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  • 1
    Electronic Resource
    Electronic Resource
    Light-cone expansion of the Dirac sea in the presence of chiral and scalar potentials (2000)
    College Park, Md. : American Institute of Physics (AIP)
    Journal of Mathematical Physics 41 (2000), S. 6689-6746 
    Details
    ISSN: 1089-7658
    Source: AIP Digital Archive
    Topics: Mathematics , Physics
    Notes: We study the Dirac sea in the presence of external chiral and scalar/pseudoscalar potentials. In preparation, a method is developed for calculating the advanced and retarded Green's functions in an expansion around the light cone. For this, we first expand all Feynman diagrams and then explicitly sum up the perturbation series. The light-cone expansion expresses the Green's functions as an infinite sum of line integrals over the external potential and its partial derivatives. The Dirac sea is decomposed into a causal and a noncausal contribution. The causal contribution has a light-cone expansion which is closely related to the light-cone expansion of the Green's functions; it describes the singular behavior of the Dirac sea in terms of nested line integrals along the light cone. The noncausal contribution, on the other hand, is, to every order in perturbation theory, a smooth function in position space. © 2000 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1290379
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  • 2
    Electronic Resource
    Electronic Resource
    Q-band ESR studies on the origin of magnetic anisotropy in tetrahalogenated semiquinones (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 83 (1985), S. 6121-6127 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The g-tensor components associated with tetrachloro-, tetrabromo-, and tetraiodo-benzosemiquinones in frozen solutions of dimethylsulfoxide and ethanol-D at 135 K have been measured by ESR spectroscopy at Q band (35 GHz). Spin-orbit interactions on the halogens make a significant contribution to the g tensor; components in the plane of the aromatic ring, (gx and gy) are enhanced, whereas the component perpendicular to the ring (gz) is decreased. Related solvent effects have been observed in the spectrum of tetraiodobenzosemiquinone; in going from dimethylsulfoxide to ethanol gx and gy were found to increase, whereas gz decreased. It is argued that the positive contribution to gx and gy arises from the participation of halogen pz orbitals in the molecular π orbitals and that the negative contribution to gz in part reflects spin density on the halogens due to σ–π interaction induced configurational admixture in the wave function. Possible participation of halogen d orbitals is discussed. The dominant part of the solvent dependence of the g tensor components is suggested to result from changes in the energy separation between the orbital of the unpaired electron and lone-pair orbitals on the halogens.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.449604
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  • 3
    Electronic Resource
    Electronic Resource
    Measurement of the HD(v′=2,J′=3) product differential cross section for the H+D2 exchange reaction at 1.55±0.05 eV using the photoloc technique (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1022-1034 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We describe a time-of-flight apparatus that uses core extraction to determine nascent product laboratory velocity distributions from which differential cross sections may be deduced. We emphasize the characterization of the instrument, the reaction conditions, and the calibration procedure. For this purpose, we have measured H-atom velocity distributions from HBr photolysis, as well as the H2(v′=4,J′=1) velocity distribution arising from the H+HBr reaction under quasi-monoenergetic collision conditions at 1.9 eV. Collisional energy spread and reagent internal state distributions were determined from the rotational and translational temperatures of the HBr photolytic precursor and the D2 diatomic reagent. The differential cross section for H+D2→HD(v′=2,J′=3)+D at 1.55±0.05 eV is presented and found to peak near 145°±10° with an approximate full width at half maximum (FWHM) of 40°. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479294
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  • 4
    Electronic Resource
    Electronic Resource
    Example of a group action determined phase transition (1985)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 82 (1985), S. 1496-1503 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The principles of the group action approach to structural phase transitions are outlined. It is assumed that all properties of the transition are determined by the action of a single physically irreducible represention of the space group of the more symmetric phase. We determine the isotropy groups using the image space of the representation. The free energy minima are determined to fourth order and to all orders using the results of Gufan and then compared. This theory is applied to Calcite (R3¯c) to determine all possible continuous commensurate phase transitions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.448948
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  • 5
    Electronic Resource
    Electronic Resource
    Electronic spectra of the chains HC2nH (n=8–13) in the gas phase (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 114 (2001), S. 2208-2212 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The intense 1Σu+←X1Σg+ electronic transition of the chains HC2nH (n=8–13) has been recorded in the gas phase using resonant two color two photon ionization spectroscopy. The gas phase frequencies lie ∼1300 cm−1 to the blue of the values in a neon matrix. These polyynes appear to be linear in both the ground and the excited state and, as for the polyynes, spectral and theoretical evidence for large bond alternation in the ground state is found indicating localized π orbitals on the C(Triple Bond)C units. These species are the longest carbon chains observed spectroscopically in the gas phase up to now. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1338530
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  • 6
    Electronic Resource
    Electronic Resource
    Differential cross sections for H+D2→HD(v′=1, J′=1,5,8)+D at 1.7 eV (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 111 (1999), S. 1035-1042 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A 1:4 mixture of HBr and D2 is expanded into a vacuum chamber, fast H atoms are generated by photolysis of HBr ca. 210 nm, and the resulting HD (v′, J′) products are detected by (2+1) resonance-enhanced multiphoton ionization (REMPI) in a Wiley–McLaren time-of-flight spectrometer. The photoloc technique allows a direct inversion of HD (v′, J′) core-extracted time-of-flight profiles into differential cross sections for the H+D2→HD(v′=1, J′=1,5,8)+D reactions at collision energies ca. 1.7 eV. The data reveal a systematic trend from narrow, completely backward scattering for HD (v′=1, J′=1) toward broader, side scattering for HD (v′=1, J′=8). A calculation based on the line of centers model with nearly elastic specular scattering accounts qualitatively for the observations. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.479191
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  • 7
    Electronic Resource
    Electronic Resource
    State-resolved differential and integral cross sections for the reaction H+D2→HD(v′=3,j′=0–7)+D at 1.64 eV collision energy (2002)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 116 (2002), S. 6634-6639 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A 212.8 nm laser initiates the reaction H+D2→HD+D in a mixture of HBr and D2. A second laser state-selectively ionizes the HD(v′=3,j′) reaction product, allowing a determination of the speed distribution and the relative cross section in a velocity-sensitive time-of-flight mass spectrometer. From these measurements we construct differential and integral cross sections for H+D2→HD(v′=3,j′=0–7)+D at 1.64±0.05 eV collision energy. Although the integral cross sections do not show any unusual features, the differential cross sections reveal forward-scattered features that have not been observed in crossed-beam experiments. An analysis of the scattering features in HD(v′=3,j′=1–4) suggests that these states are dominated by classical hard-sphere scattering. This hard-sphere (direct recoil) mechanism, however, cannot account for the dominant forward scattering observed in HD(v′=3,j′=0). © 2002 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1462576
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  • 8
    Electronic Resource
    Electronic Resource
    Forward scattering in the H+D2→HD+D reaction: Comparison between experiment and theoretical predictions (2001)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 115 (2001), S. 4534-4545 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We investigate the sensitivity of photoinitiated experiments to forward-scattering features by direct comparison of experimental angular distributions with quantum-mechanical calculations as well as by forward-convolution of theoretical and model center-of-mass differential cross sections. We find that the experimental sensitivity to forward-scattering angles depends on the instrumental velocity resolution as well as on the kinematics of the detected product channel. Explicit comparison is made between experimental HD(v′=1,2;j′) center-of-mass angular distributions at collision energies (approximate)1.6 eV (deduced from time-of-flight profiles using a single-laser, photolysis-probe approach) and quantum-mechanical calculations on the BKMP2 potential energy surface. The comparison takes into account the contributions from both slow and fast H atoms from the photolysis of HBr. We find that the contribution of the slow H atoms, which is the major source of experimental uncertainty, does not greatly affect the extraction of the angular distribution from the experimental time-of-flight profile for a specific HD(v′,j′) state. Except for HD(v′=1, j′=8) and HD(v′=2, j′=0), for which either slow H atoms or the presence of a narrow forward-scattering peak make the analysis more uncertain, the agreement between experiment and theoretical predictions is excellent. © 2001 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.1390505
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  • 9
    Electronic Resource
    Electronic Resource
    Local U(2,2) symmetry in relativistic quantum mechanics (1998)
    College Park, Md. : American Institute of Physics (AIP)
    Journal of Mathematical Physics 39 (1998), S. 6276-6290 
    Details
    ISSN: 1089-7658
    Source: AIP Digital Archive
    Topics: Mathematics , Physics
    Notes: Local gauge freedom in relativistic quantum mechanics is derived from a measurement principle for space and time. For the Dirac equation, one obtains local U(2,2) gauge transformations acting on the spinor index of the wave functions. This local U(2,2) symmetry allows a unified description of electrodynamics and general relativity as a classical gauge theory. © 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.532638
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  • 10
    Electronic Resource
    Electronic Resource
    Local order and diffusive motions in simple liquids (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 2864-2870 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: In order to study the properties of the short-range order in simple liquids, a "moving average filter'' is applied to the positions and velocities obtained from molecular dynamics simulations of L-J systems, in order to reduce the thermal motions. The fictitious systems that are obtained allow a better analysis, not only of the underlying structure, but also of the time-dependent low-frequency properties, such as diffusive motions. From the analysis of the correlation radial function g(r) and the recently proposed [S. Mazur, J. Chem. Phys. 97, 9276 (1992)] "neighborship distributions,'' P(n,r), it is shown that the face-centered-cubic lattice is inadequate to model the local order in the liquid state. To study the nature of the diffusive motions, a spatial-dependent velocity correlation function is computed. It shows significant positive values for the filtered systems. This suggests a cooperative mechanism for diffusion. However, it seems that the collective motions do not involve large clusters, but rather small clusters and linear "chains'' of molecules. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.468664
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