Showing papers on "Photoexcitation published in 2012"
TL;DR: It is shown that strongly photoexcited graphene monolayers with 35 fs pulses quasi-instantaneously build up a broadband, inverted Dirac fermion population, revealing the evolution of the transient state from a hot classical gas to a dense quantum fluid with increasing the photoexcitation.
Abstract: We show that strongly photoexcited graphene monolayers with 35 fs pulses quasi-instantaneously build up a broadband, inverted Dirac fermion population. Optical gain emerges and directly manifests itself via a negative conductivity at the near-infrared region for the first 200 fs, where stimulated emission completely compensates absorption loss in the graphene layer. Our experiment-theory comparison with two distinct electron and hole chemical potentials reproduce absorption saturation and gain at 40 fs, revealing, particularly, the evolution of the transient state from a hot classical gas to a dense quantum fluid with increasing the photoexcitation.
243 citations
TL;DR: In this paper, the authors performed time and angle-resolved photoemission spectroscopy of a prototypical topological insulator (TI) Bi_2Se_3 to study the ultrafast dynamics of surface and bulk electrons after photoexcitation.
Abstract: We perform time- and angle-resolved photoemission spectroscopy of a prototypical topological insulator (TI) Bi_2Se_3 to study the ultrafast dynamics of surface and bulk electrons after photoexcitation. By analyzing the evolution of surface states and bulk band spectra, we obtain their electronic temperature and chemical potential relaxation dynamics separately. These dynamics reveal strong phonon-assisted surface-bulk coupling at high lattice temperature and total suppression of inelastic scattering between the surface and the bulk at low lattice temperature. In this low temperature regime, the unique cooling of Dirac fermions in TI by acoustic phonons is manifested through a power law dependence of the surface temperature decay rate on carrier density.
205 citations
TL;DR: This work applies soft-x-ray ARPES to explore the 3D electron realm in a paradigm transition metal dichalcogenide VSe2 and identifies pronounced 3D warping of the Fermi surface and shows that its concomitant nesting acts as the precursor for the exotic 3D charge-density waves in VSe 2.
Abstract: The resolution of angle-resolved photoelectron spectroscopy (ARPES) in three-dimensional (3D) momentum k is fundamentally limited by ill defined surface-perpendicular wave vector k(perpendicular to) associated with the finite photoelectron mean free path. Pushing ARPES into the soft-x-ray energy region sharpens the k(perpendicular to) definition, allowing accurate electronic structure investigations in 3D materials. We apply soft-x-ray ARPES to explore the 3D electron realm in a paradigm transition metal dichalcogenide VSe2. Essential to break through the dramatic loss of the valence band photoexcitation cross section at soft-x-ray energies is the advanced photon flux performance of our synchrotron instrumentation. By virtue of the sharp 3D momentum definition, the soft-x-ray ARPES experimental band structure and Fermi surface of VSe2 show a textbook clarity. We identify pronounced 3D warping of the Fermi surface and show that its concomitant nesting acts as the precursor for the exotic 3D charge-density waves in VSe2. Our results demonstrate the immense potential of soft-x-ray ARPES to explore details of 3D electronic structure.
152 citations
TL;DR: In this article, the authors investigate the energy relaxation of hot photocarriers produced by photoexcitation of graphene through coupling to both intrinsic and remote (substrate) surface polar phonons using the Boltzmann equation approach.
Abstract: We investigate the energy relaxation of hot carriers produced by photoexcitation of graphene through coupling to both intrinsic and remote (substrate) surface polar phonons using the Boltzmann equation approach. We find that the energy relaxation of hot photocarriers in graphene on commonly used polar substrates, under most conditions, is dominated by remote surface polar phonons. We also calculate key characteristics of the energy relaxation process, such as the transient cooling time and steady-state carrier temperatures and photocarrier densities, which determine the thermoelectric and photovoltaic photoresponse, respectively. Substrate engineering can be a promising route to efficient optoelectronic devices driven by hot carrier dynamics.
123 citations
TL;DR: The dynamics of the NA exciton relaxation in Cd(33)Se( 33) semiconductor quantum dots passivated by either trimethylphosphine oxide or methylamine ligands are studied using explicit time-dependent modeling and the large extent of hybridization between electronic states of quantum dot and ligand molecules is found to strongly facilitateexciton relaxation.
Abstract: Understanding the pathways of hot exciton relaxation in photoexcited semiconductor nanocrystals, also called quantum dots (QDs), is of paramount importance in multiple energy, electronics and biological applications. An important nonradiative relaxation channel originates from the nonadiabatic (NA) coupling of electronic degrees of freedom to nuclear vibrations, which in QDs depend on the confinement effects and complicated surface chemistry. To elucidate the role of surface ligands in relaxation processes of nanocrystals, we study the dynamics of the NA exciton relaxation in Cd33Se33 semiconductor quantum dots passivated by either trimethylphosphine oxide or methylamine ligands using explicit time-dependent modeling. The large extent of hybridization between electronic states of quantum dot and ligand molecules is found to strongly facilitate exciton relaxation. Our computational results for the ligand contributions to the exciton relaxation and electronic energy-loss in small clusters are further extrap...
123 citations
TL;DR: It is proposed that Bodipy derivatives containing excited state intramolecular proton transfer (ESIPT) chromophores 7-9 do not undergo ESIPT upon photoexcitation, as well as for applications of these compounds in photovoltaics, photocatalysis and luminescent materials, etc.
Abstract: Bodipy derivatives containing excited state intramolecular proton transfer (ESIPT) chromophores 2-(2-hydroxyphenyl) benzothiazole and benzoxazole (HBT and HBO) subunits were prepared (7–10). The compounds show red-shifted UV–vis absorption (530–580 nm; e up to 50000 M–1 cm–1) and emission compared to both HBT/HBO and Bodipy. The new chromophores show small Stokes shift (45 nm) and high fluorescence quantum yields (ΦF up to 36%), which are in stark contrast to HBT and HBO (Stokes shift up to 180 nm and ΦF as low as 0.6%). On the basis of steady state and time-resolved absorption spectroscopy, as well as DFT/TDDFT calculations, we propose that 7–9 do not undergo ESIPT upon photoexcitation. Interestingly, nanosecond time-resolved transient absorption spectroscopy demonstrated that Bodipy-localized triplet excited states were populated for 7–10 upon photoexcitation; the lifetimes of the triplet excited states (τT) are up to 195 μs. DFT calculations confirm the transient absorptions are due to the triplet stat...
117 citations
TL;DR: Two-dimensional heterodyne-detected vibrational sum frequency generation (2D HD-VSFG) spectroscopy is performed for an aqueous interface for the first time to reveal the femtosecond vibrational dynamics of water at the charged interface.
Abstract: Two-dimensional heterodyne-detected vibrational sum frequency generation (2D HD-VSFG) spectroscopy is performed for an aqueous interface for the first time. The 2D HD-VSFG spectra in the OH stretch region are obtained from a positively charged surfactant/water interface with isotopically diluted water (HOD/D2O) to reveal the femtosecond vibrational dynamics of water at the charged interface. The 2D HD-VSFG spectrum is diagonally elongated immediately after photoexcitation, clearly demonstrating inhomogeneity in the interfacial water. This elongation almost disappears at 300 fs owing to the spectral diffusion. Interestingly, the 2D HD-VSFG spectrum at the 0 fs shows an oppositely asymmetric shape to the corresponding 2D IR spectrum in bulk water: The bandwidth of the bleach signal gets narrower when the pump wavenumber becomes higher. This suggests that the dynamics and mechanism of the hydrogen bond rearrangement at the charged interface are significantly different from those in bulk water.
102 citations
TL;DR: It is demonstrated that after photoexcitation aradical pair forms, becomes stabilized through proton transfer, and decays back to the protein's resting state on time scales allowing the protein, in principle, to act as a radical pair-based magnetic sensor.
Abstract: Migrating birds fly thousands of miles or more, often without visual cues and in treacherous winds, yet keep direction. They employ for this purpose, apparently as a powerful navigational tool, the photoreceptor protein cryptochrome to sense the geomagnetic field. The unique biological function of cryptochrome supposedly arises from a photoactivation reaction involving radical pair formation through electron transfer. Radical pairs, indeed, can act as a magnetic compass; however, the cryptochrome photoreaction pathway is not fully resolved yet. To reveal this pathway and underlying photochemical mechanisms, we carried out a combination of quantum chemical calculations and molecular dynamics simulations on plant (Arabidopsis thaliana) cryptochrome. The results demonstrate that after photoexcitation a radical pair forms, becomes stabilized through proton transfer, and decays back to the protein’s resting state on time scales allowing the protein, in principle, to act as a radical pair-based magnetic sensor....
101 citations
TL;DR: The UV source has been fully characterized spatially, spectrally, and temporally and its potential for time and angle-resolved photoemission is demonstrated through Fermi surface mapping and photoexcited electron dynamics in Bismuth.
Abstract: A novel experimental apparatus for time and angle-resolved photoemission on solid surfaces is presented. A 6.28 eV laser source operating at 250 kHz repetition rate is obtained by frequency mixing in nonlinear beta barium borate crystals. This UV light source has a high photon flux of 1013 photons/s with relatively low number of photons/pulse so that Fermi surface mapping over a wide region of the Brillouin zone is possible while mitigating space charge effects. The UV source has been fully characterized spatially, spectrally, and temporally. Its potential for time and angle-resolved photoemission is demonstrated through Fermi surface mapping and photoexcited electron dynamics in Bismuth. True femtosecond time resolution <65 fs is obtained while the energy resolution of 70 meV appears to be mainly limited by the laser bandwidth.
82 citations
TL;DR: In this article, the authors demonstrate ultrafast dynamical tuning of resonance in the terahertz (THz) frequency range in YBa_2Cu_3O_7-\delta (YBCO) split-ring resonator arrays excited by near-infrared femtosecond laser pulses.
Abstract: Through the integration of semiconductors or complex oxides into metal resonators, tunable metamaterials have been achieved by a change of environment using an external stimulus. Metals provide high conductivity to realize a strong resonant response in metamaterials; however, they contribute very little to the tunability. The complex conductivity in high-temperature superconducting films is highly sensitive to external perturbations, which provides new opportunities in achieving tunable metamaterials resulting directly from the resonant elements. Here we demonstrate ultrafast dynamical tuning of resonance in the terahertz (THz) frequency range in YBa_2Cu_3O_7-\delta (YBCO) split-ring resonator arrays excited by near-infrared femtosecond laser pulses. The photoexcitation breaks the superconducting Cooper pairs to create the quasiparticle state. This dramatically modifies the imaginary part of the complex conductivity and consequently the metamaterial resonance in an ultrafast timescale. We observed resonance switching accompanied with a wide range frequency tuning as a function of photoexcitation fluence, which also strongly depend on the nano-scale thickness of the superconducting films. All of our experimental results are well reproduced through calculations using an analytical model, which takes into account the SRR resistance and kinetic inductance contributed from the complex conductivity of YBCO films. The theoretical calculations reveal that the increasing SRR resistance upon increasing photoexcitation fluence is responsible for the reduction of resonance strength, and both the resistance and kinetic inductance contribute to the tuning of resonance frequency.
78 citations
Book•
16 Apr 2012
TL;DR: In this article, the two-level system was used for charge transfer and transport and other modulation techniques in the two level system, including charge transfer, pump-probe and other modulation techniques.
Abstract: 1 Introduction 2 Radiation-Matter-Interaction in the Two-Level-System 3 Molecular Exciton 4 Excited States in Solids 5 Photoexcitation Dynamics 6 Photphysics Tool Box 7 Vibrational Spectroscopy 8 Charge Transfer and Transport 9 Pump-probe and Other Modulation Techniques 10 Conclusions and Future Perspectives
TL;DR: In this paper, the influence of hot phonon effect and intervalley scattering on the hot carrier cooling rate was investigated using femtosecond time-resolved photoluminescence spectroscopy in bulk GaAs and InP, two electronically similar but vibrationally distinct semiconductors.
Abstract: The influence of hot phonon effect and intervalley scattering on the hot carrier cooling rate was investigated using femtosecond time-resolved photoluminescence spectroscopy in bulk GaAs and InP, two electronically similar but vibrationally distinct semiconductors. In both materials, a broad photoluminescence signal that extends from the band gap energy to values larger than the pump pulse energy was observed during the first few picoseconds after photoexcitation, for different excitation energies (1.7, 1.88, and 2.4 eV) at high carrier densities (>1019 cm−3). Different hot carrier relaxation times were observed in GaAs and InP for different excitation energies, demonstrating the influence of intervalley scattering phenomena in GaAs. When electrons were not energetic enough to access satellite valleys, longer decay transients were observed for InP compared with GaAs. This provides experimental evidence of the hot phonon effect in InP. Temperature transients were calculated by analyzing the topography of the two-dimensional spectra. Copyright © 2011 John Wiley & Sons, Ltd.
TL;DR: Time-resolved resonant-Raman spectroscopy is used to investigate the picosecond conformational relaxation of regioregular poly(3-hexylthiophene) (RR-P3HT) in chlorobenzene after 510 nm photoexcitation to provide a glimpse of the underlying molecular dynamics responsible for the red shift in the exciton's near-IR transient absorption occurring on the same time scale.
Abstract: We have used time-resolved resonant-Raman spectroscopy to investigate the picosecond conformational relaxation of regioregular poly(3-hexylthiophene) (RR-P3HT) in chlorobenzene after 510 nm photoexcitation. Vibrational signatures from modes along and peripheral to the exciton's backbone have been identified according to the time dependence of excited-state Raman features and from comparisons to Raman spectra of other polymer states. Measured spectral dynamics reflect initial changes in the resonance enhancement of backbone modes on a time scale of 9 ± 1 ps. In contrast, contributions from peripheral modes exhibit time-dependent decay determined only by exciton intersystem-crossing kinetics. Spectral dynamics are interpreted in terms of evolution in bond lengths along the exciton's backbone resulting from increased conjugation allowed by torsional reordering. Possible origins of peripheral features are discussed, including distorted inter-ring modes at exciton termini. Findings provide a glimpse of the underlying molecular dynamics responsible for the red shift in the exciton's near-IR transient absorption occurring on the same time scale.
TL;DR: In this paper, it was shown that ethanol quenches valence-band holes within ∼15 ps of photoexcitation, but does not quench the trapped holes responsible for the characteristic visible photoluminescence of colloidal ZnO nanocrystals.
Abstract: Photochemical charging of colloidal ZnO nanocrystals has been studied using continuous-wave and time-resolved photoluminescence spectroscopies in conjunction with electron paramagnetic resonance spectroscopy. Experiments have been performed with and without addition of alcohols as hole quenchers, focusing on ethanol. Both aerobic and anaerobic conditions have been examined. We find that ethanol quenches valence-band holes within ∼15 ps of photoexcitation, but does not quench the trapped holes responsible for the characteristic visible photoluminescence of ZnO nanocrystals. Hole quenching yields “charged” nanocrystals containing excess conduction-band electrons. The extra conduction-band electrons quench visible trap-centered luminescence via a highly effective electron/trap-state Auger-type cross-relaxation process. This Auger process is prominent even under aerobic photoexcitation conditions, particularly when samples are not stirred. Charging also reduces exciton nonradiative decay rates, resulting in i...
TL;DR: In this article, single photon emission from an epitaxially grown quantum dot at room temperature is presented, where CdSe/ZnSSe quantum dots are embedded into MgS barriers, providing dominant radiative recombination up to 300 K under continuous wave optical excitation.
Abstract: Single photon emission from an epitaxially grown quantum dot at room temperature is presented CdSe/ZnSSe quantum dots are embedded into MgS barriers, providing dominant radiative recombination up to 300 K Under continuous wave optical excitation, the autocorrelation function g(2)(t) exhibits a sharp dip at (t = 0) with g(2)(0) = 016 ± 015 at T = 300 K, revealing excellent suppression of multiphoton emission even at room temperature
TL;DR: The study illustrates emphatically that molecules in solids have physical properties different from those of isolated molecules and that their properties depend on the specific molecular environment, relevant for the understanding of the properties of molecular solid-state devices, which are increasingly used in current technology.
Abstract: The excited-state structure of [CuI[(1,10-phenanthroline-N,N′) bis(triphenylphosphine)] cations in their crystalline [BF4] salt has been determined at both 180 and 90 K by single-pulse time-resolved synchrotron experiments with the modified polychromatic Laue method. The two independent molecules in the crystal show distortions on MLCT excitation that differ in magnitude and direction, a difference attributed to a pronounced difference in the molecular environment of the two complexes. As the excited states differ, the decay of the emission is biexponential with two strongly different lifetimes, the longer lifetime, assigned to the more restricted molecule, becoming more prevalent as the temperature increases. Standard deviations in the current Laue study are very much lower than those achieved in a previous monochromatic study of a Cu(I) 2,9-dimethylphenanthroline substituted complex (J. Am. Chem. Soc. 2009, 131, 6566), but the magnitudes of the shifts on excitation are similar, indicating that lattice r...
TL;DR: The interplay of vibrational motion and electronic charge relocation in an ionic hydrogen-bonded crystal is mapped by X-ray powder diffraction with a 100 fs time resolution, demonstrating an oscillatory relocation of electronic charge with a spatial amplitude two orders of magnitude larger than the underlying vibrational lattice motions.
Abstract: The interplay of vibrational motion and electronic charge relocation in an ionic hydrogen-bonded crystal is mapped by X-ray powder diffraction with a 100 fs time resolution. Photoexcitation of the prototype material KH2PO4 induces coherent low-frequency motions of the PO4 tetrahedra in the electronically excited state of the crystal while the average atomic positions remain unchanged. Time-dependent maps of electron density derived from the diffraction data demonstrate an oscillatory relocation of electronic charge with a spatial amplitude two orders of magnitude larger than the underlying vibrational lattice motions. Coherent longitudinal optical and tranverse optical phonon motions that dephase on a time scale of several picoseconds, drive the charge relocation, similar to a soft (transverse optical) mode driven phase transition between the ferro- and paraelectric phase of KH2PO4.
TL;DR: This essay examines critically the hot electron mechanism, and proposes an alternative description based on direct charge transfer of electrons from the substrate to adsorbate, which informs how material properties of the substrate and adsorbates, as well as their interaction, influence the frequency dependent probability of photoexcitation and ultimately how light can be used to probe and control surface femtochemistry.
Abstract: In this essay we discuss the light-matter interactions at molecule-covered metal surfaces that initiate surface photochemistry. The hot-electron mechanism for surface photochemistry, whereby the absorption of light by a metal surface creates an electron-hole pair, and the hot electron scatters through an unoccupied resonance of adsorbate to initiate nuclear dynamics leading to photochemistry, has become widely accepted. Yet, ultrafast spectroscopic measurements of molecule-surface electronic structure and photoexcitation dynamics provide scant support for the hot electron mechanism. Instead, in most cases the adsorbate resonances are excited through photoinduced substrate-to-adsorbate charge transfer. Based on recent studies of the role of coherence in adsorbate photoexcitation, as measured by the optical phase and momentum resolved two-photon photoemission measurements, we examine critically the hot electron mechanism, and propose an alternative description based on direct charge transfer of electrons from the substrate to adsorbate. The advantage of this more quantum mechanically rigorous description is that it informs how material properties of the substrate and adsorbate, as well as their interaction, influence the frequency dependent probability of photoexcitation and ultimately how light can be used to probe and control surface femtochemistry.
TL;DR: In this paper, the authors investigated the dynamics following photoexcitation of Na and Li atoms located on the surface of helium nanodroplets in a joint experimental and theoretical study, and showed that the desorption of the excited alkali atom is accompanied by the creation of highly nonlinear density waves in the helium droplet that propagate at supersonic velocities.
Abstract: The dynamics following the photoexcitation of Na and Li atoms located on the surface of helium nanodroplets has been investigated in a joint experimental and theoretical study. Photoelectron spectroscopy has revealed that excitation of the alkali atoms via the (n + 1)s ← ns transition leads to the desorption of these atoms. The mean kinetic energy of the desorbed atoms, as determined by ion imaging, shows a linear dependence on excitation frequency. These experimental findings are analyzed within a three-dimensional, time-dependent density functional approach for the helium droplet combined with a Bohmian dynamics description of the desorbing atom. This hybrid method reproduces well the key experimental observables. The dependence of the observables on the impurity mass is discussed by comparing the results obtained for the 6Li and 7Li isotopes. The calculations show that the desorption of the excited alkali atom is accompanied by the creation of highly non-linear density waves in the helium droplet that propagate at supersonic velocities.
TL;DR: In this paper, the magnetic field effect of two excited-state spectroscopies in films of a prototype poly(phenylene vinylene), i.e., a soluble derivative of polyphenylene polyvinylene (MEH-PPV), was demonstrated.
Abstract: The magnetic field effect in organic light-emitting diodes, such as magnetoconductance and magnetoelectroluminescence, has been intensively explored in the last few years. Here, we demonstrate the magnetic field effect of two excited-state spectroscopies in films of a prototype \ensuremath{\pi}-conjugated polymer, i.e., a soluble derivative of poly(phenylene vinylene), [2-methoxy-5-(2\ensuremath{'}-ethylhexyloxy)-poly(p-phenylene vinylene)] (MEH-PPV); these are magnetophoto-induced absorption (MPA) and magnetophotoluminescence (MPL). We study these novel magnetic field effects in pristine MEH-PPV films, MEH-PPV films subjected to prolonged illumination, and blend of MEH-PPV with a fullerene derivative. Being spectroscopic, MPA and MPL are determined by the photoexcitation spin density and thus may unravel the occurrence of myriad spin-mixing processes in organic semiconductors that include hyperfine interaction in polaron pairs, spin-sublevel mixing in triplet excitons, triplet--triplet annihilation, and triplet--singlet collision. The recently observed ultrasmall magnetic field effect at $B$ \ensuremath{\sim}0.5 mT in organic diodes is also observed in the MPA response of MEH-PPV films that support polaron photoexcitations, thereby identifying the underlying mechanism as being due to spin mixing of polaron pairs by the hyperfine interaction.
TL;DR: In this article, the symmetry of the ligand field surrounding the metal ion surrounding the polypyridyl Fe(II) was investigated using ground-state and ultrafast time-resolved X-ray absorption methods.
Abstract: Ultrafast excited-state evolution in polypyridyl Fe(II) complexes is of fundamental interest for understanding the origins of the sub-ps spin-state changes that occur upon photoexcitation of this class of compounds as well as for the potential impact such ultrafast dynamics have on incorporation of these compounds in solar energy conversion schemes or switchable optical storage technologies. We have demonstrated that ground-state and, more importantly, ultrafast time-resolved X-ray absorption methods can offer unique insights into the interplay between electronic and geometric structure that underpins the photo-induced dynamics of this class of compounds. The present contribution examines in greater detail how the symmetry of the ligand field surrounding the metal ion can be probed using these X-ray techniques. In particular, we show that steady-state K-edge spectroscopy of the nearest-neighbour nitrogen atoms reveals the characteristic chemical environment of the respective ligands and suggests an interesting target for future charge-transfer femtosecond and attosecond spectroscopy in the X-ray water window.
TL;DR: This work designs a fluorophore that upon photoexcitation emits in either one of two distinct colors but exhibits strong antibunching between the two, demonstrating the possibility of creating room-temperature quantum emitters with higher complexity than effective two level systems via colloidal synthesis.
Abstract: Photon antibunching is ubiquitously observed in light emitted from quantum systems but is usually associated only with the lowest excited state of the emitter. Here, we devise a fluorophore that upon photoexcitation emits in either one of two distinct colors but exhibits strong antibunching between the two. This work demonstrates the possibility of creating room-temperature quantum emitters with higher complexity than effective two level systems via colloidal synthesis.
TL;DR: This work sheds light on the excess energy relaxation processes occurring immediately after photon absorption and responsible for dissipation in the photovoltaic process of light harvesting and energy storage.
Abstract: Ultrafast dynamics upon photoexcitation in a low band gap polymer for photovoltaics is investigated both experimentally and theoretically. Our work sheds light on the excess energy relaxation processes occurring immediately after photon absorption and responsible for dissipation in the photovoltaic process of light harvesting and energy storage. A peculiar non-adiabatic decay path through a conical intersection (CI) between the higher excited state S2 and the first singlet state S1 is identified by ultrafast spectroscopy and theoretical calculations. Ultrafast twisting of the initially flat conformation in S2 drives the system to the CI connecting the two potential energy surfaces, actually eliciting an internal conversion within 60 femtoseconds, followed by planarization along the adiabatic surface in S1. Relaxed potential energy profiles (PEPs) of ground and lowest excited states along a dihedral coordinate, calculated within the time dependent density functional theory (TDDFT) approach, support the S2/S1 CI mechanism. Furthermore a screening of the widely used hybrid and range separated exchange–correlation (XC) DFT functionals has been carried out finding different descriptions of S2/S1 PEPs and good agreement between experimental data and long-range corrected DFT.
TL;DR: In this article, the electron and hole populations can be described by two separate Fermi-Dirac distributions on an ultrashort 500 fs time scale, whereas on longer time scales the populations coalesce to form a single Dirac distribution at an elevated temperature.
Abstract: Graphene, a recently discovered two-dimensional form of carbon, is a strong candidate for many future electronic devices. There is, however, still much debate over how the electronic properties of graphene behave on ultrashort time scales. Here by employing the technique of time-resolved photoemission, we obtain the evolving quantum distributions of the electrons and holes: on an ultrashort 500 fs time scale, the electron and hole populations can be described by two separate Fermi–Dirac distributions, whereas on longer time scales the populations coalesce to form a single Fermi–Dirac distribution at an elevated temperature. These studies represent the first direct measure of carrier distribution dynamics in monolayer graphene after ultrafast photoexcitation.
TL;DR: In this article, femtosecond and picosecond degenerate pump-probe techniques were used to investigate the ultrafast excited state dynamics of dinaphthoporphycenes.
Abstract: Ultrafast excited state dynamics of dinaphthoporphycenes were investigated using femtosecond and picosecond degenerate pump-probe techniques at 600 nm and 800 nm, respectively. Femtosecond pump-probe data indicated photo-induced absorption at 600 nm resulting from two-photon/single photon excitation, whereas picosecond pump-probe data demonstrated photo-bleaching which was a consequence of three-photon absorption. The fastest lifetimes (100–120 fs) observed are attributed to the intramolecular vibrational relaxation, the slower ones (1–3 ps) to internal conversion, and the slowest components (7–10 ps) to non-radiative decay back to ground state. Z-scan studies in the 560–600 nm range were also carried out.
TL;DR: In this paper, the authors reported saturation of the sensitized Mn2+ photoluminescence intensity at very low continuous-wave (CW) and quasi-CW photoexcitation powers under conditions that are relevant to many of the proposed applications.
Abstract: Colloidal Mn2+-doped semiconductor nanocrystals such as Mn2+:ZnSe have attracted broad attention for potential applications in phosphor and imaging technologies. Here, we report saturation of the sensitized Mn2+ photoluminescence intensity at very low continuous-wave (CW) and quasi-CW photoexcitation powers under conditions that are relevant to many of the proposed applications. Time-resolved photoluminescence measurements and kinetic modeling indicate that this saturation arises from an Auger-type nonradiative cross relaxation between an excited Mn2+ ion and an exciton within the same nanocrystal. A lower limit of k = 2 × 1010 s–1 is established for the fundamental rate constant of the Mn2+(4T1)-exciton cross relaxation.
TL;DR: Monitoring the ejection yield as a function of excitation wavelength can be used to obtain the optical spectrum of hemin(+) - the here obtained spectrum is slightly narrower and shifted to the blue.
Abstract: We report on a method by which mass/charge selected ions are picked up from a linear ion trap by liquid helium droplets. The size distributions of the doped droplets are measured via acceleration experiments. Depending on the source temperature, droplet sizes ranging from tens of thousands to several million helium atoms are obtained. Droplets doped with hemin, an iron containing porphyrin molecule, in the charge state +1 are then investigated using laser spectroscopy. It is observed that excitation with UV/VIS light can lead to ejection of the ion from the droplet. For doped droplets with a median size of ∼150 000 helium atoms, the absorption of two photons at 380 nm is needed for ejection to become efficient. When droplets become smaller, the ejection efficiency is observed to strongly increase. Monitoring the ejection yield as a function of excitation wavelength can be used to obtain the optical spectrum of hemin+. Compared to the spectrum of free gas-phase hemin+ at room temperature, the here obtained spectrum is slightly narrower and shifted to the blue.
TL;DR: The thermal spin transition, the photoexcitation, and the subsequent spin relaxation in the mixed crystal series of the covalently linked two-dimensional network {[Zn(1-x)Fe(x)(bbtr)(3)](ClO(4))(2)}(∞) (x = 0.02-1, bbtr =1,4-di(1,2,3-triazol-1-yl)-butane)
Abstract: The thermal spin transition, the photoexcitation, and the subsequent spin relaxation in the mixed crystal series of the covalently linked two-dimensional network {[Zn1-xFex(bbtr)3](ClO4)2}∞ (x = 0.02–1, bbtr =1,4-di(1,2,3-triazol-1-yl)-butane) are discussed. In the neat compound, the thermal spin transition with a hysteresis of 13 K is accompanied by a crystallographic phase transition (Kusz, J.; Bronisz, R.; Zubko, M.; Bednarek, H. Chem. Eur. J.2011, 17, 6807). In contrast, the diluted crystals with x ≤ 0.1 stay essentially in the high-spin state down to low temperatures and show typical first order relaxation kinetics upon photoexcitation, and the structural phase transition is well separated from the spin transition. With increasing Fe(II) concentration, steeper thermal transitions and sigmoidal relaxation curves indicate increasingly important cooperative effects. Already at x = 0.38, the spin relaxation is governed by cooperative interactions between Fe(II) centers, and the crystallographic phase tra...
TL;DR: In this paper, the carrier extraction mechanism in Si solar cells with Ge quantum dots (QDs), which enable the optical absorption of photons with energies below the band gap of the host, was investigated.
Abstract: We report studies of the carrier extraction mechanism in Si solar cells with Ge quantum dots (QDs), which enable the optical absorption of photons with energies below the band gap of the host. Photocurrent measurements revealed that the photocurrent in the QD solar cells increased superlinearly with increasing excitation intensity under strong photoexcitation, which differed greatly from the behavior of Si solar cells without Ge QDs. This nonlinear photocurrent generation indicates that the carrier extraction efficiency from QDs is enhanced under strong photoexcitation by nonlinear carrier extraction processes, such as two-step photon absorption and hot carrier generation via Auger recombination.
TL;DR: The protein environment was found to enhance the bond length alternation (BLA) of the retinyl chain and blueshift the first absorption maxima of the protonated Schiff base in the BC and Rh models relative to the chromophore in the gas phase, thus improving the quantum yield of the photoexcitation process.
Abstract: Retinal is the photon absorbing chromophore of rhodopsin and other visual pigments, enabling the vertebrate vision process. The effects of the protein environment on the primary photoexcitation pro...