and simple corrections were applied to the potential energy surface to improve the agreement between theory and experiment for the cross section of H2O. Spectra for D2O and vibrationally excited water molecules calculated with all three corrected potential energy surfaces were in good agreement with experiments. A comparison between calculated OH(X) or OD(X) vibrational distributions, and recent kinetic energy release measurements of the H or D atoms produced in the 157.6 nm photodissociation of water and its isotopomers [Yang et al., J. Chem. Phys. 113, 10597 (2000)], however, suggests that the Leiden surface is more accurate than the two other surfaces. © 2001 American Institute of Physics.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.1370946
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  • 9
    Digitale Medien
    Digitale Medien
    An ab initio potential energy surface for the study of rotationally inelastic OH–H2 collisions (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 99 (1993), S. 3836-3846 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: A new two-configuration self-consistent field (SCF) plus dispersion potential for the interaction of OH with H2 is presented. The ground state of OH is a 2Π state leading to a two component potential which is diagonal in the adiabatic electronic basis in which the quantum chemical calculations are done. The transformation between the adiabatic basis and the electronic basis used in collision calculations is described. The transformed potential matrix is Hermitian in this electronic basis when OH–H2 configurations with no plane of symmetry are considered. The orientation dependence of the elements of the potential matrix is given in the form of a spherical expansion and the results are compared with previous potential calculations. The comparison is discussed in terms of the dependence on the orientation of the H2 molecule.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.466130
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  • 10
    Digitale Medien
    Digitale Medien
    The NaZn excimer: Blue–green band (1993)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 98 (1993), S. 4672-4679 
    Details
    ISSN: 1089-7690
    Quelle: AIP Digital Archive
    Thema: Physik , Chemie und Pharmazie
    Notizen: Electronically excited NaZn excimers have been produced in a photochemical reaction of Na2(C 1Πu) with Zn(4 1S0). The Na–Zn vapor mixture was prepared by introducing a controlled stream of sodium vapor into zinc vapor generated in a heat-pipe oven. A new band (bound–free transition) was observed in the region from 450 to 487 nm. Potential curves of the eight lowest electronic states of NaZn were calculated. On the basis of the calculations, the observed emission is interpreted as resulting from the 2 2Π–X 2Σ+ transition. The conditions favorable for the observation of the photochemical reaction are discussed.
    Materialart: Digitale Medien
    URL: http://dx.doi.org/10.1063/1.464971
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