10 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
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
Ultrafast kinetics of linkage isomerism in Na<sub>2</sub>[Fe(CN)<sub>5</sub>NO] aqueous solution revealed by time-resolved photoelectron spectroscopy
The kinetics of ultrafast photoinduced structural changes in linkage isomers is investigated using Na2[Fe(CN)5NO] as a model complex. The buildup of the metastable side-on configuration of the NO ligand, as well as the electronic energy levels of ground, excited, and metastable states, has been revealed by means of time-resolved extreme UV (XUV) photoelectron spectroscopy in aqueous solution, aided by theoretical calculations. Evidence of a short-lived intermediate state in the isomerization process and its nature are discussed, finding that the complete isomerization process occurs in less than 240 fs after photoexcitation
Injection Kinetics and Electronic Structure at the N719/TiO<sub>2</sub> Interface Studied by Means of Ultrafast XUV Photoemission Spectroscopy
The method of transient
XUV photoemission spectroscopy is developed
to investigate the ultrafast dynamics of heterogeneous electron transfer
at the interface between the N719 ruthenium dye complex and TiO<sub>2</sub> nanoparticles. XUV light from high-order harmonic generation
is used to probe the electron density distribution among the ground
and excited states at the interface after its exposure to a pump laser
pulse of 530 nm wavelength. A spectral decomposition of the transient
emission signal is used to follow the population and decay dynamics
of the involved transient states individually. By comparing results
obtained for the N719/TiO<sub>2</sub> and N719/FTO interfaces, we
can unambiguously reveal the kinetics of electrons injected to TiO<sub>2</sub> from the singlet metal-to-ligand charge-transfer (MLCT) excited
state of the dye. With the developed approach, we characterize both
the kinetic constants and the absolute binding energies of the singlet
and triplet MLCT states of the dye and the state of electrons injected
to the conduction band of TiO<sub>2</sub>. The energy levels of the
singlet and triplet states are found to lie 0.7 eV above and 0.2 eV
below the conduction band minimum, respectively. This energetic structure
gives rise to a strong driving force for injection from the singlet
state and a slow electron transfer from the triplet state, the latter
being possible due to a partial overlap of the triplet state band
of N719 and the conduction band of TiO<sub>2</sub>