Showing papers on "Photoexcitation published in 2004"
TL;DR: The reversible charging and discharging effects associated with photoexcitation of a TiO2 shell in a Ag@TiO2 composite are described and the charging of the silver core is associated with the shift in the surface plasmon band from 460 to 430 nm.
Abstract: The reversible charging and discharging effects associated with photoexcitation of a TiO2 shell in a Ag@TiO2 composite are described. The photoinduced charge separation in the TiO2 shell is followed by electron injection into the silver core. Interestingly, the charging of the silver core is associated with the shift in the surface plasmon band from 460 to 430 nm. The stored electrons are discharged upon exposure of the charged Ag@TiO2 composite to an electron acceptor. As the electrons from the silver core are discharged, the original surface plasmon absorption of the Ag core is restored.
288 citations
TL;DR: A first-principles computational approach is applied to study a light-sensitive molecular switch that can convert between a trans and a cis configuration upon photoexcitation and finds that the conductance of the two isomers varies dramatically.
Abstract: We apply a first-principles computational approach to study a light-sensitive molecular switch. The molecule that comprises the switch can convert between a trans and a cis configuration upon photoexcitation. We find that the conductance of the two isomers varies dramatically, which suggests that this system has potential application as a molecular device. A detailed analysis of the band structure of the metal leads and the local density of states of the system reveals the mechanism of the switch.
251 citations
TL;DR: It is found that the dynamics of the ultrafast carrier dynamics in single-walled carbon nanotubes are dependent on excitation intensity and the electronic transitions initially excited.
Abstract: Ultrafast carrier dynamics in individual semiconducting single-walled carbon nanotubes was studied by femtosecond transient absorption and fluorescence measurements. After photoexcitation of the second van Hove singularity of a specific tube structure, the relaxation of electrons and holes to the fundamental band edge occurs to within 100 fs. The fluorescence decay from this band is dependent on the excitation density and can be rationalized by exciton annihilation theory. In contrast to fluorescence, the transient absorption has a distinctly different time and intensity dependence for different tube structures, suggesting a branching to emissive and trap states following photoexcitation.
177 citations
TL;DR: Predominantly excitons, with near-unity quantum efficiency, are generated on excitation, while only a very small fraction of free charges are initially excited, consistent with rapid hot exciton dissociation.
Abstract: We determine the efficiencies for the formation of excitons and charge carriers following ultrafast photoexcitation of a semiconducting polymer (MEH-PPV). The simultaneous, quantitative determination of exciton and charge photoyields is achieved through subpicosecond studies of both the real and the imaginary components of the complex conductivity over a wide frequency range. Predominantly excitons, with near-unity quantum efficiency, are generated on excitation, while only a very small fraction ($l{10}^{\ensuremath{-}2}$) of free charges are initially excited, consistent with rapid ($\ensuremath{\sim}100\text{ }\text{ }\mathrm{fs}$) hot exciton dissociation. These initial charges are very short lived, decaying on subpicosecond time scales.
174 citations
TL;DR: In this paper, the conduction electron energy exchanges were investigated in gold and silver nanoparticles with average size ranging from 2 to 26 nm, embedded in different matrices, and the experimental studies were performed by following the internal thermalization dynamics of photoexcited nonequilibrium electrons with a femtosecond pump-probe technique.
Abstract: The conduction electron energy exchanges are investigated in gold and silver nanoparticles with average size ranging from 2 to 26 nm, embedded in different matrices. The experimental studies were performed by following the internal thermalization dynamics of photoexcited nonequilibrium electrons with a femtosecond pump-probe technique. The probe wavelength dependent measurements are in qualitative agreement with the results of a theoretical model based on bulk metal electron kinetics and band-structure modeling. In both metals, the measured electron thermalization times are close to the bulk ones for nanoparticles larger than 10 nm and sharply decrease for smaller ones. The results are independent of the nanoparticle environment and synthesis technique showing that the observed size behavior reflects an increase of the efficiency of the electron-electron energy exchanges in small nanoparticles. It is in agreement with a simple model based on a bulk metal approach of the electron kinetics modified to introduce surface effects. The observed increase of the electron-electron interaction with size reduction is ascribed to reduction of the screening of the Coulomb interaction by the conduction and core electrons close to the nanoparticle surface.
157 citations
TL;DR: In this article, the frequency dependence of the plasma resonant intensity in the terahertz range for a short gate-length InGaP∕InGaAs∕GaAs pseudomorphic high-electron-mobility transistor was observed.
Abstract: We have observed the frequency dependence of the plasma resonant intensity in the terahertz range for a short gate-length InGaP∕InGaAs∕GaAs pseudomorphic high-electron-mobility transistor. The plasma resonance excitation was performed by means of interband photoexcitation using the difference-frequency component of a photomixed laser beam. Under sufficient density of two-dimensional (2D) conduction electrons (>1012cm−2) and a moderate modulation index (the ratio of the density of photoexcited electrons to the initial density of the 2D electrons) we clearly observed the plasma-resonant peaks at 1.9 and 5.8THz corresponding to the fundamental and third-harmonic resonance at room temperature, which is in good agreement with theory.
154 citations
TL;DR: In this paper, a model of cold excitons was proposed for studying collective states and many-body phenomena in a system of cold bosons, including pattern formation and macroscopically ordered exciton states.
Abstract: Bound electron?hole pairs?excitons?are light Bose particles with a mass comparable to or smaller than that of the free electron. Since the quantum degeneracy temperature scales inversely with the mass, it is anticipated that Bose?Einstein condensation of an exciton gas can be achieved at temperatures of about 1?K, orders of magnitude larger than the micro-Kelvin temperatures employed in atomic condensation. High quantum degeneracy temperatures and the possibility to control exciton density by laser photoexcitation make cold excitons a model system for studies of collective states and many-body phenomena in a system of cold bosons. Experimentally, an exciton temperature well below 1?K is achieved in a gas of indirect excitons in coupled quantum-well semiconductor heterostructures. Here, we overview phenomena in the cold exciton gases: condensation, pattern formation, and macroscopically ordered exciton states.
134 citations
TL;DR: In this article, a detailed exploration of pump-probe stimulated emission (SE) and time-resolved photoluminescence (PL) dynamics of conjugated polymer films under identical excitation conditions is presented.
Abstract: Despite the enormous versatility of conjugated polymers for use in optoelectronic devices and the correspondingly large number of research studies on their photophysics, understanding of the fundamental nature of the primary photoexcitations remains highly controversial. Part of the reason for this controversy stems from the fact that the photophysics of conjugated polymer films depends sensitively on the excitation intensity, making it difficult to compare the results of different experiments such as pump-probe stimulated emission (SE) and time-resolved photoluminescence (PL), which usually are performed at excitation intensities that differ by orders of magnitude. In this paper, we present a detailed exploration of the pump-probe SE and time-resolved PL dynamics of poly(2-methoxy 5-$[{2}^{\ensuremath{'}}$-ethylhexyloxy]-p-phenylene vinylene) (MEH-PPV) films under identical excitation conditions. Using an optically triggered streak camera, we are able to simultaneously measure the PL and SE dynamics as a function of both excitation intensity and emission wavelength. Although the SE and PL dynamics of dilute MEH-PPV solutions are identical, we find that even at relatively low excitation intensities, the PL and SE dynamics of MEH-PPV films are different, uncovering the presence of an interchain excited-state absorbing species that has dynamics distinct from the intrachain exciton. The number of interchain absorbing species increases nonlinearly with excitation intensity, suggesting that interchain species are formed both directly upon photoexcitation and as a by-product of exciton-exciton annihilation (E-EA). A comparison of the emission dynamics of pristine and intentionally oxidized MEH-PPV films leads us to conclude that there are multiple types of interchain species; we propose that the directly excited interchain species are likely aggregates or excimers, while the interchain species produced by E-EA are best assigned to polaron pairs. We also find that the wavelength dependence of the SE and PL from MEH-PPV films is complex, both because energy migration leads to a dynamic redshift of the exciton emission and because the rate of E-EA is higher for hot (blue-emitting) excitons than for thermalized (red-emitting) excitons. All the results are compared in detail to previous work, providing a means to resolve many of the apparently contradictory ideas in the literature concerning the photophysics of conjugated polymer films.
123 citations
TL;DR: The processes involved in ultrafast electron injection from a photoexcited novel coumarin dye (NKX-2311) into the conduction band of a nanocrystalline ZnO film were investigated by observing the femtosecond transient absorption in the visible-to-infrared range (600−5000 nm) as discussed by the authors.
Abstract: The processes involved in ultrafast electron injection from a photoexcited novel coumarin dye (NKX-2311), an efficient photosensitizer for TiO2-based dye-sensitized solar cells, into the conduction band of a nanocrystalline ZnO film were investigated by observing the femtosecond transient absorption in the visible-to-infrared range (600−5000 nm). After photoexcitation of adsorbed NKX-2311 dye, the stimulated emission and absorption of the singlet excited dye decayed with a 500-fs time constant. These were followed by rises in absorptions of the oxidized dye and conduction-band electrons, indicating a direct electron-injection process. In addition, indirect electron-injection processes involving intermediates were identified. The intermediates showed stimulated emission at longer wavelengths than that of the excited dye; very broad absorptions in the near-IR region (900−1300 nm) were observed immediately after excitation, and they decayed with 1- and 10-ps time constants, leading to further rises in the ab...
122 citations
TL;DR: In this article, it was shown that 532 and 1064 nm laser photoexcitation of trapped electrons generated by 355 nm photolysis of aqueous titania (TiO2) nanoparticles causes rapid photobleaching of their absorbance band in the visible and near-IR.
Abstract: It is shown that 532 and 1064 nm laser photoexcitation of trapped electrons generated by 355 nm photolysis of aqueous titania (TiO2) nanoparticles causes rapid photobleaching of their absorbance band in the visible and near-IR. This photobleaching occurs within the duration of the laser pulse (3 ns fwhm); it is caused by photoinduced electron detrapping followed by rapid recombination of the resulting free electron and a trapped hole. The quantum yield for the electron photobleaching is ca. 0.28 for 532 nm and ca. 0.024 for 1064 nm photoexcitation. Complete separation of the spectral contributions from trapped electron and hole is demonstrated using glycerol as a selective hole scavenger. When glycerol is added to the solution, some light-absorbing holes are scavenged promptly within the duration of the 355 nm photoexcitation pulse, some are scavenged at a slower rate over the first 200 ns after the 355 nm pulse, and the rest are not scavenged, even at high concentration of the scavenger (>10 vol. %). A r...
121 citations
TL;DR: In this article, the authors report studies on several methylated uracils and thymines and water complexes in the gas phase using resonantly enhanced multiphoton ionization (REMPI) and laser-induced fluorescence (LIF) spectroscopy.
Abstract: We report studies on several methylated uracils and thymines and thymine−water complexes in the gas phase using resonantly enhanced multiphoton ionization (REMPI) and laser-induced fluorescence (LIF) spectroscopy. Results from two different REMPI experiments provided strong evidence that, after photoexcitation to the S1 state, bare molecules were funneled into and trapped in a dark state via fast internal conversion. Lifetimes of this dark state were determined to be tens to hundreds of nanoseconds, depending on the internal energy and the degree of methyl substitution. The mass spectra of hydrated thymine clusters demonstrated dependence on the excitation wavelength, and the gradual loss of the ion signal with increasing water content across the absorption region of the S1 state indicated a reduced lifetime of the S1 state by the water solvent. In addition, the lifetime of the dark state also decreased gradually as thymine became more hydrated. On the basis of these results, we conclude that, in water so...
TL;DR: Experimental observation of freely propagating pulses of terahertz radiation produced by optical rectification of femtosecond pulses at metal surfaces opens a qualitatively new way of investigation of nonlinear phenomena atMetal surfaces.
Abstract: The emission of freely propagating terahertz (THz) radiation coming from optical rectification at metallic surfaces has been detected and characterized for the first time to the authors’ knowledge. The observed THz transients are induced through nonlinear electronic processes at gold and silver surfaces on intense pulsed optical photoexcitation and exhibit a peak electric field of as much as 200 V/cm. This finding opens a qualitatively new way to investigate nonlinear phenomena at metal surfaces and also can be exploited for the development of new THz emitters.
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TL;DR: In this article, it was shown that 532 nm and 1064 nm laser photoexcitation of trapped electrons generated by 355 nm photolysis of aqueous titania (TiO2) nanoparticles causes rapid photobleaching of their absorbance band in the visible and near IR.
Abstract: It is shown that 532 nm and 1064 nm laser photoexcitation of trapped electrons generated by 355 nm photolysis of aqueous titania (TiO2) nanoparticles causes rapid photobleaching of their absorbance band in the visible and near IR. This photobleaching occurs within the duration of the laser pulse (3 ns FWHM); it is caused by photoinduced electron detrapping followed by rapid recombination of the resulting free electron and a trapped hole. The quantum yield for the electron photobleaching is ca. 0.28 for 532 nm and ca. 0.024 for 1064 nm photoexcitation. Complete separation of the spectral contributions from trapped electron and hole is demonstrated using glycerol as a selective hole scavenger. When glycerol is added to the solution, some light-absorbing holes are scavenged promptly within the duration of the 355 nm photoexcitation pulse, some are scavenged at a slower rate over the first 200 ns after the 355 nm pulse, and the rest are not scavenged, even at high concentration of the scavenger (> 10 vol %). A reaction with chemi- and physi- sorbed glycerol would account for the prompt and the slow hole decay, respectively. The implications of these results are discussed.
TL;DR: In this paper, the authors used time-resolved transient absorption spectroscopy to determine quantum yields for electron photodetachment in 193 nm and (where possible) 248 nm laser excitation of miscellaneous aqueous anions, including hexacyanoferrate(II), sulfate, halide anions (Cl-, Br-, and I-), pseudohalide (OH-, HS-, and CNS-).
Abstract: Time-resolved transient absorption spectroscopy has been used to determine quantum yields for electron photodetachment in 193 nm and (where possible) 248 nm laser excitation of miscellaneous aqueous anions, including hexacyanoferrate(II), sulfate, halide anions (Cl-, Br-, and I-), pseudohalide anions (OH-, HS-, and CNS-), and several common inorganic anions for which no quantum yields have been reported heretofore: SO32-, NO2-, NO3-, ClO3-, and ClO4-. Molar extinction coefficients for these anions and photoproducts of electron detachment from these anions at the excitation wavelengths were also determined. These results are discussed in the context of recent ultrafast kinetic studies and compared with the previous data obtained by product analyses. We suggest using electron photodetachment from the aqueous halide and pseudohalide anions as actinometric standards for time-resolved studies of aqueous photosystems in the UV.
TL;DR: In this article, the effect of Dushinskii rotations on the vibrational population created in an excited electronic state through photoexcitation was investigated, and it was shown that rotation, position shifts, and frequency shifts on the absorption spectrum and average vibrational energy in the excited state generally decrease cooling.
Abstract: We present a theoretical study of the effect of Dushinskii rotations on the vibrational population created in an excited electronic state through photoexcitation. Special attention is given to the effect of Dushinskii rotations on the possibility of cooling the vibrational population in the excited state, relative to the thermal distribution in the ground state. The absorption spectrum and corresponding average energy in the excited state are calculated using a closed-form expression for the harmonic correlation function between the ground and excited electronic states, which includes the effects of Dushinskii rotations, equilibrium position shifts, and frequency shifts between the excited- and ground-electronic-state normal modes. We investigate numerically the separate and joint effects of rotation, position shifts, and frequency shifts on the absorption spectrum and average vibrational energy in the excited electronic state. We find that, although the Dushinskii rotations generally diminish the cooling...
TL;DR: Ultrafast reorientation of the antiferromagnetic order parameter of NiO is triggered by a photoinduced transient modification of the magnetic anisotropy and evidenced by the subsequent interference of the hard- and easy-axis states.
Abstract: Photoexcitation of antiferromagnetic NiO leads to ultrafast reorientation of Ni2+ spins due to change of the magnetic anisotropy. Recovery of the magnetic ground state occurs as coherent oscillation of the antiferromagnetic order parameter between hard- and easy-axis states manifesting itself as quantum beating. The coherence time is approximately 1 ns with the beating frequency being determined by the anisotropy energy.
TL;DR: In this paper, electron injection and back-electron transfer dynamics of quinizarin (Qz) adsorbed on TiO2 and ZrO2 nanoparticles has been studied by femtosecond transient absorption spectroscopy in the visible and near-IR region.
Abstract: Electron injection and back-electron-transfer (BET) dynamics of quinizarin (Qz) adsorbed on TiO2 and ZrO2 nanoparticles has been studied by femtosecond transient absorption spectroscopy in the visible and near-IR region. A good fraction of Qz forms a charge-transfer (CT) complex while being adsorbed on the TiO2 or ZrO2 nanoparticles surface. Following photoexcitation of Qz/TiO2 and Qz/ZrO2 systems, electron injection into the nanoparticles has been confirmed for both the systems by direct detection of electron in the nanoparticle and cation radical of Qz (Qz.+). The dynamics of BET from TiO2 and ZrO2 to the parent cation has been measured by monitoring the decay kinetics of Qz•+ and electron in the nanoparticles and it is found to be multiexponential. As S1 state of Qz lies below the conduction band edge of ZrO2 so electron injection from S1 state into the nanoparticle is not thermodynamically possible. However, the detection of Qz•+ as well as injected electrons in the case of Qz/ZrO2 system confirms tha...
TL;DR: The relationship between the molecular structures and their TPA spectral behaviors are discussed, and the saturation behavior of TPA transition can be observed obviously in AF-350 and AF-380 solutions that exhibit much higher nonlinear absorptivity than the other chromophores investigated.
Abstract: Degenerate two-photon absorption (TPA) spectral properties of five AFX chromophore solutions have been studied using a single and spectrally dispersed sub-picosecond white-light continuum beam. In a specially designed optical configuration, optical pathways inside the sample solution for different spectral components of the focused continuum beam were spatially separated from each other. Thus, the nondegenerate TPA processes coming from different spectral components can be eliminated, and the direct nonlinear absorption spectrum attributed to degenerate TPA processes can be readily obtained. Using this new technique, the complete TPA spectra for these five highly two-photon-active compounds (AF-380, AF-350, AF-295, AF-270, and AF-50) were obtained in the spectral range from 600 to 950 nm on an absolute scale of TPA cross section. The relationship between the molecular structures and their TPA spectral behaviors are discussed. In general the measured TPA spectra are not identical with the linear absorption...
TL;DR: Investigation of ultrafast processes in photoexcited N-salicylideneaniline with femtosecond time-resolved resonance-enhanced multiphoton ionization spectroscopy reveals that an internal conversion to the S(1)(n,pi( *)) state of the enol form may provide opposite effect on the quantum yield of photochromic products.
Abstract: Ultrafast processes in photoexcited N-salicylideneaniline have been investigated with femtosecond time-resolved resonance-enhanced multiphoton ionization spectroscopy. The ion signals via the S1(n,π*) state of the enol form as well as the proton-transferred cis-keto form emerge within a few hundred femtoseconds after photoexcitation to the first S1(π,π*) state of the enol form. This reveals that two ultrafast processes, excited-state intramolecular proton transfer (ESIPT) reaction and an internal conversion (IC) to the S1(n,π*) state, occur on a time scale less than a few hundred femtoseconds from the S1(π,π*) state of the enol form. The rise time of the transient corresponding to the production of the proton-transferred cis-keto form is within 750 fs when near the red edge of the absorption is excited, indicating that the ESIPT reaction occurs within 750 fs. The decay time of the S1(π,π*) state of the cis-keto form is 8.9 ps by exciting the enol form at 370 nm, but it dramatically decreases to be 1.5–1.6...
TL;DR: The geometry changes on excitation differ for the two symmetry-independent molecules, but are in the same direction as calculated for an isolated reference molecule, although the flattening distortion in the crystal is significantly smaller, implying that the reorganization energy is greatly affected by the confining medium.
Abstract: Using a stroboscopic technique, in which the molecule is repeatedly excited and the structural change is probed more than 5000 times per second immediately after excitation, we performed a 16 K time-resolved single-crystal study of the microsecond lifetime triplet state of the Cu(I)phenanthroline derivative[Cu(I)(dmp)(dppe)][PF6] (dppe = 1,2-bis(diphenylphosphino)ethane). The geometry changes on excitation differ for the two symmetry-independent molecules, but are in the same direction as calculated for an isolated reference molecule, although the flattening distortion in the crystal is significantly smaller, implying that the reorganization energy is greatly affected by the confining medium.
TL;DR: In this article, the authors used transient absorption spectroscopy to determine quantum yields for electron photodetachment in 193 nm and (where possible) 248 nm laser excitation of miscellaneous aqueous anions, including hexacyanoferrate(II), sulfate, sulfite, halide anions (Cl-, Br-, and I-), pseudohalide (OH-, HS-, CNS-), and several common inorganic anions for which no quantum yields have been reported heretofore.
Abstract: Time resolved transient absorption spectroscopy has been used to determine quantum yields for electron photodetachment in 193 nm and (where possible) 248 nm laser excitation of miscellaneous aqueous anions, including hexacyanoferrate(II), sulfate, sulfite, halide anions (Cl-, Br-, and I-), pseudohalide anions (OH-, HS-, CNS-), and several common inorganic anions for which no quantum yields have been reported heretofore: SO3=, NO2-, NO3-, ClO3- and ClO4-. Molar extinction coefficients for these anions and photoproducts of electron detachment from these anions at the excitation wavelengths were also determined. These results are discussed in the context of recent ultrafast kinetic studies and compared with the previous data obtained by product analyses. We suggest using electron photodetachment from the aqueous halide and pseudohalide anions as actinometric standard for time-resolved studies of aqueous photosystems in the UV.
TL;DR: In this article, the intramolecular photoinduced charge separation within an oligo(p-phenylene vinylene) and fulleropyrrolidine dyad with four phenyl rings (OPV4-C60) was investigated with femtosecond pump-probe spectroscopy in solvents of different polarity and in the solid state.
Abstract: The intramolecular photoinduced charge separation within an oligo(p-phenylene vinylene)–fulleropyrrolidine dyad with four phenyl rings (OPV4-C60) has been investigated with femtosecond pump-probe spectroscopy in solvents of different polarity and in the solid state. In solution, photoexcitation of the OPV4 moiety of OPV4-C60 results in an ultrafast (<190 fs) singlet energy transfer reaction, creating the fullerene singlet excited state. In polar solvents, the ultrafast energy transfer is followed in the picosecond time domain by an intramolecular electron transfer. In accordance with Marcus theory, the rates for forward and backward intramolecular electron transfer in OPV4-C60 are influenced by the polarity of the solvent. In the solid state the photophysics of OPV4-C60 is dramatically different. In thin films, the forward electron transfer proceeds within 500 fs, irrespective of which chromophore is photoexcited. The increased rate for charge separation in the solid state is attributed to a more favorable orientation of the donor and acceptor that results in an intermolecular electron transfer. In the films, energy and electron transfer processes compete at the earliest moments after photoexcitation. In the solid state, the photogenerated electrons and holes have long lifetimes as a result of migration of these charges to thermodynamically more favorable sites in the film.
TL;DR: In this article, femtosecond transient absorption studies of [Os(bpy)3]2+ have been performed to gain new insights into the excited-state dynamics via the π−π* transition on the bpy-anion at 370 nm.
Abstract: Femtosecond transient absorption studies of [Os(bpy)3]2+ have been performed to gain new insights into the excited-state dynamics. Experiments were performed for a series of excitation wavelengths throughout both the singlet and triplet metal-to-ligand charge transfer (MLCT) bands. The dynamics are probed via the π−π* transition on the bpy- anion at 370 nm. Time scales for interligand electron transfer (ILET) and vibrational cooling are extracted from the amplitude and anisotropy of the excited state absorptions. The ILET rate has been measured as a function of temperature and, from these data, estimates for the electron transfer activation energy and ligand−ligand coupling are made. Our data also provide insight into the vibrational cooling dynamics within these large complexes, suggesting that vibrational cooling occurs only when exciting into the singlet MLCT state, after intersystem crossing. It appears that photoexcitation within the triplet MLCT absorption produces molecules that are vibrationally c...
TL;DR: In this paper, the authors describe time-resolved x-ray diffraction experiments which reveal changes in the atomic structure with a time resolution of a few hundred femtoseconds.
Abstract: When intense femtosecond laser pulses are focused on solid targets short-lived microplasmas are formed which emit bursts of x-rays with kilovolt photon energies. Under the proper conditions x-ray pulses as short as a few hundred femtoseconds can be produced. These x-ray pulses enable ultrafast x-ray spectroscopy using pump–probe schemes where the x-ray pulses serve as probe pulses. This article describes time-resolved x-ray diffraction experiments which reveal changes in the atomic structure with a time resolution of a few hundred femtoseconds. In particular, we have studied solid-to-liquid phase transitions in semiconductors induced by femtosecond photoexcitation and the accompanying thermoacoustic phenomena. We were able to monitor the changes in the atomic position underlying a coherent optical phonon mode. These and a number of other lattice dynamics experiments discussed here demonstrate the feasibility and usefulness of ultrafast time-resolved x-ray diffraction. Future applications in many other fields of science can be foreseen.
TL;DR: Static and time-resolved magneto-optical spectra of the ferromagnetic semiconductor (Ga,Mn)As show that a pulsed photoexcitation with a fluence of 10 microJ/cm(2) is equivalent to the application of an external magnetic field of about 1 mT, which relaxes with a decay time of 30 ps.
Abstract: Static and time-resolved magneto-optical spectra of the ferromagnetic semiconductor (Ga,Mn)As show that a pulsed photoexcitation with a fluence of 10 microJ/cm(2) is equivalent to the application of an external magnetic field of about 1 mT, which relaxes with a decay time of 30 ps. This relaxation is attributed to the spin relaxation of electrons in the conduction band and is found to be not affected by interactions with Mn ions.
TL;DR: Charge-transfer-to-solvent reactions of hydroxide induced by 200 nm monophotonic or 337 and 389 nm biphotonic excitation of this anion in aqueous solution have been studied by means of pump-probe ultrafast laser spectroscopy and semianalytical theory of Shushin for diffusion controlled reactions in the central force field was used to model the geminate dynamics.
Abstract: Charge-transfer-to-solvent reactions of hydroxide induced by 200 nm monophotonic or 337 and 389 nm biphotonic excitation of this anion in aqueous solution have been studied by means of pump–probe ultrafast laser spectroscopy. Transient absorption kinetics of the hydrated electron, eaq−, have been observed, from a few hundred femtoseconds out to 600 ps, and studied as function of hydroxide concentration and temperature. The geminate decay kinetics are bimodal, with a fast exponential component (∼13 ps) and a slower power “tail” due to the diffusional escape of the electrons. For the biphotonic excitation, the extrapolated fraction of escaped electrons is 1.8 times higher than for the monophotonic 200 nm excitation (31% versus 17.5% at 25 °C, respectively), due to the broadening of the electron distribution. The biphotonic electron detachment is very inefficient; the corresponding absorption coefficient at 400 nm is <4 cm TW−1 M−1 (assuming unity quantum efficiency for the photodetachment). For [OH−] between 10 mM and 10 M, almost no concentration dependence of the time profiles of solvated electron kinetics was observed. At higher temperature, the escape fraction of the electrons increases with a slope of 3×10−3 K−1 and the recombination and diffusion-controlled dissociation of the close pairs become faster. Activation energies of 8.3 and 22.3 kJ/mol for these two processes were obtained. The semianalytical theory of Shushin for diffusion controlled reactions in the central force field was used to model the geminate dynamics. The implications of these results for photoionization of water are discussed.
TL;DR: In this paper, the authors use the intense, 5-ns-long, excitation pulses provided by the fourth harmonic of a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser to induce a strong high-energy shift of the photoluminescence of a 7.8nm wide GaN/Al 0.15Ga0.85N single quantum well.
Abstract: We use the intense, 5-ns-long, excitation pulses provided by the fourth harmonic of a neodymium-doped yttrium aluminum garnet (Nd:YAG) laser to induce a strong high-energy shift of the photoluminescence of a 7.8-nm-wide GaN/Al0.15Ga0.85N single quantum well. We follow the complex relaxation dynamics of the energy and of the intensity of this emission, by using a time-resolved photoluminescence setup. We obtain excellent agreement between our experimental results and those of our finite-element modeling of the time-dependent energy and oscillator strength. The model, based on a self-consistent solution of the Schrodinger and Poisson equations, accounts for the three important sources of energy shifts: (1) the screening of the electric field present along the growth axis of the well, by accumulation of electron-hole dipoles, (2) the band-gap renormalization induced by many-body interactions, and (3) the filling of the conduction and valence bands.
TL;DR: In this paper, the vanishing resistance of two-dimensional electron system exhibits vanishing resistance under photoexcitation at low temperatures in the vicinity of B=[4/(4j+1)]Bf, where B f = 2πfm ∗ /e, m ∗ is an effective mass, e is the charge, and f is the microwave frequency.
Abstract: The high mobility two-dimensional electron system exhibits vanishing resistance under photoexcitation at low temperatures in the vicinity of B=[4/(4j+1)]Bf. Here, the characteristic magnetic field B f =2πfm ∗ /e, m ∗ is an effective mass, e is the charge, and f is the microwave frequency. In this report, we present further experimental results that illustrate the phenomenology of the effect.
TL;DR: In this paper, the lifetime of the 1s2s 2 2p 22 D, 2 P and 2 S autoionizing states of C + was determined by measurement of the natural linewidth.
Abstract: Lifetimes for K-shell vacancy states in atomic carbon have been determined by measurement of the natural linewidth of the 1s → 2p photoexcited states of C + ions. The K-shell vacancy states produced by photoionization of atomic carbon are identical to those produced by 1s → 2p photoexcitation of a C + ion: 1s2s 2 2p 22 D, 2 P, and 2 S autoionizing states occur in both cases. These vacancy states stabilize by emission of an electron to produce C 2+ ions. Measurements are reported for the lifetime of the 1s2s 2 2p 22 D, 2 P and 2 S autoionizing states of C + :6 .3± 0.9 fs, 11.2 ± 1.1 fs and 5.9 ± 1.3 fs respectively. Knowledge of such lifetimes is important for comparative studies of the lifetimes of Kshell vacancies in carbon-containing molecules, benchmarking theory, and interpreting satellite x-ray spectra from astrophysical sources such as x-ray binaries. Absolute cross sections were measured for both ground-state and metastable-state ions providing a stringent test of state-of-the-art theoretical calculations. Carbon is ubiquitous in nature and is the building block of life. This atom in its various stages of ionization has relatively few electrons, and is thus amenable to theoretical study. Lifetimes
TL;DR: It is concluded that the dissociation of DPCP generates the S(2) state of DPA that probably has a cis-bent structure and the assignment of the "330-cm(-1) vibration" is attributed to a vibration involving the displacement of the CO group as well as the deformation of the Ph-C[Double Bond]C-Ph skeleton.
Abstract: Reaction dynamics and coherent nuclear motions in the photodissociation of diphenylcyclopropenone (DPCP) were studied in solution by time-resolved absorption spectroscopy. Subpicosecond transient absorption spectra were measured in the visible region with excitation at the second absorption band of DPCP. The obtained spectra showed a new short-lived band around 480 nm immediately after photoexcitation, which is assignable to the initially populated S2 state of DPCP before the dissociation. The dissociation takes place from this excited state (the precursor of the reaction) with a time constant of 0.2 ps, and the excited state of diphenylacetylene (DPA) is generated as the reaction product. The transient absorption after the dissociation decayed with a time constant of 8 ps that is very close to the S2-state lifetime of DPA, but the spectrum of this 8-ps component was different from the S2 absorption observed with direct photoexcitation of DPA. We conclude that the dissociation of DPCP generates the S2 state of DPA that probably has a cis-bent structure. At later delay times (>30 ps), the transient absorption signals are very similar to those obtained by direct photoexcitation of DPA. This confirmed that the electronic relaxation from the S2 state of the product DPA occurs in a similar manner to that of DPA itself, i.e., the internal conversion to the S1 state and subsequent intersystem crossing to the T1 state. In order to examine the coherent nuclear dynamics in this dissociation reaction, we carried out time-resolved absorption measurements for the 480-nm band with 70 fs resolution. It was found that an underdamped oscillatory modulation with a 0.1-ps period is superposed on the decay of the precursor absorption. This indicates that DPCP exhibits a coherent nuclear motion having a ∼330-cm−1 frequency in the dissociative excited state. Based on a comparison with the measured and calculated Raman spectra of ground-state DPCP, we discuss the assignment of the “330-cm−1 vibration” and attribute it to a vibration involving the displacement of the CO group as well as the deformation of the Ph–C=C–Ph skeleton. We consider that this motion is closely related to the reaction coordinate of the photodissociation of DPCP.