Showing papers on "Photoexcitation published in 1998"
TL;DR: In this paper, femtosecond pump−probe spectroscopy was used to time resolve the injection of electrons into nanocrystalline TiO2 film electrodes under ambient conditions following photoexcitation of the adsorbed dye, [Ru(4,4,dicarboxy-2,2‘-bipyridine)2(NCS)2] (N3).
Abstract: We have used femtosecond pump−probe spectroscopy to time resolve the injection of electrons into nanocrystalline TiO2 film electrodes under ambient conditions following photoexcitation of the adsorbed dye, [Ru(4,4‘-dicarboxy-2,2‘-bipyridine)2(NCS)2] (N3). Pumping at one of the metal-to-ligand charge-transfer absorption peaks and probing the absorption by injected electrons in the TiO2 at 1.52 μm and in the range of 4.1−7.0 μm, we have directly observed the arrival of electrons injected into the TiO2 film. Our measurements indicate an instrument-limited ∼50 fs upper limit on the electron injection time. We have compared the infrared transient absorption for noninjecting systems consisting of N3 in ethanol and N3 adsorbed to films of nanocrystalline Al2O3 and ZrO2 and found no indication of electron injection at probe wavelengths in the mid-IR (4.1−7.0 μm).
280 citations
TL;DR: In this paper, the power-dependent relaxation dynamics of photoexcited charge carriers in a number of II-VI semiconductor quantum dots have been studied using femtosecond laser spectroscopy.
Abstract: The power-dependent relaxation dynamics of photoexcited charge carriers in a number of II-VI semiconductor quantum dots have been studied using femtosecond laser spectroscopy. The dynamics are obtained via excitation of the quantum dots with high power 390 nm pulses of 150 fs duration, and probing of the photoexcited species by monitoring the change in absorption at 790 nm as a function of time. Particles with vastly differing surfaces, sizes, electronic structures, and solvents all show a fast 1.5–4 picosecond decay component which grows in with power, a 17 ps (CdSe) or 50 ps (CdS and Cd0.5Zn0.5S) decay component, and some transient absorption persisting beyond 600 ps. The power-dependent component for CdSe quantum dots in glass has a 1.5 ps decay time constant, while for the liquid dispersed CdS and Cd0.5Zn0.5S quantum dots it has 2–4 ps decay time constants. This variation in the time constant is due to its power dependence, the time constant decreases with increasing power. It is also shown that the power-dependent decay is only weakly dependent on surface, size, and electronic structure. With the assistance of a power-dependent nanosecond fluorescence study, we have assigned the power-dependent decay primarily to exciton–exciton annihilation. This decay mechanism becomes dominant under high intensity excitation where multiple photoexcited charge carriers are created in each quantum dot, leading to trap state saturation and an accumulation of band edge excitons. Auger recombination may also play an important role at very high excitation intensities.
96 citations
TL;DR: In this article, the authors investigated the underlying plasmon dynamics in time-resolved experiments on the emitted submillimeter wave radiation and found that the plasmor damping is due to phonon scattering.
Abstract: Femtosecond laser excitation of bulk $n$-doped GaAs leads to coherent plasmon excitations that emit intense submillimeter wave radiation. We investigated the underlying plasmon dynamics in time-resolved experiments on the emitted THz radiation. The results show that THz pulses are emitted due to the coherent oscillation of the extrinsic electron plasma in the bulk. The plasmon excitations are started by single-sided screening of the surface depletion field after photoexcitation. Temperature-dependent measurements indicate that the plasmon damping is due to phonon scattering.
94 citations
70 citations
TL;DR: In this article, the main manifestations of polaron pairs in the generation of free charge carriers, delayed luminescence, intra-chain triplet exciton generation, recombination of free carriers, persistence of the photoconductivity, displacement current in the transient photoconductivities experiments, and spin-dependent reactions are discussed.
64 citations
TL;DR: In this paper, a theory to describe nonequilibrium electronic surface crossing during vibrational relaxation induced by ultrafast photoexcitation is developed and applied to the primary electron transfer (ET) in bacterial photosynthetic reaction centers.
Abstract: A theory to describe nonequilibrium electronic surface crossing during vibrational relaxation induced by ultrafast photoexcitation is developed and applied to the primary electron transfer (ET) in bacterial photosynthetic reaction centers. As a key concept, we define on a microscopic basis the angle between two reaction coordinates each representing the environmental nuclear displacements coupled to the initial photoexcitation (to the P* state) and to the subsequent ET processes, respectively. The “cross-spectral” density function, whose integral intensity gives the cosine of this angle, is also defined to give a consistent (nonphenomenological) description of the vibrational coherence and its dephasing. In the application to the primary ET in bacterial photosynthesis, we find (1) the time-dependent ET rate exhibits marked oscillation at low temperatures due to the nonequilibrium vibrational coherence in the P* state. However, it does not contribute very much to accelerate the primary ET rate with respect...
64 citations
01 Jun 1998
TL;DR: In this paper, the branching ratio for forming ground I(2P3/2) and spin-orbit excited I(1 1/2 ) atoms was determined at 24 different wavelengths in the range 201.0-303.0 nm, whilst detailed angular distributions for both fragmentation channels have been measured at five wavelengths within this range.
Abstract: The near ultraviolet photodissociation of hydrogen iodide has been investigated in detail using the technique of H Rydberg atom photofragment translational spectroscopy. Branching ratios for forming ground I(2P3/2) and spin-orbit excited I(2P1/2) atoms have been determined at 24 different wavelengths in the range 201.0–303.0 nm, whilst detailed angular distributions for both fragmentation channels have been measured at five wavelengths within this range. The branching ratio measurements confirm previous measures of this ratio at excitation wavelengths λ<250 nm, but at longer wavelengths show a significantly larger relative yield of spin-orbit excited iodine (I*) atoms than hitherto reported. The ground state H+I products are confirmed as arising as a result of a perpendicular (i.e., ΔΩ=1) electronic excitation but, again in contradiction to the conventional wisdom, the angular distribution measurements show the I* atoms to arise almost exclusively (ca. 93%) via a parallel (i.e., ΔΩ=0) photoexcitation pathway. The observations are discussed in the light of available information regarding the number and nature of the various repulsive excited states contributing to the near UV absorption spectrum of HI.
62 citations
TL;DR: In this paper, an ultrafast time-resolved magneto-optical pump probe is applied to study spin-relaxation processes in a ferromagnetic CoPt 3 alloy film.
Abstract: An ultrafast time-resolved magneto-optical pump probe is applied to study spin-relaxation processes in a ferromagnetic ${\mathrm{CoPt}}_{3}$ alloy film. Following spin-selective photoexcitation with circularly polarized femtosecond pulses, transient Kerr ellipticity tracks the evolution of the nonequilibrium spin system. We isolate two distinct processes: a subpicosecond component due to the relaxation of coherently spin polarized electrons, and a slower component (\ensuremath{\sim}10 psec) associated with the evolution of the thermalized spin distribution and the photoinduced transient magnetization.
62 citations
TL;DR: In this paper, the short-time charge transfer evolution following photoexcitation in mixed valence compounds is studied using path integral calculations, and the effects of preparing an initial state that is not a rovibrational state of the acceptor potential energy surface is also examined.
Abstract: The short-time charge transfer evolution following photoexcitation in mixed valence compounds is studied using path integral calculations. Due to the large nonadiabatic coupling, path integral calculations using direct path summation techniques are inadequate, and charge transfer dynamics can only be computed using a transfer matrix technique developed by Makri and Makarov. The resulting relaxation is considerably slower than that predicted by low-order perturbation theory. The effects of the solvent on the decay process, and the validity of the golden rule to predict the dynamics of the decay process are investigated. The effects of preparing an initial state that is not a rovibrational state of the acceptor potential energy surface is also examined. These exact calculations show that the large electronic mixing gives rise to very fast oscillations in the electronic state population as the wave function oscillates coherently between the donor and acceptor. This is followed by a slower relaxation induced ...
57 citations
51 citations
TL;DR: In this article, three pyrazine derivatives were probed by emission and transient absorption spectroscopy, and the triplet excited pyrazines undergo rapid intermolecular energy transfer to a monofunctionalized fullerene derivative.
Abstract: Excited-state properties of three different pyrazine derivatives 4−6 were probed by emission and transient absorption spectroscopy. They display emission maxima at 464 (4), 417 (5), and 515 nm (6) that are red-shifted with respect to their strong UV ground-state absorption and formed with overall quantum yields (Φ) of 0.156, 0.22, and 0.13, respectively. Once photoexcited, these triplet excited pyrazines undergo rapid intermolecular energy transfer to a monofunctionalized fullerene derivative (7) with bimolecular rate constants ranging from 3.64 × 109 M-1 s-1 (6) to 1.1 × 1010 M-1 s-1 (4). The product of these bimolecular energy-transfer reactions is in all cases the fullerene triplet excited state. Functionalization of pristine C60 with the investigated pyrazine derivatives promotes the UV−vis absorption characteristics and, in turn, improves the light-harvesting efficiency of the resulting dyads 1−3 relative to pristine C60. Photoexcitation of the pyrazine moieties in dyads 1−3 leads to the formation of...
TL;DR: In this paper, the vibrational relaxation process of trans-stilbene in the lowest electronically excited singlet state after photoexcitation has been studied by picosecond time-resolved anti-Stokes Raman spectros.
Abstract: The vibrational relaxation process of trans-stilbene in the lowest electronically excited singlet state after photoexcitation has been studied by picosecond time-resolved anti-Stokes Raman spectros...
TL;DR: The synthesis and study of a rigid "coplanar" noncovalent electron-transfer model system that juxtaposes a novel donor and acceptor via a three-point hydrogen bonding interaction is described.
Abstract: Described in this paper is the synthesis and study of a rigid “coplanar” noncovalent electron-transfer model system. This putative noncovalent complex juxtaposes a novel donor (chlorin) and acceptor (naphthalene diimide) via a three-point hydrogen bonding interaction (CDCl3, Ka = 364 ± 47 M-1). It was studied by steady state fluorescence, time-resolved luminescence, and transient absorption methods. The results of the studies are consistent with (1) forward intraensemble electron transfer (ET) taking place rapidly following photoexcitation of the chlorin donor at 575 nm (kET = 7.6 × 108 s-1; ΔGcs ∼ −457 mV; Φ = 0.91) and (2) back electron transfer occurring even more rapidly.
TL;DR: In this paper, a single crystal of conjugated sexithiophene oligomer shows stimulated emission when excited with a low-energy photonic pulse, interpreted in terms of the excitonic energy diagram of the crystal, which presents a four-level pathway for the photoexcitation and emission, similar to the one observed in classical inorganic-based laser materials.
Abstract: Single crystal of conjugated sexithiophene oligomer shows stimulated emission when excited with a low-energy photonic pulse This phenomenon is interpreted in terms of the excitonic energy diagram of the crystal, which presents a four-level pathway for the photoexcitation and emission, similar to the one observed in classical inorganic-based laser materials
TL;DR: In this paper, a simple method to measure a rate of the translational temperature increase after the non-radiative transition with a few ps time resolution is presented, which uses an acoustic peak shift of the transient grating signal, which can be determined accurately, and is very sensitive to rates and the relative amount of the thermal energy from the non radiative transition.
Abstract: A simple method to measure a rate of the translational temperature increase after the nonradiative transition with a few ps time resolution is presented. This method uses an acoustic peak shift of the transient grating signal, which can be determined accurately, and is very sensitive to rates and the relative amount of the thermal energy from the nonradiative transition. Using this method, the thermalization rate after the photoexcitation of trans-azobenzene to the S2(ππ*) state is measured. The acoustic peak shift indicates that the temperature of the solvents rises within less than ∼3 ps after the nonradiative transition of the S2→S1 process. The thermalization after the S1→S0 transition completes faster than the decay of the S1 state (16 ps in acetonitrile and 18 ps in ethanol). The fast energy transfer from the solute to the solvent is discussed.
TL;DR: In this paper, the authors studied molecule reorientation initiated by absorption of resonant light, in D2 azo dye doped nematic liquid crystals, where the photoexcitation of the dye subsystem creates a torque on the host, that is not collinear with the traditional dielectric torque.
TL;DR: In this article, an approach for measuring reflected light signal on crystalline or powder samples of thermochromic materials, such as ''spin-crossover'' materials, has been developed in conjunction with a helium cryostat enabling measurements from cryogenic to ambient temperatures.
Abstract: Apparatus for measuring a reflected light signal on crystalline or powder samples of thermochromic materials, such as `spin-crossover' materials, has been developed in conjunction with a helium cryostat enabling measurements from cryogenic to ambient temperatures. Depending on the intensity and wavelength of the incident light and optical properties of the sample, reflectivity measurements provide information about a relatively thin surface layer of the investigated compound. The thermal spin transition or photoexcitation from the low-spin state, which is usually coloured, to the high-spin state, which is white or transparent, is detected by a change in the relative intensity as shown on examples of crystalline powders (btr = bis - triazole). To compare the situation in the bulk, simultaneous magnetic measurements were performed.
TL;DR: In this article, photoexcitation of electrons from excited vibrational states of the ground electronic (valence band) state to a nearby excited electronic (SiO2 gap) state is argued that the latter excitation can result in H0 formation at elevated temperatures.
Abstract: The annihilation of mobile protons in thin SiO2 films by capture of ultraviolet-excited electrons has been analyzed for temperatures between 77 and 500 K. We observe a strong increase in proton annihilation with increasing temperature, and derive an activation energy for electron capture of about 0.2 eV. Based on quantum chemical [(OH)3Si]2–O–H+ cluster calculations, we suggest photoexcitation of electrons from excited vibrational states of the ground electronic (valence band) state to a nearby excited electronic (SiO2 gap) state. It is argued that the latter excitation can result in H0 formation at elevated temperatures.
TL;DR: In this paper, the authors measured the excitation photon energy dependence of the resonance Raman spectra of a solvatochromic dye, phenol blue (PB) in various solvents from nonpolar to dipolar ones, including supercritical fluids of trifluoromethane and carbon dioxide.
Abstract: We have measured the excitation photon energy dependence of the resonance Raman spectra of a solvatochromic dye, phenol blue (PB) in various solvents from nonpolar to dipolar ones, including supercritical fluids of trifluoromethane and carbon dioxide. We have found that the band peak positions of the C=N stretching mode of PB appear to change with the excitation laser frequency in polar solvents. On the other hand, no dependence of the band peak positions on the excitation frequency is found in nonpolar solvents. The peak positions of the C=O stretching mode also show small excitation energy dependence in chloroform and in methanol. The extent of the excitation energy dependence is correlated with the amount of the fluctuation of the local field on PB molecule exerted by the solvent, which had been estimated from the bandwidths of the absorption and the Raman spectra of PB in solution. The IR absorption spectra of PB are measured in chloroform and carbon tetrachloride, and compared with the Raman spectra ...
TL;DR: In this article, a polyaniline containing azo groups was synthesized using horseradish peroxidase catalyzed oxidative free radical coupling of 4,4‘-diaminoazobenzene in the presence of hydrogen peroxide.
Abstract: A novel polyaniline containing azo groups has been synthesized using horseradish peroxidase catalyzed oxidative free radical coupling of 4,4‘-diaminoazobenzene in the presence of hydrogen peroxide. The spectral changes observed indicate polymerization accompanied by trans−cis isomerization, leading to structural constraints in the growing polymer chain. This soluble azo polymer consists of azo groups in the main chain as well as in the side chain. Upon photoexcitation of the polymer in solution, the chromophores undergo structural photoisomerization. The polymer relaxes to a conformation having spectral features identical to those of the conformation prior to photoexcitation if the excitation is carried out within the trans absorption band. Photoexcitation at the cis absorption band or thermal kinetic randomization of the chromophore conformation in the polymer in solution results in relaxation of the structural constraints, which leads to a different constrained conformation.
TL;DR: The theory behind the anisotropy of fluorescence has been extended to include the case of residual polarization following energy transfer between fluorophores, and here the theory is further extended to accommodate two-photon excitation.
Abstract: Fluorescence resonance energy transfer (FRET) is a technique now widely applied to probe biological and other complex systems for the determination of fluorophore separation and structure Recently the theory behind the anisotropy of fluorescence has been extended to include the case of a residual polarization following energy transfer between fluorophores, and here the theory is further extended to accommodate two-photon excitation This reveals not only novel polarization characteristics but also a distance dependence whose analysis does not require a priori knowledge of the donor–acceptor spectral overlap The two-photon FRET anisotropy results mirror their one-photon counterparts, in terms of fluorophore separation characteristics and also their relationship to the anisotropies for isolated fluorophores Moreover, the two-photon results are not restricted to a plane polarized input, results being given for both plane and circularly polarized pump radiation
TL;DR: In this article, the authors applied the technique of synchronization between the pulses of a linear dye laser, synchronously pumped by a mode-locked laser and the synchrotron radiation pulses from the SuperACO storage ring in Orsay (France) has been applied to study the photoionization of the Xe * and Xe* excited states in the energy region between the and thresholds.
Abstract: The technique of synchronization between the pulses of a linear dye laser, synchronously pumped by a mode-locked laser, and the synchrotron radiation pulses from the SuperACO storage ring in Orsay (France) has been applied to study the photoionization of the Xe* and Xe* excited states in the energy region between the and thresholds. The experimental spectra are dominated by strong transitions to the Xe* autoionization resonances, which are characterized by typical asymmetric Fano-type line profiles. The energy positions of the n = 4, 7 - 13 resonances have been measured. For the and (n = 7 - 9) resonances, the asymmetry parameter q and the linewidth could be determined upon photoexcitation from the intermediate state 5d and 6d , respectively. As a general behaviour of the investigated excitations, the oscillator strengths for transitions were found to exceed by far those for the, also dipole-allowed, transitions.
TL;DR: In this article, electron spin resonance spectra are reported of fullerene Q60 and C70 thin films after photoexcitation, where the spin-dependent recombination of positive and negative charge carriers (polarons) leads to a decrease in the conductivity under resonance conditions.
Abstract: Electrically detected electron spin resonance spectra are reported of fullerene Q60 and C70 thin films after photoexcitation. The spin-dependent recombination of positive and negative charge carriers (polarons) leads to a decrease in the conductivity under resonance conditions. The spectra recorded are simulated within a radical pair model including different g factors and g anisotropy of the polarons and exchange (J) and dipolar (D) coupling.
TL;DR: In this article, a two-component gas of excitons and biexcitons in a GaAs quantum well was examined and the predicted signature of Bose-Einstein statistics was found: a saturation of the exciton density with a continued growth of the bieciton density as the pair density is increased.
Abstract: Intense photoexcitation of a GaAs quantum well produces a dense gas of free excitons that can pairwise combine to form biexcitons. At sufficiently high density where interparticle spacing is comparable to the thermal deBroglie wavelength, such a two-component gas confined to two-dimensional motion may exhibit quantum-statistical behavior. In this paper, we theoretically show how quantum statistics modifies the equilibrium-density relationship between excitons and biexcitons from that of a classical square law. We also experimentally examine a gas of excitons and biexcitons in GaAs quantum wells and find the predicted signature of Bose-Einstein statistics: a saturation of the exciton density with a continued growth of the biexciton density, as the pair density is increased. For a two-dimensional excitonic gas at a temperature of 5 K inside a 100-\AA{} GaAs quantum well, the calculated pair density at the onset of excitonic saturation is only about $1\ifmmode\times\else\texttimes\fi{}{10}^{11}{\mathrm{cm}}^{\ensuremath{-}2},$ a density readily attained by photoexcitation with 5-ps laser pulses focused to a $3\ensuremath{-}\ensuremath{\mu}\mathrm{m}$ spot. Concurrent with the saturation behavior with increasing density, we observe a gradual broadening and blueshifting of the luminescence peaks, indicating the onset of many-particle effects. Also, the deduced ratio of the total radiative rate of a biexciton to that of an exciton is considerably smaller than what might be expected from simple kinetic arguments. Thus, it seems that a rigorous understanding of the spectral line shapes and luminescence intensities is needed to support our interpretation of Bose-Einstein statistics in this system.
TL;DR: The dynamics of electron−hole (e−h) pairs created by photoexcitation of charge-transfer (CT) states in films and solutions of oligomeric poly-Nepoxypropylcarbazole (PEPCz) doped with electron-acce...
Abstract: The dynamics of electron−hole (e−h) pairs created by photoexcitation of charge-transfer (CT) states in films and solutions of oligomeric poly-N-epoxypropylcarbazole (PEPCz) doped with electron-acce...
TL;DR: In this article, a synchrotron radiation source was utilized for X-ray photoelectron spectroscopic analysis of a nitrobenzaldimine-formed monolayer.
Abstract: A synchrotron radiation source was utilized for X-ray photoelectron spectroscopic analysis of a nitrobenzaldimine-formed monolayer. The N(1s) peak intensity for the nitro group becomes reduced upon X-ray irradiation, while C(1s) and O(1s) peaks are invariant. This observation indicates that the nitro group is cleaved selectively, leaving the phenyl ring intact in the layer. The cleavage rate is measured as a function of photon energy and normalized with the photon flux. The cleavage is first-order to the concentration of the nitro group. The rate constant is independent of the incident photon energy, suggesting that the cleavage is not associated with a direct photoexcitation of atomic core electrons. Electrons ejected by the X-ray are proposed as the most viable cause for the bond cleavage. The molecules remaining on the irradiated surface were analyzed with gas chromatography−mass spectrometry coupled with the solid-phase microextraction method after hydrolyzing the imine bond. It is found that the amou...
TL;DR: In this paper, it was shown that the 1 kq rate of the photoexcitation reaction is 5 orders of magnitude higher than that found for the same abstraction process by the T1(﷿ﷷ�*) state.
Abstract: ) (3.0 ( 0.7) 10 10 s -1 and 1 kq ) (6.0 ( 1.4) 10 10 s -1 , respectively. This 1 kq rate value is 5 orders of magnitude higher than that found for the same abstraction process by the T1(﷿﷿*) state. The mechanism of this surprisingly fast process is discussed on the basis of a comparison of these data and those obtained previously from picosecond transient absorption. Although the reaction is nondiffusional and takes place within H-bonded 4,4′-bipyridine/alcohol complexes, it results from the abstraction of an alkyl hydrogen of the alcohol donor and does not involve the H-bond. It is an example of reaction with a very low structural barrier where the rate is governed in part by the dynamics of reorientation of the alcohol molecules in the solvent cage following the photoexcitation.
TL;DR: It is observed that some amino acid residues have strong interactions with the chromophore, and these residues are conserved in PYPs from three different species of bacteria, indicating the biological importance of these residues.
Abstract: Molecular dynamics simulations were carried out to study what happens in a photoreceptor protein, photoactive yellow protein (PYP), immediately after the vertical transition of the chromophore from the ground to the excited state. A photon absorption simulation was performed to investigate the movement of amino acid residues upon photoexcitation. To calculate the excited state of the chromophore, SCF-CI calculation was carried out with INDO/S Hamiltonian. We observed that some amino acid residues have strong interactions with the chromophore. Most of these amino acid residues are conserved in PYPs from three different species of bacteria. This observation indicates the biological importance of these residues.
TL;DR: In this paper, the S 2p photoabsorption spectrum of gas-phase SO2 was studied at high resolution (ΔE≅30 meV) using synchrotron radiation.
Abstract: The S 2p photoabsorption spectrum of gas-phase SO2 was studied at high resolution (ΔE≅30 meV) using synchrotron radiation. The spectrum contains excitations to both valence orbitals and Rydberg states. The lowest core-to-valence excitation, S 2p−13b1, was found to be fivefold split, and vibrational subbands originating from the symmetric-stretching mode could be resolved. A Franck–Condon analysis of these five states reveals different geometries depending on the symmetry of the total electronic wave function in the final state. In the Rydberg region, both the spin–orbit splitting (≅1.20 eV) and the ligand-field splitting (≅95 meV) were resolved.
TL;DR: In this paper, the authors used time-of-flight mass spectroscopy to study the relaxation dynamics of HCl following photoexcitation in the vicinity of the Cl K edge using monochromatic synchrotron radiation.
Abstract: Time-of-flight mass spectroscopy was used to study the relaxation dynamics of HCl following photoexcitation in the vicinity of the Cl K edge ({approximately}2.8keV) using monochromatic synchrotron radiation. At the lowest resonant excitation to the 6{sigma}{sup {asterisk}} antibonding orbital, almost half of the excited molecules decay by emission of a neutral H atom, mostly in coincidence with a highly charged Cl{sup n+} ion. The present work demonstrates that neutral-atom emission can be a significant decay channel for excited states with very short lifetimes (1 fs). {copyright} {ital 1998} {ital The American Physical Society}