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DDC: 540 × Subject: Infrared devices ×

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    Metallofullerene photoswitches driven by photoinduced fullerene-to-metal electron transfer
    (Cambridge : RSC, 2021) Zalibera, Michal; Ziegs, Frank; Schiemenz, Sandra; Dubrovin, Vasilii; Lubitz, Wolfgang; Savitsky, Anton; Deng, Shihu H.M.; Wang, Xue-Bin; Advoshenko, Stanislav M.; Popov, Alexey A.
    We report on the discovery and detailed exploration of the unconventional photo-switching mechanism in metallofullerenes, in which the energy of the photon absorbed by the carbon cage π-system is transformed to mechanical motion of the endohedral cluster accompanied by accumulation of spin density on the metal atoms. Comprehensive photophysical and electron paramagnetic resonance (EPR) studies augmented by theoretical modelling are performed to address the phenomenon of the light-induced photo-switching and triplet state spin dynamics in a series of YxSc3−xN@C80 (x = 0–3) nitride clusterfullerenes. Variable temperature and time-resolved photoluminescence studies revealed a strong dependence of their photophysical properties on the number of Sc atoms in the cluster. All molecules in the series exhibit temperature-dependent luminescence assigned to the near-infrared thermally-activated delayed fluorescence (TADF) and phosphorescence. The emission wavelengths and Stokes shift increase systematically with the number of Sc atoms in the endohedral cluster, whereas the triplet state lifetime and S1–T1 gap decrease in this row. For Sc3N@C80, we also applied photoelectron spectroscopy to obtain the triplet state energy as well as the electron affinity. Spin distribution and dynamics in the triplet states are then studied by light-induced pulsed EPR and ENDOR spectroscopies. The spin–lattice relaxation times and triplet state lifetimes are determined from the temporal evolution of the electron spin echo after the laser pulse. Well resolved ENDOR spectra of triplets with a rich structure caused by the hyperfine and quadrupolar interactions with 14N, 45Sc, and 89Y nuclear spins are obtained. The systematic increase of the metal contribution to the triplet spin density from Y3N to Sc3N found in the ENDOR study points to a substantial fullerene-to-metal charge transfer in the excited state. These experimental results are rationalized with the help of ground-state and time-dependent DFT calculations, which revealed a substantial variation of the endohedral cluster position in the photoexcited states driven by the predisposition of Sc atoms to maximize their spin population.
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    Electronic transitions and dielectric function tensor of a YMnO3 single crystal in the NIR-VUV spectral range
    (London [u.a.] : Royal Society of Chemistry, 2014) Schmidt-Grund, R.; Richter, S.; Ebbinghaus, S.G.; Lorenz, M.; Bundesmann, C.; Grundmann, M.
    We present optical properties in the near-infrared to vacuum-ultraviolet spectral range of hexagonal YMnO3. The high-quality (110)-oriented bulk single crystal was grown by the optical floating zone technique. We have determined the tensor of the dielectric function by means of Mueller matrix ellipsometry in the wide spectral range (0.5-9.15) eV. For the spectral range below 5.4 eV, we present much more precise data compared to previous reports. For higher energies no experimental reports were given previously. The experimental dielectric function of YMnO3 agrees generally with theoretical calculations. We found the well known transitions involving hybridized oxygen-Mn states and Mn-3d states to be spectrally localized with a homogeneous Lorentzian lineshape. At energies above these transitions, we observe pseudo-transparent points where for each of the principal diagonal elements of the dielectric function tensor the imaginary part approaches zero but at different photon energies. These are followed at the onset of the high-absorption spectral range by parabolic direct band-band transitions which have not been reported so far.
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    Two-color two-dimensional terahertz spectroscopy: A new approach for exploring even-order nonlinearities in the nonperturbative regime
    (Melville, NY : American Institute of Physics, 2021) Woerner, Michael; Ghalgaoui, Ahmed; Reimann, Klaus; Elsaesser, Thomas
    Nonlinear two-dimensional terahertz (2D-THz) spectroscopy at frequencies of the emitted THz signal different from the driving frequencies allows for exploring the regime of (off-)resonant even-order nonlinearities in condensed matter. To demonstrate the potential of this method, we study two phenomena in the nonlinear THz response of bulk GaAs: (i) The nonlinear THz response to a pair of femtosecond near-infrared pulses unravels novel fourth- and sixth-order contributions involving interband shift currents, Raman-like excitations of transverse-optical phonon and intervalence-band coherences. (ii) Transient interband tunneling of electrons driven by ultrashort mid-infrared pulses can be effectively controlled by a low-frequency THz field with amplitudes below 50 kV/cm. The THz field controls the electron–hole separation modifying decoherence and the irreversibility of carrier generation.