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Reorientational dynamics of organic cations in perovskite-like coordination polymers
摘要： Here we report the dynamics of organic cations as guest molecules in a perovskite host-framework. The molecular motion of CH3NH3+ (MAFe), (CH3)2NH2+ (DMAFe) and (CH3)3NH+ (TrMAFe) in the cage formed by KFe(CN)63? units was studied using a combination of experimental methods: (i) thermal analysis, (ii) dielectric and electric studies, (iii) optical observations, (iv) EPR and 1H NMR spectroscopy and (v) quasielastic neutron scattering (QENS). In the case of MAFe and TrMAFe, the thermal analysis reveals one solid-to-solid phase transition (PT) and two PTs for the DMAFe crystal. A markedly temperature-dependent dielectric constant indicates the tunable and switchable properties of the complexes. Also, their semiconducting properties are confirmed by a dc conductivity measurement. The broadband dielectric relaxation is analyzed for the TrMAFe sample in the frequency range of 100 Hz–1 GHz. QENS shows that we deal rather with the localized motion of the cation than a diffusive one. Three models, which concern the simultaneous rotation of the CH3 and/or NH3 group, π-flips and free rotations of the organic cation, are used to fit the elastic incoherent structure factor. The 1H NMR spin–lattice relaxation time for all compounds under study, as well as the second moments, has been measured in a wide temperature range. In all studied samples, the temperature dependence of the second moment of the proton NMR line indicated the gradual evolution of the molecular movements from the rigid state up to a highly disordered one.
关键词： quasielastic neutron scattering,phase transitions,perovskite,coordination polymers,dielectric properties,NMR spectroscopy,dynamics,organic cations
更新于2025-11-14 17:28:48
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Active Metasurface Devices for Terahertz Wave Front Modulation
摘要： Terahertz (THz) radiation, which is sandwiched between the infrared and microwave in the electromagnetic spectrum, has become more and more attractive due to its special properties. Compared with the quick development of THz sources and detectors, THz functional devices are lagging behind, especially the devices for wave front modulation. Metasurface, a kind of artificial materials, is enable effective manipulation of the amplitude, phase and polarization state of electromagnetic waves with subwavelength spatial resolution. It has been used for ultrathin planar lenses, complex light field generation, spin orbital angular momentum coupling, and metasurface holograms. However, the function of this kind of metasurface based devices are fixed once they are fabricated, the spatial THz modulation which can dynamically modulate the wave front of THz radiation is demand in high resolution THz imaging and THz communication. In this presentation, we present a novel approach for generating arbitrary wave fronts of a THz beam by dynamically creating metasurface structures through illuminating a super thin silicon wafer with femtosecond laser, which is spatially modulated to result in an array of reconfigurable subwavelength resonators. The wave front of the THz beam is then determined by forming spatial profiles of the Pancharatnam-Berry scattering phase by dynamically controlling the resonator orientation. Proof-of-concept experiments demonstrate that streaming holographic images and lenses of variable focal length can be realized in real time. The theoretical expectations and experimental results are shown in Fig. 1, the generated patterns are matching with the theoretical exceptions. The reconfigurable scheme demonstrated here is convenient and fast, and may lead to advances in a host of THz applications.
关键词： Pancharatnam-Berry scattering phase,reconfigurable subwavelength resonators,wave front modulation,metasurface,Terahertz
更新于2025-09-12 10:27:22
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Improved molecular structure retrieval method for photoelectron holography
摘要： Photoelectron holography is a method that retrieves molecular structure information from photoelectron diffraction patterns generated by x-ray free-electron lasers, which have the promise of following dynamic molecular structure evolutions with angstrom spatial and femtosecond temporal resolutions (the so-called molecular movie). However, an additional phase is introduced when the photoelectron is scattered by an atom, and this scattering phase turns into a spatial error when the position of the atom is retrieved using a Fourier-type transform. We propose a method to remove, or greatly suppress, this error, with demonstrations using numerical examples.
关键词： x-ray free-electron lasers,Fourier-type transform,molecular structure retrieval,scattering phase,photoelectron holography
更新于2025-09-12 10:27:22
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Photon-momentum transfer in diatomic molecules: An <i>ab initio</i> study
摘要： For a molecule, the two-center interference and the molecular scattering phase of the electron are important for almost all the processes that may occur in a laser field. In this study, we investigate their effects in the transfer of linear photon momentum to the ionized electron by absorbing a single photon. The time-dependent Schr?dinger equation of H2+ is numerically solved in the multipolar gauge in which the electric quadrupole term and the magnetic dipole term are explicitly expressed. This allows us to separate the contributions of the two terms in the momentum transfer. For different configurations of the molecular and the laser orientation, the transferred momentum to the electron is evaluated at different internuclear distances with various photon energies and two-center interferences are identified in the whole region. At small electron energies and small internuclear distances, we find significant deviations from the prediction of the classical double-slit model due to the strong mediation of the Coulomb potential. Finally, even for a large internuclear distance, our results show that a varying molecular scattering phase is important at all electron energies, which is beyond the simple prediction of the linear combination of the atomical orbitals.
关键词： diatomic molecules,molecular scattering phase,ab initio study,photon-momentum transfer,two-center interference
更新于2025-09-04 15:30:14