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Search Results (4)

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Keywords = pump/probe photoemission

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7 pages, 1407 KB  
Article
Pump-Probe X-ray Photoemission Spectroscopy of Free-Standing Graphane
by Roberto Costantini, Dario Marchiani, Maria Grazia Betti, Carlo Mariani, Samuel Jeong, Yoshikazu Ito, Alberto Morgante and Martina Dell’Angela
Condens. Matter 2023, 8(2), 31; https://doi.org/10.3390/condmat8020031 - 27 Mar 2023
Cited by 2 | Viewed by 3899
Abstract
Free-standing nanoporous graphene was hydrogenated at about 60 at.% H uptake, as determined by the emerging of the sp3 bonding component in the C 1s core level investigated by high-resolution X-ray photoelectron spectroscopy (XPS). Fully unsupported graphane was investigated by XPS under [...] Read more.
Free-standing nanoporous graphene was hydrogenated at about 60 at.% H uptake, as determined by the emerging of the sp3 bonding component in the C 1s core level investigated by high-resolution X-ray photoelectron spectroscopy (XPS). Fully unsupported graphane was investigated by XPS under optical excitation at 2.4 eV. At a laser fluence of 1.6 mJ/cm2, a partial irreversible dehydrogenation of the graphane was observed, which could be attributed either to the local temperature increase or to a photo-induced softening of the H-to-C stretching mode. The sub-ns dynamics of the energy shift and peak broadening of the C 1s core level revealed two different decay constants: 210 ps and 130 ps, respectively, the former associated with photovoltage dynamics and the latter with thermal heating on a time scale comparable with the synchrotron temporal resolution. Full article
(This article belongs to the Special Issue Experimental Ideas for Novel FEL Facilities Based on Plasma Acceleration)
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9 pages, 1227 KB  
Article
Light-Induced Charge Accumulation in PTCDI/Pentacene/Ag(111) Heterojunctions
by Roberto Costantini, Albano Cossaro, Alberto Morgante and Martina Dell’Angela
Chemistry 2021, 3(3), 744-752; https://doi.org/10.3390/chemistry3030053 - 13 Jul 2021
Cited by 1 | Viewed by 3503
Abstract
The incorporation of singlet fission (SF) chromophores in solar cells is expected to bring significant increases in the power conversion efficiency thanks to multiexciton generation. However, efficient charge generation in the device is determined by the energy level alignment (ELA) between the active [...] Read more.
The incorporation of singlet fission (SF) chromophores in solar cells is expected to bring significant increases in the power conversion efficiency thanks to multiexciton generation. However, efficient charge generation in the device is determined by the energy level alignment (ELA) between the active materials, which should favor exciton transport and separation under illumination. By combining ultraviolet photoemission spectroscopy and optical differential reflectance measurements, we determine the ELA in a prototypical SF heterojunction between pentacene (Pc) and perylene-tetracarboxylic-diimide (PTCDI) grown on Ag(111). Time-resolved X-ray photoelectron spectroscopy on such a system reveals light-induced modifications of the ELA; by measuring the transient shift of the core level photoemission lines we observe an accumulation of long-lived holes in the PTCDI within the first hundred picoseconds after the optical pump. Full article
(This article belongs to the Special Issue A Special Issue in Honor of Professor Josef Michl)
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19 pages, 6569 KB  
Article
Picosecond Lifetime Hot Electrons in TiO2 Nanoparticles for High Catalytic Activity
by Bochao Li, Hao Li, Chang Yang, Boyu Ji, Jingquan Lin and Toshihisa Tomie
Catalysts 2020, 10(8), 916; https://doi.org/10.3390/catal10080916 - 10 Aug 2020
Cited by 7 | Viewed by 3643
Abstract
A large number of studies have examined the origins of high-catalytic activities of nanoparticles, but very few have discussed the lifetime of high-energy electrons in nanoparticles. The lifetime is one of the factors determining electron transfer and thus catalytic activity. Much of the [...] Read more.
A large number of studies have examined the origins of high-catalytic activities of nanoparticles, but very few have discussed the lifetime of high-energy electrons in nanoparticles. The lifetime is one of the factors determining electron transfer and thus catalytic activity. Much of the lifetime of electrons reported in the literature is too short for a high transfer-efficiency of photo-excited electrons from a catalyst to the attached molecules. We observed TiO2 nanoparticles using the femtosecond laser two-color pump-probe technique with photoemission electron microscopy having a 40 nm spatial resolution. A lifetime longer than 4 ps was observed together with a fast decay component of 100 fs time constant when excited by a 760 nm laser. The slow decay component was observed only when the electrons in an intermediate state pumped by the fundamental laser pulse were excited by the second harmonic pulse. The electronic structure for the asymmetry of the pump-probe signal and the origin of the two decay components are discussed based on the color center model of the oxygen vacancy. Full article
(This article belongs to the Special Issue Photo-Induced Electron Transfer Kinetics in Catalysis)
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8 pages, 234 KB  
Article
Constant Matrix Element Approximation to Time-Resolved Angle-Resolved Photoemission Spectroscopy
by James K. Freericks and H. R. Krishnamurthy
Photonics 2016, 3(4), 58; https://doi.org/10.3390/photonics3040058 - 8 Nov 2016
Cited by 8 | Viewed by 3995
Abstract
We discuss several issues associated with employing a constant matrix element approximation for the coupling of light to multiband electrons in the context of time-resolved angle-resolved photoemission spectroscopy (TR-ARPES). In particular, we demonstrate that the “constant matrix element approximation” —even when reasonable—only holds [...] Read more.
We discuss several issues associated with employing a constant matrix element approximation for the coupling of light to multiband electrons in the context of time-resolved angle-resolved photoemission spectroscopy (TR-ARPES). In particular, we demonstrate that the “constant matrix element approximation” —even when reasonable—only holds for specific choices of the one-electron basis, and changing to other bases, requires including nonconstant corrections to the matrix element. We also discuss some simplifying approximations, where a constant matrix element is employed in multiple bases, and the consequences of this further approximation (especially with respect to the calculated TR-ARPES signal becoming negative). We also discuss issues related to gauge invariance of the final spectra. Full article
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