15 research outputs found
Ultrafast dynamics in polymeric carbon nitride thin films probed by time-resolved EUV photoemission and UV-Vis transient absorption spectroscopy
The ground- and excited-state electronic structures of four polymeric carbon nitride (PCN) materials have been investigated using a combination of photoemission and optical absorption spectroscopy. To establish the driving forces for photocatalytic water-splitting reactions, the ground-state data was used to produce a band diagram of the PCN materials and the triethanolamine electron scavenger, commonly implemented in water-splitting devices. The ultrafast charge-carrier dynamics of the same PCN materials were also investigated using two femtosecond-time-resolved pump–probe techniques: extreme-ultraviolet (EUV) photoemission and ultraviolet-visible (UV-Vis) transient absorption spectroscopy. The complementary combination of these surface- and bulk-sensitive methods facilitated photoinduced kinetic measurements spanning the sub-picosecond to few nanosecond time range. The results show that 400 nm (3.1 eV) excitation sequentially populates a pair of short-lived transient species, which subsequently produce two different long-lived excited states on a sub-picosecond time scale. Based on the spectro-temporal characteristics of the long-lived signals, they are assigned to singlet-exciton and charge-transfer states. The associated charge-separation efficiency was inferred to be between 65% and 78% for the different studied materials. A comparison of results from differently synthesized PCNs revealed that the early-time processes do not differ qualitatively between sample batches, but that materials of more voluminous character tend to have higher charge separation efficiencies, compared to exfoliated colloidal materials. This finding was corroborated via a series of experiments that revealed an absence of any pump-fluence dependence of the initial excited-state decay kinetics and characteristic carrier-concentration effects that emerge beyond few-picosecond timescales. The initial dynamics of the photoinduced charge carriers in the PCNs are correspondingly determined to be spatially localised in the immediate vicinity of the lattice-constituting motif, while the long-time behaviour is dominated by charge-transport and recombination processes. Suppressing the latter by confining excited species within nanoscale volumes should therefore affect the usability of PCN materials in photocatalytic devices
Charge transfer to solvent dynamics in iodide aqueous solution studied at ionization threshold
We explore the early-time electronic relaxation in NaI aqueous solution
exposed to a short UV laser pulse. Rather than initiating the charge transfer
reaction by resonant photoexcitation of iodide, in the present time-resolved
photoelectron spectroscopy study the charge-transfer-to-solvent (CTTS) states
are populated via electronic excitation above the vacuum level. By analyzing
the temporal evolution of electron yields from ionization of two transient
species, assigned to CTTS and its first excited state, we determine both their
ultrafast population and relaxation dynamics. Comparison with resonant-
excitation studies shows that the highly excited initial states exhibit
similar relaxation characteristics as found for resonant excitation.
Implications for structure and dynamical response of the hydration cage are
discussed
Time-of-flight electron spectrometer for a broad range of kinetic energies
A newly constructed time-of-flight electron spectrometer of the magnetic
bottle type is characterized for electron detection in a broad range of
kinetic energies. The instrument is designed to measure the energy spectra of
electrons generated from liquids excited by strong laser fields and photons in
the range of extreme ultra violet and soft X-rays. Argon inner shell electrons
were recorded to calibrate the spectrometer and investigate its
characteristics, such as energy resolution and collection efficiency. Its
energy resolution ΔE/E of 1.6% allows resolving the Ar 2p spin orbit structure
at kinetic energies higher than 100 eV. The collection efficiency is
determined and compared to that of the spectrometer in its field-free
configuration
Electron affinity of Li: A state-selective measurement
We have investigated the threshold of photodetachment of Li^- leading to the
formation of the residual Li atom in the state. The excited residual
atom was selectively photoionized via an intermediate Rydberg state and the
resulting Li^+ ion was detected. A collinear laser-ion beam geometry enabled
both high resolution and sensitivity to be attained. We have demonstrated the
potential of this state selective photodetachment spectroscopic method by
improving the accuracy of Li electron affinity measurements an order of
magnitude. From a fit to the Wigner law in the threshold region, we obtained a
Li electron affinity of 0.618 049(20) eV.Comment: 5 pages,6 figures,22 reference
Photodetachment study of the 1s3s4s ^4S resonance in He^-
A Feshbach resonance associated with the 1s3s4s ^{4}S state of He^{-} has
been observed in the He(1s2s ^{3}S) + e^- (\epsilon s) partial photodetachment
cross section. The residual He(1s2s ^{3}S) atoms were resonantly ionized and
the resulting He^+ ions were detected in the presence of a small background. A
collinear laser-ion beam apparatus was used to attain both high resolution and
sensitivity. We measured a resonance energy E_r = 2.959 255(7) eV and a width
\Gamma = 0.19(3) meV, in agreement with a recent calculation.Comment: LaTeX article, 4 pages, 3 figures, 21 reference
Charge Transfer Dynamics at Dye-Sensitized ZnO and TiO<sub>2</sub> Interfaces Studied by Ultrafast XUV Photoelectron Spectroscopy
Interfacial charge transfer from photoexcited ruthenium based N3 dye molecules into ZnO thin films received controversial interpretations. To identify the physical origin for the delayed electron transfer in ZnO compared to TiO2, we probe directly the electronic structure at both dye semiconductor interfaces by applying ultrafast XUV photoemission spectroscopy. In the range of pump probe time delays between 0.5 to 1.0 amp; 8201;ps, the transient signal of the intermediate states was compared, revealing a distinct difference in their electron binding energies of 0.4 amp; 8201;eV. This finding strongly indicates the nature of the charge injection at the ZnO interface associated with the formation of an interfacial electron cation complex. It further highlights that the energetic alignment between the dye donor and semiconductor acceptor states appears to be of minor importance for the injection kinetics and that the injection efficiency is dominated by the electronic couplin