Proceedings of XII International Symposium on Nuclei in the Cosmos — PoS(NIC XII), 2013
PoS(NIC XII)145 Stars with M ≤ 2.3M ⊙ are considered Li depletion sites during their early and la... more PoS(NIC XII)145 Stars with M ≤ 2.3M ⊙ are considered Li depletion sites during their early and late evolutionary stages. Indeed 7 Li, synthesized through electron-captures on 7 Be, is burnt into two alpha particles, when the H-shell burns below convective envelopes of Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) stars. Furthermore, Li abundances observed in the spectra of these stars cover a wide range of values, which is difficult to be explained by stellar models, both Li-rich and Li-poor objects being observed. Difficulties arise in measuring very low Li abundances in O-rich AGBs and the lack of knowledge about the physical causes of extra-mixing, but the main source of uncertainty in investigating Li nucleosynthesis concerns the 7 Be lifetime , as the available estimations actually are valid only for the Sun [1]. Since in RGB and AGB H-shell burning temperatures and densities might be respectively up to five times higher and five orders of magnitudes lower than in the solar core, using for giants a 7 Be lifetime , extrapolated from the one valid in solar condition is at least hazardous. We present the consequence on Li nucleosynthesis of a new 7 Be lifetime estimation computed by a theoretical approach inherited from quantum chemistry. Extra-mixing models integrated with this new datum well reproduce the trends of Li destruction observed in RGBs and are suitable to account for Li observations in AGBs, explaining also the amount of nuclei from C to Al.
Summary Described here are new computational procedures which have been developed for obtaining ... more Summary Described here are new computational procedures which have been developed for obtaining the continuum orbital in Auger problems. A Lippmann-Schwinger (LS) equation is solved for the inner, atomic or molecular region and in the asymptotic region the LS orbital is matched with the eigenfunctions of the long-range Hamiltonian. A detailed analysis of the form of the continuum orbital for various
Summary Characteristics and advantages of a basis set of Hermite Gaussian functions modulated by... more Summary Characteristics and advantages of a basis set of Hermite Gaussian functions modulated by plane waves (THGF) are discussed and analysed in a specific case. The expressions of the prototype integrals necessary in variational calculations for bound and continuum states are given.
We analyze the 55 134 Cs → 56 134 Ba and 55 135 Cs → 56 135 Ba β − decays, which are crucial prod... more We analyze the 55 134 Cs → 56 134 Ba and 55 135 Cs → 56 135 Ba β − decays, which are crucial production channels for Ba isotopes in asymptotic giant branch (AGB) stars. We calculate, starting from relativistic quantum mechanics, the effects of multichannel scattering onto weak decays, including nuclear and electronic excited states (ESs) populated above ≃10 keV, for both parent and daughter nuclei. We find increases in the half-lives for T > 108 K (by more than a factor of 3 for 134Cs) as compared to previous works based on systematics. We also discuss our method in view of these previous calculations. An important impact on half-lives comes from nuclear ES decays, while including electronic temperatures yields further increases of about 20% at energies of 10–30 keV, typical of AGB stars of moderate mass (M ≲ 8 M ⊙). Despite properly considering these effects, the new rates remain sensitively lower than the Takahashi & Yokoi values, implying longer half-lives at least above 8–9 k...
Angle resolved-Auger-photoelectron coincidence spectroscopy (AR-APECS) has been exploited to inve... more Angle resolved-Auger-photoelectron coincidence spectroscopy (AR-APECS) has been exploited to investigate the role that electron correlation plays in the exchange-coupling at the ferromagnetic/antiferromagnetic interface of a Fe/CoO bilayer grown on Ag(001). The effective correlation energy U eff , usually employed to assess the energy distribution of core-valence-valence Auger spectra, has been experimentally determined for each possible combination of the orbital (e g or t 2g) and the spin (majority or minority) of the two valence electrons involved in the Auger decay. Coulomb and exchange interactions have been identified and compared with the result obtained on the Fe/Ag system. The presented analysis reveals in the Fe/CoO interface an enhancement of the Coulomb interaction for the e g orbital and of the exchange interaction for the t 2g orbital with respect to the Fe/Ag case, that can be associated to the stronger electron
Context. Electron-capture on 7Be is the main production channel for 7Li in several astrophysical ... more Context. Electron-capture on 7Be is the main production channel for 7Li in several astrophysical environments. Theoretical evaluations have to account for not only the nuclear interaction, but also the processes in the plasma in which 7Be ions and electrons interact. In recent decades several estimates were presented, pointing out that the theoretical uncertainty in the rate is in general of a few percent. Aims. In the framework of fundamental solar physics, we consider a recent evaluation for the 7Be+e− rate, which has not been used up to now, in the estimate of neutrino fluxes. Methods. We analyzed the effects of the new assumptions on standard solar models (SSMs) and compared the results obtained by adopting the revised 7Be+e− rate to those obtained by that reported in a widely used compilation of reaction rates (ADE11). Results. We found that new SSMs yield a maximum difference in the efficiency of the 7Be channel of about −4% with respect to what is obtained with the previously...
In most cases RGB and AGB stars with M≤ 2M destroy Li (which is instead synthesized trough electr... more In most cases RGB and AGB stars with M≤ 2M destroy Li (which is instead synthesized trough electron-captures on 7 Be). This occurs through the combined operation of mixing processes and proton captures, when H-burning operates close to the envelope. Observed Li abundances are however difficult to explain, as they cover a wide spread. Various uncertainties affect model attempts, but so far the largest one concerns the processes of bound and free ecaptures on 7 Be, hence its lifetime , whose known estimates are valid only for solar conditions. RGB and AGB stages have temperatures and densities below the envelope covering a wide range and differing from solar by up to a factor of five for T and up to five orders of magnitudes for ρ, hence extrapolations are unreliable. Recently, we presented an estimate of the 7 Be half-life based on a fully quantistic method that goes beyond the Debye-Hückel approximation. Here we discuss its consequences on Li nucleosynthesis in low mass AGB stars.
The theoretical framework of a method that utilizes a projected potential operator to construct s... more The theoretical framework of a method that utilizes a projected potential operator to construct scattering wave functions is presented. Theorems and spectral properties of a Hamiltonian with the potential energy operator represented in terms of L2(~3)-functions are derived. The computational advantages offered by the method for calculating spectroscopic quantities, like resonance energies, decay probabilities and photoionization cross-sections, are discussed.
Summary Screening effects due to the presence of an excited electron, acting as spectator during... more Summary Screening effects due to the presence of an excited electron, acting as spectator during the decay process of autoionizing and shake-up states, are analysed in the specific case of the Auger spectrum of LiF molecule. Decreasing of total decay probability and enhancement of the energy of the emitted electrons results as the major effects that can be ascribed to electronic
Summary Analytic expressions of prototype integrals for electronic-energy calculations involving... more Summary Analytic expressions of prototype integrals for electronic-energy calculations involving plane waves and Hermite-Gaussian functions of every order are explicitly derived. A few notes on the use of such a mixed basis set in solid-state and molecular physics are given.
A general expression for predicting vibrational profiles of a molecular Auger spectrum is derived... more A general expression for predicting vibrational profiles of a molecular Auger spectrum is derived. The relationship between a "theoretical" cross section and an experimental spectrum is discussed and specific procedures for implementing cross-section calculations in the Born-Oppenheimer approximation are proposed. The carbon and oxygen K-LL Auger spectra of CO are reproduced on the entire energy range of interest and specific spectral regions are analyzed in greater detail. The results are compared with experimental data.
A general approach, recently proposed for treating multichannel scattering processes, is summariz... more A general approach, recently proposed for treating multichannel scattering processes, is summarized. Applications to coreionization and autoionization spectra of the CO molecule are presented and the results compared with experimental spectra. The presence of asymmetric (Fano) profiles in the autoionization spectrum of the CO is discussed.
We show with angle-resolved photoemission spectroscopy that a new energy band appears in the elec... more We show with angle-resolved photoemission spectroscopy that a new energy band appears in the electronic structure of electron doped hydrogenated monolayer graphene (H-graphene). Its occupation can be controlled with the hydrogen amount and allows for tuning of graphene's doping level. Our calculations of the electronic structure of H-graphene suggest that this state is largely composed from hydrogen 1s orbitals and remains extended for low H coverages despite the random chemisorption of H. Further evidence for the existence of a hydrogen state is provided by X-ray absorption studies of undoped H-graphene which are clearly showing the emergence of an additional state in the vicinity of the π *-resonance.
Journal of Electron Spectroscopy and Related Phenomena, 2001
In this work, the angular correlation of the two electrons produced in the cascade of Ne 1s−1 3p→... more In this work, the angular correlation of the two electrons produced in the cascade of Ne 1s−1 3p→Ne+ 2s−12p−1 (1P) 3p+eA (E=778.4 eV)→Ne++ 2p−2 (1D, 3P)+eC (E=22.3, 25.7 eV) has been measured for the first time in a coincidence experiment. These measurements have been complemented by the non-coincidence angular distributions and energy spectra of both resonant Auger and second step
The proximity effect, which arises at the interface between two fermionic superfluids with differ... more The proximity effect, which arises at the interface between two fermionic superfluids with different critical temperatures, is examined with a non-local (integral) equation whose kernel contains information about the size of Cooper pairs that leak across the interface. This integral approach avoids reference to the boundary conditions at the interface that would be required with a differential approach. The temperature dependence of the pair penetration depth on the normal side of the interface is determined over a wide temperature range, also varying the inter-particle coupling along the BCS-BEC crossover independently on both sides of the interface. Conditions are found for which the proximity effect is optimized in terms of the extension of the pair penetration depth.
With reference to the broad and narrow Fano-Feshbach resonances of 6Li at about 822 G and 543 G, ... more With reference to the broad and narrow Fano-Feshbach resonances of 6Li at about 822 G and 543 G, we show that for the broad resonance a molecular coupled-channel calculation can be mapped with excellent accuracy onto an effective single-channel problem with a contact interaction. This occurs for a wide enough range of the magnetic field, that the full BCS-BEC crossover can be realized with a typical trap. For the narrow resonance, the mapping onto a single-channel problem and the realization of the BCS-BEC crossover are restricted to too narrow a range of the magnetic field to obtain them in practice. In this way, the BCS-BEC crossover for Fermi atoms with the broad resonance is placed on the same footing as the corresponding crossover for different physical systems.
Proceedings of XII International Symposium on Nuclei in the Cosmos — PoS(NIC XII), 2013
PoS(NIC XII)145 Stars with M ≤ 2.3M ⊙ are considered Li depletion sites during their early and la... more PoS(NIC XII)145 Stars with M ≤ 2.3M ⊙ are considered Li depletion sites during their early and late evolutionary stages. Indeed 7 Li, synthesized through electron-captures on 7 Be, is burnt into two alpha particles, when the H-shell burns below convective envelopes of Red Giant Branch (RGB) and Asymptotic Giant Branch (AGB) stars. Furthermore, Li abundances observed in the spectra of these stars cover a wide range of values, which is difficult to be explained by stellar models, both Li-rich and Li-poor objects being observed. Difficulties arise in measuring very low Li abundances in O-rich AGBs and the lack of knowledge about the physical causes of extra-mixing, but the main source of uncertainty in investigating Li nucleosynthesis concerns the 7 Be lifetime , as the available estimations actually are valid only for the Sun [1]. Since in RGB and AGB H-shell burning temperatures and densities might be respectively up to five times higher and five orders of magnitudes lower than in the solar core, using for giants a 7 Be lifetime , extrapolated from the one valid in solar condition is at least hazardous. We present the consequence on Li nucleosynthesis of a new 7 Be lifetime estimation computed by a theoretical approach inherited from quantum chemistry. Extra-mixing models integrated with this new datum well reproduce the trends of Li destruction observed in RGBs and are suitable to account for Li observations in AGBs, explaining also the amount of nuclei from C to Al.
Summary Described here are new computational procedures which have been developed for obtaining ... more Summary Described here are new computational procedures which have been developed for obtaining the continuum orbital in Auger problems. A Lippmann-Schwinger (LS) equation is solved for the inner, atomic or molecular region and in the asymptotic region the LS orbital is matched with the eigenfunctions of the long-range Hamiltonian. A detailed analysis of the form of the continuum orbital for various
Summary Characteristics and advantages of a basis set of Hermite Gaussian functions modulated by... more Summary Characteristics and advantages of a basis set of Hermite Gaussian functions modulated by plane waves (THGF) are discussed and analysed in a specific case. The expressions of the prototype integrals necessary in variational calculations for bound and continuum states are given.
We analyze the 55 134 Cs → 56 134 Ba and 55 135 Cs → 56 135 Ba β − decays, which are crucial prod... more We analyze the 55 134 Cs → 56 134 Ba and 55 135 Cs → 56 135 Ba β − decays, which are crucial production channels for Ba isotopes in asymptotic giant branch (AGB) stars. We calculate, starting from relativistic quantum mechanics, the effects of multichannel scattering onto weak decays, including nuclear and electronic excited states (ESs) populated above ≃10 keV, for both parent and daughter nuclei. We find increases in the half-lives for T > 108 K (by more than a factor of 3 for 134Cs) as compared to previous works based on systematics. We also discuss our method in view of these previous calculations. An important impact on half-lives comes from nuclear ES decays, while including electronic temperatures yields further increases of about 20% at energies of 10–30 keV, typical of AGB stars of moderate mass (M ≲ 8 M ⊙). Despite properly considering these effects, the new rates remain sensitively lower than the Takahashi & Yokoi values, implying longer half-lives at least above 8–9 k...
Angle resolved-Auger-photoelectron coincidence spectroscopy (AR-APECS) has been exploited to inve... more Angle resolved-Auger-photoelectron coincidence spectroscopy (AR-APECS) has been exploited to investigate the role that electron correlation plays in the exchange-coupling at the ferromagnetic/antiferromagnetic interface of a Fe/CoO bilayer grown on Ag(001). The effective correlation energy U eff , usually employed to assess the energy distribution of core-valence-valence Auger spectra, has been experimentally determined for each possible combination of the orbital (e g or t 2g) and the spin (majority or minority) of the two valence electrons involved in the Auger decay. Coulomb and exchange interactions have been identified and compared with the result obtained on the Fe/Ag system. The presented analysis reveals in the Fe/CoO interface an enhancement of the Coulomb interaction for the e g orbital and of the exchange interaction for the t 2g orbital with respect to the Fe/Ag case, that can be associated to the stronger electron
Context. Electron-capture on 7Be is the main production channel for 7Li in several astrophysical ... more Context. Electron-capture on 7Be is the main production channel for 7Li in several astrophysical environments. Theoretical evaluations have to account for not only the nuclear interaction, but also the processes in the plasma in which 7Be ions and electrons interact. In recent decades several estimates were presented, pointing out that the theoretical uncertainty in the rate is in general of a few percent. Aims. In the framework of fundamental solar physics, we consider a recent evaluation for the 7Be+e− rate, which has not been used up to now, in the estimate of neutrino fluxes. Methods. We analyzed the effects of the new assumptions on standard solar models (SSMs) and compared the results obtained by adopting the revised 7Be+e− rate to those obtained by that reported in a widely used compilation of reaction rates (ADE11). Results. We found that new SSMs yield a maximum difference in the efficiency of the 7Be channel of about −4% with respect to what is obtained with the previously...
In most cases RGB and AGB stars with M≤ 2M destroy Li (which is instead synthesized trough electr... more In most cases RGB and AGB stars with M≤ 2M destroy Li (which is instead synthesized trough electron-captures on 7 Be). This occurs through the combined operation of mixing processes and proton captures, when H-burning operates close to the envelope. Observed Li abundances are however difficult to explain, as they cover a wide spread. Various uncertainties affect model attempts, but so far the largest one concerns the processes of bound and free ecaptures on 7 Be, hence its lifetime , whose known estimates are valid only for solar conditions. RGB and AGB stages have temperatures and densities below the envelope covering a wide range and differing from solar by up to a factor of five for T and up to five orders of magnitudes for ρ, hence extrapolations are unreliable. Recently, we presented an estimate of the 7 Be half-life based on a fully quantistic method that goes beyond the Debye-Hückel approximation. Here we discuss its consequences on Li nucleosynthesis in low mass AGB stars.
The theoretical framework of a method that utilizes a projected potential operator to construct s... more The theoretical framework of a method that utilizes a projected potential operator to construct scattering wave functions is presented. Theorems and spectral properties of a Hamiltonian with the potential energy operator represented in terms of L2(~3)-functions are derived. The computational advantages offered by the method for calculating spectroscopic quantities, like resonance energies, decay probabilities and photoionization cross-sections, are discussed.
Summary Screening effects due to the presence of an excited electron, acting as spectator during... more Summary Screening effects due to the presence of an excited electron, acting as spectator during the decay process of autoionizing and shake-up states, are analysed in the specific case of the Auger spectrum of LiF molecule. Decreasing of total decay probability and enhancement of the energy of the emitted electrons results as the major effects that can be ascribed to electronic
Summary Analytic expressions of prototype integrals for electronic-energy calculations involving... more Summary Analytic expressions of prototype integrals for electronic-energy calculations involving plane waves and Hermite-Gaussian functions of every order are explicitly derived. A few notes on the use of such a mixed basis set in solid-state and molecular physics are given.
A general expression for predicting vibrational profiles of a molecular Auger spectrum is derived... more A general expression for predicting vibrational profiles of a molecular Auger spectrum is derived. The relationship between a "theoretical" cross section and an experimental spectrum is discussed and specific procedures for implementing cross-section calculations in the Born-Oppenheimer approximation are proposed. The carbon and oxygen K-LL Auger spectra of CO are reproduced on the entire energy range of interest and specific spectral regions are analyzed in greater detail. The results are compared with experimental data.
A general approach, recently proposed for treating multichannel scattering processes, is summariz... more A general approach, recently proposed for treating multichannel scattering processes, is summarized. Applications to coreionization and autoionization spectra of the CO molecule are presented and the results compared with experimental spectra. The presence of asymmetric (Fano) profiles in the autoionization spectrum of the CO is discussed.
We show with angle-resolved photoemission spectroscopy that a new energy band appears in the elec... more We show with angle-resolved photoemission spectroscopy that a new energy band appears in the electronic structure of electron doped hydrogenated monolayer graphene (H-graphene). Its occupation can be controlled with the hydrogen amount and allows for tuning of graphene's doping level. Our calculations of the electronic structure of H-graphene suggest that this state is largely composed from hydrogen 1s orbitals and remains extended for low H coverages despite the random chemisorption of H. Further evidence for the existence of a hydrogen state is provided by X-ray absorption studies of undoped H-graphene which are clearly showing the emergence of an additional state in the vicinity of the π *-resonance.
Journal of Electron Spectroscopy and Related Phenomena, 2001
In this work, the angular correlation of the two electrons produced in the cascade of Ne 1s−1 3p→... more In this work, the angular correlation of the two electrons produced in the cascade of Ne 1s−1 3p→Ne+ 2s−12p−1 (1P) 3p+eA (E=778.4 eV)→Ne++ 2p−2 (1D, 3P)+eC (E=22.3, 25.7 eV) has been measured for the first time in a coincidence experiment. These measurements have been complemented by the non-coincidence angular distributions and energy spectra of both resonant Auger and second step
The proximity effect, which arises at the interface between two fermionic superfluids with differ... more The proximity effect, which arises at the interface between two fermionic superfluids with different critical temperatures, is examined with a non-local (integral) equation whose kernel contains information about the size of Cooper pairs that leak across the interface. This integral approach avoids reference to the boundary conditions at the interface that would be required with a differential approach. The temperature dependence of the pair penetration depth on the normal side of the interface is determined over a wide temperature range, also varying the inter-particle coupling along the BCS-BEC crossover independently on both sides of the interface. Conditions are found for which the proximity effect is optimized in terms of the extension of the pair penetration depth.
With reference to the broad and narrow Fano-Feshbach resonances of 6Li at about 822 G and 543 G, ... more With reference to the broad and narrow Fano-Feshbach resonances of 6Li at about 822 G and 543 G, we show that for the broad resonance a molecular coupled-channel calculation can be mapped with excellent accuracy onto an effective single-channel problem with a contact interaction. This occurs for a wide enough range of the magnetic field, that the full BCS-BEC crossover can be realized with a typical trap. For the narrow resonance, the mapping onto a single-channel problem and the realization of the BCS-BEC crossover are restricted to too narrow a range of the magnetic field to obtain them in practice. In this way, the BCS-BEC crossover for Fermi atoms with the broad resonance is placed on the same footing as the corresponding crossover for different physical systems.
Uploads
Papers by S. Simonucci