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The oxidation and spin state of a metal-organic molecule determine its chemical reactivity and magnetic properties. Here, we demonstrate the reversible control of the oxidation and spin state in a single Fe porphyrin molecule in the force field of the tip of a scanning electron tunneling microscope. Within the regimes of half-integer and integer spin state, we can further track the evolution of the magnetocrystalline anisotropy. Our experimental results are corroborated by density functional theory and wave function theory. This combined analysis allows us to draw a complete picture of the molecular states over a large range of intramolecular deformations.
The non-Born-Oppenheimer quantum dynamics of pp mu and pd mu molecular ions excited by ultrashort, superintense VUV laser pulses polarized along the molecular axis (z) is studied by the numerical solution of the time-dependent Schrodinger equation within a three-dimensional (3D) model, including the internuclear distance R and muon coordinates z and rho, a transversal degree of freedom. It is shown that in both pp mu and pd mu, muons approximately follow the applied laser field out of phase. After the end of the laser pulse, expectation values < z >, < p >, and < R > demonstrate "post-laser-pulse" oscillations in both pp mu and pd mu. In the case of pd mu, the post-laser-pulse oscillations of < z > and < R > appear as shaped "echo pulses." Power spectra, which are related to high-order harmonic generation (HHG), generated due to muonic and nuclear motion are calculated in the acceleration form. For pd mu it is found that there exists a unique characteristic frequency omega(pd mu)(osc) representing both frequencies of post-laser-pulse muonic oscillations and the frequency of nuclear vibrations, which manifest themselves by very sharp maxima in the corresponding power spectra of pd mu. The homonuclear pp p. ion does not possess such a unique characteristic frequency. The "exact" dynamics and power, and HHG spectra of the 3D model are compared with a Born-Oppenheimer, fixed-nuclei model featuring interesting differences: postpulse oscillations are absent and HHG spectra are affected indirectly or directly by nuclear motion.
Pump-probe near edge X-ray absorption fine structure (PP-NEXAFS) spectra of molecules offer insight into valence-excited states, even if optically dark. In PP-NEXAFS spectroscopy, the molecule is "pumped" by UV or visible light enforcing a valence excitation, followed by an X-ray "probe" exciting core electrons into (now) partially empty valence orbitals. Calculations of PP-NEXAFS have so far been done by costly, correlated wavefunction methods which are not easily applicable to medium-sized or large molecules. Here we propose an efficient, first principles method based on density functional theory in combination with the transition potential and Delta SCF methodology (TP-DFT/Delta SCF) to compute molecular ground state and PP-NEXAFS spectra. We apply the method to n ->pi* pump/O-K-edge NEXAFS probe spectroscopy of thymine (for which both experimental and other theoretical data exist) and to n -> pi* or pi -> pi* pump/N-K-edge NEXAFS probe spectroscopies of trans-and cis-azobenzene. Published by AIP Publishing.