Fast Track Communication

Abstract Photoionization of Mg 3s is studied near the Cooper minimum in dipole channels using the relativistic-random-phase approximation. While the importance of first-order nondipole effects on photoelectron angular distributions at low energies is well known, it is reported here for the first time that in the energy region near the dipole Cooper minimum, quadrupole transitions are not just important, but actually dominate the total photoionization cross section. Studies of dipole-dipole, dipole-quadrupole and quadrupole-quadrupole interference terms in the photoelectron angular distribution show that in the region of dipole Cooper minimum even the calculation of the dipole angular distribution parameter, β, requires the inclusion of quadrupole channels. The significance of second-order [O(k 2 r 2 )] nondipole terms, primarily due to the contributions from electric quadrupole-quadrupole interference terms at photon energy as low as ∼11 eV, are shown to induce dramatic changes in the photoelectron angular distribution over a small energy range

A theoretical study on the formation of iodine oxide aggregates and monohydrates

Biotic and abiotic emissions of molecular iodine and iodocarbons from the sea or the ice surface and the intertidal zone to the coastal/polar marine boundary layer lead to the formation of iodine oxides, which subsequently nucleate forming iodine oxide particles (IOPs). Although the link between coastal iodine emissions and ultrafine aerosol bursts is well established, the details of the nucleation mechanism have not yet been elucidated. In this paper, results of a theoretical study of a range of potentially relevant aggregation reactions of different iodine oxides, as well as complexation with water molecules, are reported. Thermochemical properties of these reactions are obtained from high level ab initio correlated calculations including spin-orbit corrections. The results show that the nucleation path most likely proceeds through dimerisation of I2O4. It is also shown that water can hinder gas-to-particle conversion to some extent, although complexation with key iodine oxides does not remove enough of these to stop IOP formation. A consistent picture of this process emerges from the theoretical study presented here and the findings of a new laboratory study reported in the accompanying paper (Gomez Martin et al., 2013)

Investigations on compton scattering: New directions

Although inelastic (Compton) scattering of a photon off a free electron was well understood about 80 years ago, inelastic scattering off bound electrons remains an incompletely understood process. The availability of synchrotron light sources has led a great enhancement in the precision of experimental measurements involving this process. As a result, approximations made in obtaining numerical predictions of physical observables are being reexamined by theorists. In this article, we present a comparison of experimental measurements to theoretical predictions to assess the need for future advances in both experiment and theory

Stereoelectronic effects and the kinetic acidities of diastereotopic hydrogens

A general qualitative treatment is proposed which accounts for the relative reactivities of diastereotopic hydrogens adjacent to a heteroatom in proton transfer, hydride transfer, and hydrogen atom transfer reactions. This treatment focuses on the structures of the reactive intermediates formed in such reactions, which can be predicted or understood in terms of qualitative molecular orbital arguments, and on the manner in which these intermediates will undergo readdition of H+, H\uaf, or H\ub7. The procedure has been tested, for the specific case of proton transfer reactions, by a direct computational approach to the energy differences between diastereomeric transition states of reactions HO\uaf + CH3X \u2192 H2O + \uafCH2X (X = sulfide, sulfoxide, sulfone, sulfonium). The computations support the qualitative treatment, and are in all cases in near quantitative agreement with the extensive experimental data concerning the diastereotopic selectivities exhibited in these systems.On propose une m\ue9thode qualitative g\ue9n\ue9rale qui tient compte des r\ue9activit\ue9s relatives des hydrog\ue8nes diast\ue9r\ue9otopiques adjacents \ue0 un h\ue9t\ue9roatome lors de r\ue9actions de transfert de protons, d'hydrures ou d'atomes d'hydrog\ue8ne. Cette m\ue9thode met l'accent \ue0 la fois sur la structure des interm\ue9diaires r\ue9actifs form\ue9s au cours de telles r\ue9actions, qui peuvent \ueatre pr\ue9vus ou compris en termes d'arguments qualitatifs d'orbitales mol\ue9culaires, et sur la fa\ue7on dont ces interm\ue9diaires peuvent subir une nouvelle addition de H+, H\uaf ou H\ub7. On a v\ue9rifi\ue9 cette m\ue9thode, dans le cas sp\ue9cifique de r\ue9actions de transfert de proton, par une technique de calculs directs des diff\ue9rences d'\ue9nergie entre les \ue9tats de transition diast\ue9r\ue9oisom\ue8res des r\ue9actions: HO\uaf + CH3X \u2192 H2O + \uafCH2X (X = sulfure, sulfoxyde, sulfone, sulfonium). Les calculs sont en accord avec l'approche qualitative et, dans tous les cas, ils sont aussi en accord quasi-quantitatif avec les donn\ue9es exp\ue9rimentales extensives relatives aux s\ue9lectivit\ue9s diast\ue9r\ue9otopiques observ\ue9es dans ces syst\ue8mes.NRC publication: Ye

Asymmetry and the shift of the Compton profile

We show that the conventionally defined asymmetry of the Compton profile (CP) is, to a large extent, simply a shift of CP. Compton scattering is widely used in studying the electron momentum distribution (EMD) of complex systems. Extraction of information about the EMD is based on an impulse approximation (IA) description of the process. In IA the scattering from bound electrons is described as scattering from the EMD of free electrons. Most often the angular and energy distributions of scattered photons (doubly differential cross sections (DDCS)) is measured and presented in terms of CP, which is just the DDCS normalized by a kinematical factor. The deviations of measured CP from the IA results are conventionally described as an asymmetry of CP about the IA peak position. IA predicts CP to be symmetric. We have examined the discrepancy between IA predictions (and the corresponding relativistic version of IA, RIA) and more rigorous approaches ( A 2 and S-matrix), using independent particle approximations for the description of the bound state of electrons. In the nonrelativistic region (in which many measurements of CP are performed) we find that the conventional asymmetry can largely be understood as the shift of the peak position. The true asymmetry with respect to the shifted peak position is in fact much smaller. RIA has similar properties to IA, except that for atoms with high nuclear charge the p ⇒ · A ⇒ interaction may modify the shift and limit the utility of description as a shift

Limitations on the validity of impulse approximation in Compton scattering

The validity of impulse approximation (IA), which is commonly used in the description of Compton scattering of photons from atomic electrons, is discussed with particular attention to the kinematical region in which the photon momentum transfer k is not much larger than the average bound electron momentum a of a given shell. IA can be justified in the Compton peak region of the spectrum if a/k≪1. However, for the doubly differential cross-section of photon–atom scattering (ejected electrons not observed) IA is commonly used, and viewed as adequate, while only requiring that a/k<1. In addition to a general discussion of the validity of IA (and the relativistic version RIA) for doubly and triply differential cross-sections, in this paper, we are particularly concerned with (1) the asymmetry around the IA peak of the Compton profile and (2) the contribution of the p→·A→ interaction term (neglected in IA) in the peak region for a/k<1. We argue that the observed asymmetry of the Compton profile is to a large extent just a shift of the IA profile. We find that p→·A→ contribution to the peak region for a/k≈1 is important only for scattering from high Z K-shells

Compton scattering revisited

We review the standard theory of Compton scattering from bound electrons, and we describe recent findings that require modification of the usual understanding, noting the nature of consequences for experiment. The subject began with Compton and scattering from free electrons. Experiment actually involved bound electrons, and this was accommodated with the use of impulse approximation (IA), which described inelastic scattering from bound electrons in terms of scattering from free electrons. This was good for the Compton peak but failed for soft final photons. The standard theory was formalized by Eisenberger and Platzman (EP) [1970. Phys. Rev. A 2, 415], whose work also suggested why impulse approximation was better than one would expect, for doubly differential cross sections (DDCS), but not for triply differential cross sections (TDCS). A relativistic version of IA (RIA) was worked out by Ribberfors [1975. Phys. Rev. B 12, 2067]. And Surić et al. [1991. Phys. Rev. Lett. 67, 189] and Bergstrom et al. [1993. Phys. Rev. A 48, 1134] developed a full relativistic second order S-matrix treatment, not making impulse approximation, but within independent particle approximation (IPA). Newer developments in the theory of Compton scattering include: (1) Demonstration that the EP estimates of the validity of IA are incorrect, although the qualitative conclusion remains unchanged; IA is not to be understood as the first term in a standard series expansion. (2) The greater validity of IA for DDCS than for the TDCS, which when integrated give DDCS, is related to the existence of a sum rule, only valid for DDCS. (3) The so-called “asymmetry” of a Compton profile is primarily to be understood as simply the shift of the peak position in the profile; symmetric and anti-symmetric deviations from a shifted Compton profile are very small, except for high Z inner shells where further p ⇒ · A ⇒ effects come into play. (4) Most relativistic effects, except at low energies, are to be understood in terms of simple kinematic modifications of nonrelativistic IA, plus using a relativistic charge density for high Z inner shell states; these shift the peak and change its height. However, for high Z, corrections to RIA persist in the peak region, even at extreme relativistic energies (correction of about 15% for Z = 92 )