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248. Imaging superatomic molecular orbitals in a 60 molecule through four 800-nm photons.

Author

Zhang, G. P., Zhu, H. P., Bai, Y. H., Bonacum, J., Wu, X. S., and George, Thomas F.

Subjects
MOLECULAR orbitals, PHOTONS, BLOCKS (Building materials), WAVELENGTHS, FULLERENES, ATOMIC clusters
Abstract

Superatomic molecular orbitals (SAMOs) in 60 are ideal building blocks for functional nanostructures. However, imaging them spatially in the gas phase has been unsuccessful. It is found experimentally that if 60 is excited by an 800-nm laser, the photoelectron casts an anisotropic velocity image, but the image becomes isotropic if excited at a 400-nm wavelength. This diffuse image difference has been attributed to electron thermal ionization, but more recent experiments (800 nm) reveal a clear nondiffuse image superimposed on the diffuse image, whose origin remains a mystery. Here we show that the nondiffuse anisotropic image is the precursor of the f SAMOs. We predict that four 800-nm photons can directly access the 1f SAMO, and with one more photon, can image the orbital, with the photoelectron angular distribution having two maxima at 0° and 180° and two humps separated by 56.5°. Since two 400-nm photons only resonantly excite the spherical 1s SAMO and four 800-nm photons excite the anisotropic 1f SAMO, our finding gives a natural explanation of the nondiffuse image difference, complementing the thermal scenario. [ABSTRACT FROM AUTHOR]

Published
2015
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249. Calibrating the α parameter of convective efficiency using observed stellar properties.

Author

Wu, X. S., Alexeeva, S., Mashonkina, L., Wang, L., Zhao, G., and Grupp, F.

Subjects
STELLAR atmospheres, STAR formation, CONVECTIVE clouds, SPECTRAL energy distribution, GIANT stars, BALMER line
Abstract

Context. Synthetic model atmosphere calculations are still the most commonly used tool when determining precise stellar parameters and stellar chemical compositions. Besides three-dimensional models that consistently solve for hydrodynamic processes, one dimensional models that use an approximation for convective energy transport play the major role. Aims. We use modern Balmer-line formation theory as well as spectral energy distribution (SED) measurements for the Sun and Procyon to calibrate the model parameter that describes the effciency of convection in our 1D models. Convection was calibrated over a significant range in parameter space, reaching from F-K along the main sequence and sampling the turno and giant branch over a wide range of metallicities. This calibration was compared to theoretical evaluations and allowed an accurate modeling of stellar atmospheres. Methods. We used Balmer-line fitting and SED fits to determine the convective effciency parameter α. Both methods are sensitive to the structure and temperature stratification of the deeper photosphere. Results. While SED fits do not allow a precise determination of the convective parameter for the Sun and Procyon, they both favor values significantly higher than 1.0. Balmer-line fitting, which we find to be more sensitive, suggests that the convective effciency parameter α is ≈2.0 for the main sequence and quickly decreases to ≈1.0 for evolved stars. These results are highly consistent with predictions from 3D models. While the values on the main sequence fit predictions very well, measurements suggest that the decrease of convective effciency as stars evolve to the giant branch is more dramatic than predicted by models. [ABSTRACT FROM AUTHOR]

Published
2015
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