9 research outputs found
Femtosekunden-Photoelektronenspektroskopie an isolierten Multianionen
The ultrafast excited state dynamics and electronic structure of molecular multianions are studied via femtosecond time-resolved photoelectron spectroscopy in gas phase. Investigations on isolated molecules give direct access to intrinsic photophysical properties fading out any disturbing influences of the local environment - with remarkable effects for multiply charged species. Excited state electron tunneling detachment is highlighted as a unique pathway in the photophysics of multianions
Highly efficient soft x-ray spectrometer for transient absorption spectroscopy with broadband table-top high harmonic sources
We present a novel soft x-ray spectrometer for ultrafast absorption spectroscopy utilizing table-top femtosecond high-order harmonic sources. Where most commercially available spectrometers rely on spherical variable line space gratings with a typical efficiency on the order of 3% in the first diffractive order, this spectrometer, based on a Hettrick–Underwood design, includes a reflective zone plate as a dispersive element. An improved efficiency of 12% at the N K-edge is achieved, accompanied by a resolving power of 890. The high performance of the soft x-ray spectrometer is further demonstrated by comparing nitrogen K-edge absorption spectra from calcium nitrate in aqueous solution obtained with our high-order harmonic source to previous measurements performed at the electron storage ring facility BESSY II
Conformer-selective photoelectron spectroscopy of alpha-lactalbumin derived multianions in the gas phase
International audienceWe have recorded conformer-selective, gas-phase photoelectron spectra of a-lactalbumin derived multianions generated by electrospraying solutions of both the native protein and its denatured form (as prepared by breaking the sulfur-sulfur bonds by chemical reduction). Three different groups of gas-phase multianion conformers have been observed and characterized. Highly-folded and partially-unfolded structures are obtained from solutions of the native protein. Only highly-elongated conformers are observed upon electrospraying the denatured protein. Adiabatic detachment energies were determined at several negative charge states for each conformer group. In comparison to highly-elongated conformations, highly-folded structures show a steeper decrease of electron binding energy with increasing negative charge. By comparing experimental detachment energies for highly-elongated structures with the predictions of a simple electrostatic model calculation, we have determined the effective dielectric shielding constant
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Highly efficient soft x-ray spectrometer for transient absorption spectroscopy with broadband table-top high harmonic sources
We present a novel soft x-ray spectrometer for ultrafast absorption spectroscopy utilizing table-top femtosecond high-order harmonic sources. Where most commercially available spectrometers rely on spherical variable line space gratings with a typical efficiency on the order of 3% in the first diffractive order, this spectrometer, based on a Hettrick-Underwood design, includes a reflective zone plate as a dispersive element. An improved efficiency of 12% at the N K-edge is achieved, accompanied by a resolving power of 890. The high performance of the soft x-ray spectrometer is further demonstrated by comparing nitrogen K-edge absorption spectra from calcium nitrate in aqueous solution obtained with our high-order harmonic source to previous measurements performed at the electron storage ring facility BESSY II
Resonant tunneling through the repulsive Coulomb barrier of a quadruply charged molecular anion
Multiply charged anions possess a repulsive Coulomb barrier (RCB) against electron emission, thus allowing for long-lived metastable species with negative electron binding energies. For the prototypical multianion, bisdisulizole tetra-anion, we demonstrate that electronically excited states supported by the RCB can undergo resonant tunneling. The dynamics of this process was investigated by one-photon photoelectron imaging and femtosecond pump-probe photoelectron spectroscopy and confirmed by theoretical calculations. Efficient resonant tunneling emission of electrons from the excited states of multianions may be common for systems with sufficiently large RCB. This may provide new opportunities to study electron emission dynamics in complex systems. Multiply charged anions (MCAs) are common in the condensed phase Photoelectron spectroscopy (PES) has been an important technique to probe the RCB and electronic stability of MCAs Here we report a direct observation of a resonant tunneling state in the bisdisulizole tetra-anion [BDSZ 4− , see The PES experiment was performed with an electrospray PES apparatus equipped with a magnetic-bottle electron analyze
Electronic Structure Changes of an Aromatic Amine Photoacid along the Forster Cycle
Photoacids show a strong increase in acidity in the first electronic excited state, enabling real-time studies of proton transfer in acid-base reactions, proton transport in energy storage devices and biomolecular sensor protein systems. Several explanations have been proposed for what determines photoacidity, ranging from variations in solvation free energy to changes in electronic structure occurring along the four stages of the Forster cycle. Here we use picosecond nitrogen K-edge spectroscopy to monitor the electronic structure changes of the proton donating group in a protonated aromatic amine photoacid in solution upon photoexcitation and subsequent proton transfer dynamics. Probing core-to-valence transitions locally at the amine functional group and with orbital specificity, we clearly reveal pronounced electronic structure, dipole moment and energetic changes on the conjugate photobase side. This result paves the way for a detailed electronic structural characterization of the photoacidity phenomenon
Photochemical Formation and Electronic Structure of an Alkane σ-Complex from Time-Resolved Optical and X-ray Absorption Spectroscopy
C–H bond activation reactions with transition metals typically proceed via the formation of alkane σ-complexes, where an alkane C–H σ-bond binds to the metal. Due to the weak nature of metal–alkane bonds, σ-complexes are challenging to characterize experimentally. Here, we establish the complete pathways of photochemical formation of the model σ-complex Cr(CO)5-alkane from Cr(CO)6 in octane solution and characterize the nature of its metal–ligand bonding interactions. Using femtosecond optical absorption spectroscopy, we find photoinduced CO dissociation from Cr(CO)6 to occur within the 100 fs time resolution of the experiment. Rapid geminate recombination by a fraction of molecules is found to occur with a time constant of 150 fs. The formation of bare Cr(CO)5 in its singlet ground state is followed by complexation of an octane molecule from solution with a time constant of 8.2 ps. Picosecond X-ray absorption spectroscopy at the Cr L-edge and O K-edge provides unique information on the electronic structure of the Cr(CO)5-alkane σ-complex from both the metal and ligand perspectives. Based on clear experimental observables, we find substantial destabilization of the lowest unoccupied molecular orbital upon coordination of the C–H bond to the undercoordinated Cr center in the Cr(CO)5-alkane σ-complex, and we define this as a general, orbital-based descriptor of the metal–alkane bond. Our study demonstrates the value of combining optical and X-ray spectroscopic methods as complementary tools to study the stability and reactivity of alkane σ-complexes in their role as the decisive intermediates in C–H bond activation reactions
Electronic Fingerprint of the Protonated Imidazole Dimer Probed by X‑ray Absorption Spectroscopy
Protons in low-barrier
superstrong hydrogen bonds are
typically
delocalized between two electronegative atoms. Conventional methods
to characterize such superstrong hydrogen bonds are vibrational spectroscopy
and diffraction techniques. We introduce soft X-ray spectroscopy to
uncover the electronic fingerprints for proton sharing in the protonated
imidazole dimer, a prototypical building block enabling effective
proton transport in biology and high-temperature fuel cells. Using
nitrogen core excitations as a sensitive probe for the protonation
status, we identify the X-ray signature of a shared proton in the
solvated imidazole dimer in a combined experimental and theoretical
approach. The degree of proton sharing is examined as a function of
structural variations that modify the shape of the low-barrier potential
in the superstrong hydrogen bond. We conclude by showing how the sensitivity
to the quantum distribution of proton motion in the double-well potential
is reflected in the spectral signature of the shared proton
Photochemical Formation and Electronic Structure of an Alkane -Complex from Time-Resolved Optical and X-ray Absorption Spectroscopy
C-H bond activation reactions with transition metals typically proceed via the formation of alkane -complexes, where an alkane C-H -bond binds to the metal. Due to the weak nature of metal-alkane bonds, -complexes are challenging to characterize experimentally. Here, we photochemically prepare the model -complex Cr(CO)5-alkane from Cr(CO)6 in octane solution and characterize the nature of its metal-ligand bonding interactions. Using femtosecond optical absorption spectroscopy, we find photo-induced CO dissociation from Cr(CO)6 to occur within the 100 fs time-resolution of the experiment. Rapid geminate recombination by a fraction of molecules is found to occur with a time constant of 150 fs. The formation of bare Cr(CO)5 in its singlet ground state is followed by complexation of an octane molecule from solution with a time constant of 8.2 ps. Picosecond X-ray absorption spectroscopy at the Cr L-edge and O K-edge provides unique information on the electronic structure of the Cr(CO)5-alkane -complex both from the metal and ligand perspectives. We find substantial destabilization of the lowest unoccupied molecular orbital upon coordination of the C-H bond to the undercoordinated Cr center in the Cr(CO)5-alkane -complex, accompanied with rehybridization between metal and ligand orbitals. Our study demonstrates the value of combining optical and X-ray spectroscopic methods as complementary tools to study the properties of alkane -complexes as the decisive intermediates in C-H bond activation reactions