Showing papers on "Photoexcitation published in 1992"
TL;DR: The observed photoinduced phase transition was found to be mediated by photogeneration of charge carriers (and not of excitons) and the extremely high efficiency is perhaps due to cooperative interaction between the locally photoconverted domains.
Abstract: It was observed that bi-directional phase changes can be triggered by genuine photoexcitation between two phases with different bond structures in polydiacetylenes The observed photoinduced phase transition was found to be mediated by photogeneration of charge carriers (and not of excitons) The extremely high efficiency is perhaps due to cooperative interaction between the locally photoconverted domains
180 citations
TL;DR: In this paper, the authors show clear and conclusive experimental evidence for the enhancement of superconductivity in GdBa2Cu3Ox and YBa2cu3Ox thin films by photoexcitation.
Abstract: We show clear and conclusive experimental evidence for the enhancement of superconductivity in GdBa2Cu3Ox and YBa2Cu3Ox thin films by photoexcitation. Upon laser illumination the critical temperature increases while the resistivity of the material decreases. The relaxation back to the original state is very slow, of the order of days at room temperature. The existence of this effect opens the possibility of fabricating weak‐link devices with in situ tunable superconducting characteristics.
114 citations
TL;DR: In this article, the authors employed picosecond pump-probe techniques in conjunction with a tandem time-of-flight mass spectrometer to investigate the caging dynamics of photodissociated I2− solvated with a specific number of CO2 molecules.
Abstract: We have employed picosecond pump–probe techniques in conjunction with a tandem time‐of‐flight mass spectrometer to investigate the caging dynamics of photodissociated I2− solvated with a specific number of CO2 molecules. In this paper, we report the observation of a recurrence at ≊2 ps in the I2− absorption recovery, a feature which is attributed to coherent I...I− nuclear motion following I2− photoexcitation.
84 citations
TL;DR: The results show that photogenerated exciton polarons are formed within 300 fs; they subsequently decay in the nanosecond time domain by 1D diffusion toward recombination centers.
Abstract: We have studied the photoexcitation dynamics in electrochemically polymerized polythiophene thin films using transient photomodulation (PM) spectroscopy measured from 100 fs to 20 ms in the spectral range from 0.25 to 2.2 eV, and by the novel technique of absorption-detected magnetic resonance (ADMR). Our results show that photogenerated exciton polarons are formed within 300 fs; they subsequently decay in the nanosecond time domain by 1D diffusion toward recombination centers The dominant excitations after about 250 ns are charged polarons and bipolarons, which are identified by optical transitions correlated with a spin-1/2 ADMR signal at {ital g}=2.003.
78 citations
TL;DR: In this paper, atomic data are used in conjunction with the statistical equilibrium code of Dufton (1977) to calculate relative C III and Si III level populations, and hence emission-line strengths for a range of electron temperatures and densities.
Abstract: Atomic data are used in conjunction with the statistical equilibrium code of Dufton (1977) to calculate relative C III and Si III level populations, and hence emission-line strengths for a range of electron temperatures and densities. It is assumed that photoexcitation and deexcitaton rates are negligible in comparison with the corresponding collisional rates, that ionization to and recombination from other ionic levels are slow compared with bound-bound rates, and that all transitions are optically thin. The observed values of R1 and R2 for several planetary nebulae and a symbiotic star, measured from high-resolution spectra obtained with the IUE satellite, lead to electron densities that are compatible, and are also in good agreement with those deduced from line ratios in other species.
71 citations
TL;DR: In this paper, a transform-limited Raman spectra of S 1 trans-stilbene (S 1 t SB) in heptane was obtained with 3 ps and 2 cm −1 ) picosecond pulses from a high repetition-rate amplified synchronously-pumped dye laser system.
68 citations
TL;DR: An LS coupling R-matrix approach was used in a seventeen target term expansion for Fe I, including 3d7, 3d6(5D)4S, 3ding6(4p)4p and 3d5(6S)4s2 configurations.
Abstract: Photoexcitation and photoionization processes are examined for ground and excited terms of Fe I, Fe II, Fe III and Fe IV. An LS coupling R-matrix approach is used in a seventeen target term expansion for Fe I, including 3d7, 3d6(5D)4S, 3d6(5D)4p and 3d5(6S)4s2 configurations; and sixteen term expansions for Fe II, Fe III and Fe IV, including 3d6, 3d5 and 3d4 configurations respectively. Data for even and odd LS terms having S
59 citations
TL;DR: This result confirms the theoretically predicted relationship between the exciton radiative recombination lifetime and the homogenous linewidth (and coherence area) of theexciton state.
Abstract: Resonant photoexcitation of a predominantly homogeneously broadened exciton line in a GaAs quantum well results in a nonexponential time-resolved photoluminescence decay transient due to the decrease in the exciton-exciton scattering contribution to the homogeneous linewidth, ${\mathrm{\ensuremath{\Gamma}}}_{\mathit{h}}$, as the exciton population decays. This result confirms the theoretically predicted relationship between the exciton radiative recombination lifetime and the homogenous linewidth (and coherence area) of the exciton state.
59 citations
TL;DR: The technique of H(D) atom photofragment translational spectroscopy has been applied to the photodissociation of HCN(DCN) at 121.6 nm and no evidence for any contribution from the product channel yielding H/D atoms in conjunction with ground state CN(X 2∑+) fragments is discerned.
Abstract: The technique of H(D) atom photofragment translational spectroscopy has been applied to the photodissociation of HCN(DCN) at 121.6 nm. Analyses of the H(D) atom time‐of‐flight spectra reveal the partner CN fragment to be formed predominantly in its A 2Π excited electronic state. Branching into the H/D+CN(B 2∑+) product channel accounts for a few percent of the total fragment yield, but we discern no evidence for any contribution from the product channel yielding H/D atoms in conjunction with ground state CN(X 2∑+) fragments. The majority of the CN (A) fragments are formed in their v’=0 level but with a markedly bimodal rotational state population distribution. This bimodality has been rationalized in the light of the available information regarding the form of the potential energy surface of the excited 1Π state of HCN(DCN) populated following photoexcitation at 121.6 nm.
57 citations
TL;DR: An induced absorption on the high-energy side of spectral hole burning in band-to-band transitions in GaAs is observed in the early stage of photoexcitation by an intense femtosecond pump pulse.
Abstract: We report the observation of an induced absorption on the high-energy side of spectral hole burning in band-to-band transitions in GaAs. This effect appears in the early stage of photoexcitation by an intense femtosecond pump pulse. This feature can be explained by edge singularities in the presence of a «nonequilibrium Fermi sea»
51 citations
TL;DR: In this paper, the authors used steady state photoinduced absorption to study the long-lived photoexcitations in the conjugated polymer poly(2-methoxy, 5-(2′-ethylhexyloxy)- p -phenylene vinylene) (MEH-PPV) as a dilute component in ultrahigh molecular weight polyethylene (PE) oriented by gel-processing.
TL;DR: The large number of active electrons in a C 60 cluster and the strong Coulomb interaction among them leads to a rich spectrum of collective states that compare very well with photoexcitation and electron-energy-loss-spectroscopy data on gas targets in the high-energy domain.
Abstract: The large number of active electrons in a C 60 cluster and the strong Coulomb interaction among them leads to a rich spectrum of collective states. States with total angular momentum up to L=8h have a strong collective character. The equivalent of both π and σ plasmons in graphite are predicted. In the low-energy region a strong dynamical screening is obtained, in good agreement with measured absorption spectra. The theoretical results compare very well with photoexcitation and electron-energy-loss-spectroscopy data on gas targets in the high-energy domain
TL;DR: A comparative study of the optical properties and photoexcitations of solid-state and solution samples of poly-3-alkylthiophenes is reported, indicating two different photoexcitation pathways.
Abstract: We report a comparative study of the optical properties and photoexcitations of solid-state and solution samples of poly-3-alkylthiophenes. The \ensuremath{\pi}-electron delocalization and backbone conformational properties are investigated by resonant and preresonant Raman scattering. Photoexcited electronic states are studied by cw photomodulation spectroscopy. The intensity and chopper-frequency dependence of the photoinduced electronic bands indicate two different photoexcitation pathways. An interchain mechanism takes place in the solid while in solution a weak solvent-polymer charge transfer is proposed.
TL;DR: In this paper, a novel instrument is described capable of recording gas electron diffraction (GED) patterns of excited molecular states or transient species with pulsed electron beams, which can be used to determine the structures of reactive species.
Abstract: A novel instrument is described capable of recording gas electron diffraction (GED) patterns of excited molecular states or transient species with pulsed electron beams. The system incorporates (1) a pulsed optical beam for electronic excitation of materials under study, (2) a synchronously pulsed source of 30–50 keV electrons in a space‐charge‐limited beam, (3) necessary vacuum environment and sample‐handling capabilities, and (4) detection and signal processing equipment using an on‐line procedure developed at the University of Arkansas. Data obtained for several test gases demonstrate successful operation of the instrument. The 193 nm laser photofragmentation of carbon disulfide, CS2, is described in detail. In agreement with a recent time‐of‐flight mass spectrometric study of the same process, carbon monosulfide was observed as the reaction product. This study is the first quantitatively successful joint exercise of on‐line multichannel GED data recording and a stroboscopic electron source. The method is expected to be generally useful in determining the structures of excited molecular states or reactive species, and it may enable studies of time‐dependent phenomena. The current time resolution is ∼20 ns.
TL;DR: In this paper, temperature dependent absorption cross-sections for acetone and n-butanone have been measured over the temperature range 260-360 K for 260 nm < λ < 360 nm.
TL;DR: In this article, a new spectral feature is observed in each of the curves near the threshold for the removal of an na1 electron from CH3X (n=4, 6, and 8, respectively, for CH3F, CH3Cl, and CH3Br ).
Abstract: Ion‐pair formation from photoexcited halomethanes, CH3X*→X−+CH+3 (X=F, Cl, Br) has been studied by measuring photodissociation efficiency curves of X− using synchrotron radiation in the 9.9–27.5 eV photon energy range. A new spectral feature is observed in each of the curves near the threshold for the removal of an na1 electron from CH3X (n=4, 6, and 8, respectively, for CH3F, CH3Cl, and CH3Br ). This feature, composed of two or three peaks in each case, is interpreted as resulting from photoexcitation to the Rydberg states converging to CH3X+(C 2A1), which then predissociate into ion pairs through avoided potential energy surface crossings. The interpretation is based on the results of the inner‐shell electron energy loss study by Brion and co‐workers and the photoabsorption study by Hochmann and co‐workers. Peak features are also observed in the X− efficiency curves near the ionization threshold for CH3X+(X 2E). The origins of these peaks are also discussed.
TL;DR: In this article, the ESR spin-trapping technique was used to detect active oxygen radicals by photoexcitation of powdery semiconductors (TiO2, WO3, CdS, and Fe2O3) in water and aqueous H2O2 solutions using 5,5-dimethyl-1 pyrroline N-oxide (DMPO) as spin trap.
Abstract: The ESR spin-trapping technique was used to detect active oxygen radicals by photoexcitation of powdery semiconductors (TiO2, WO3, CdS, and Fe2O3) in water and aqueous H2O2 solutions using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as spin trap. Superoxide ion (O2−), hydroperoxyl radical (·O2H) and hydroxyl radical (·OH) were detected under photoexcitation. The mechanism for the generation of active oxygen radicals is discussed referring to the energy band structures of the semiconductors and the redox potentials of the oxygen radicals.
TL;DR: In this article, the angular distribution parameters of the N5-02,3O2,3 Auger lines have been measured between the 4d5/2 and 4d3/2 thresholds of Xe using the CIS technique.
Abstract: The angular distribution parameters beta of the N5-02,3O2,3 Auger lines has been measured between the 4d5/2 and the 4d3/2 thresholds of Xe using the CIS technique. The radiationless decay parameter alpha 2 for each of the N5-O2,3O2.3 Auger lines and the alignment parameter A20 for the intermediate ion were also determined. Comparisons with previous values of beta and alpha 2 for the N5-O2.3O2.3 Auger lines have been made. In addition, shakeoff to the various states of the Xe2+ 5p4 ion has been measured following 4d photoexcitation and found to be small. These results support the contention that near-zero kinetic energy electrons observed following similar photoexcitation in the rare gases result primarily from a two-step process rather than the direct process of shakeoff.
TL;DR: In this paper, the mass and kinetic energy resolved cluster ions were photodissociated in the entrance to a time-of-flight (TOF) device of variable length and the subsequent deflection of all ions allowed for time resolved measurements to be undertaken on the neutral photofragments.
Abstract: Following the photoexcitation of argon cluster ions, Ar+n for n in the range 4–25, kinetic energy release measurements have been undertaken on the fragments using two quite separate techniques. For Ar+4–Ar+6, fragment ion kinetic energy spectra were recorded at 532 nm in a crossed beam apparatus as a function of the angle of polarization of the laser radiation with respect to the incident ion beam. Only Ar+ from Ar+4 was observed to exhibit a polarization dependence together with a comparatively high kinetic energy release. The principal fragment ion Ar+2 was found both to emerge with a low kinetic energy release and to display no dependence on the angle of polarization of the radiation. In a second series of experiments, mass and kinetic energy resolved cluster ions were photodissociated in the entrance to a time‐of‐flight (TOF) device of variable length. The subsequent deflection of all ions allowed for time resolved measurements to be undertaken on the neutral photofragments. Following the absorption o...
TL;DR: Since the quantum yields for photoisomerization and luminescence of bilirubin bound to serum albumin at room temperature are both low, internal conversion processes, rather than Z→E configurational isomerizations, are probably the major pathways for deactivation of photo‐excited bilirube.
Abstract: The quantum yield, phi ZE, for configurational photoisomerization (4Z,15Z----4Z,15E) of bilirubin bound non-covalently to human serum albumin was determined (at 23 +/- 2 degrees C) by laser excitation and chromatographic analysis of products. Values obtained for photoexcitation at 465 nm were about one-half those previously reported. The quantum yield was dependent on excitation wavelength, decreasing from a value of 0.109 +/- 0.010 for excitation at 457.9 nm to a value of 0.054 +/- 0.005 for excitation at 514.5 nm. The wavelength dependence is consistent with rapid transfer of excitation energy between the two non-identical pyrromethenone chromophores of bilirubin in the singlet excited state. Since the quantum yields for photoisomerization and luminescence of bilirubin bound to serum albumin at room temperature are both low, internal conversion processes, rather than Z----E configurational isomerizations, are probably the major pathways for deactivation of photo-excited bilirubin.
TL;DR: Ab initio studies of the 2p(3a 1 )-6a 1 and 2p-3b 2 photoexcitation resonances of H 2 S have been carried out in order to support interpretations of recent photoelectron and autoionization spectra using synchrotron-radiation excitation.
Abstract: Ab initio studies of the 2p(3a 1 )-6a 1 and 2p(3a 1 )-3b 2 photoexcitation resonances of H 2 S have been carried out in order to support interpretations of recent photoelectron and autoionization spectra using synchrotron-radiation excitation. The full H 2 S * S+H 2 and H 2 S * S+H+H potential-energy surfaces as well as the resonant energies for autoionization (Auger) decay of core-excited states in the fragment HS species have been obtained by multiconfiguration self-consistent-field calculations
TL;DR: Theoretical and experimental results for the 257 nm photolysis of methyl iodide adsorbed on an MgO(100) crystal were compared in this article, where the I*/I branching ratio and the velocity and angular distributions of both photofragments were predicted by the theory.
Abstract: Theoretical and experimental results are compared for the 257 nm photolysis of methyl iodide adsorbed on an MgO(100) crystal. Molecular‐dynamics calculations treat CH3I as a pseudodiatomic molecule and describe the geometry and the vibrational and librational frequencies of ground state CH3I on the surface of a solid at 125 K. The simulations modeled the photodissociation dynamics of the adsorbed species. The photoexcitation of CH3I at 257 nm is to the 3Q0 state which is, in turn, coupled to the 1Q1 state. The electronic surface coupling allows for two dissociation pathways, producing either ground‐ or excited‐state iodine atoms in concert with ground‐state methyl radicals. The I*/I branching ratio and the velocity and angular distributions of both photofragments are predicted by the theory. A comparison is made between these predictions and experimental observation of the I*/I branching ratio, the velocity distribution of the methyl fragment, and the internal state distribution of the methyl. A substanti...
TL;DR: The photo-induced absorption spectra of conjugated polymers poly[3-methoxy-6-(2-ethylhexyloxy) phenylene vinylene] (MEH-PPV) and poly[2,5-thienylene vinylenes] (PTV) have been measured using 100 fs laser pulses and white-light continuum generation as mentioned in this paper.
TL;DR: In this paper, a spontaneous ion pairing between soluble anionic porphyrins (TPPS and Up) and cationic bipyridinium salts in water is described.
Abstract: Spontaneous ion pairing occurs between soluble anionic porphyrins (TPPS and Up) and cationic bipyridinium salts in water. After photoexcitation (532 nm) of the porphyrin component, nonfluorescent ion pairs are formed by fast electron transfer (<100 fs) from the porphyrin donor to the bipyridinium acceptor. The system returns to the ground state by reverse electron transfer without dissociation into separated radical ions. The reverse electron transfer reactions occur over hundreds of picoseconds, and the rate constants are shown to form a homogeneous set of data which demonstrate an inverted region in the driving force range 0.8-1.7 eV. These results are discussed in terms of adiabatic and nonadiabatic electron-transfer theory. 24 refs., 2 figs.
TL;DR: In this paper, it was shown that hole centers produced by charge-transfer photoexcitation in W 10 O 32 4- abstract H from alkanes as do O -in numerous solid-state and molecular analogues.
Abstract: O - hole centers produced by charge-transfer photoexcitation in W 10 O 32 4- abstract H from alkanes as do O - in numerous solid-state and molecular analogues. Upon protonation, forming H 2 W 10 O 32 4- this activity increases, and this has been modeled with atom superposition and electron delocalization molecular orbital (ASED-MO) calculations by shifting the polyopxometalate valence bands by an amount suggested by the observed decrease in first reduction potential
TL;DR: In this article, photoexcitation of the Kr 3d to np resonances and photoionization of the 3d inner shell have been studied by measuring the emitted electrons with energies from 0 to 3 eV and from0 to 12 eV, respectively.
Abstract: Photoexcitation of the Kr 3d to np resonances and photoionization of the 3d inner shell have been studied by measuring the emitted electrons with energies from 0 to 3 eV and from 0 to 12 eV, respectively. Intense discrete structure due to the Kr+ to Kr2+ transitions has been observed in photoelectron spectra. Assignments are proposed based on the spectator shake model and by analysis of effective quantum numbers of proposed Kr+ states. The observed energy shift and asymmetric line broadening, related to the post-collision interaction, of the Kr+ 3d-1(2D512.3) ionic states have been compared with previous experimental results and with theoretical predictions.
TL;DR: Ionic fragments of BF 3 produced by the B 1sa 2 photoexcitation have been mass analyzed in coincidence with the resonance spectator B KVV Auger electrons.
Abstract: Ionic fragments of BF 3 produced by the B 1sa 2 photoexcitation have been mass analyzed in coincidence with the resonance spectator B KVV Auger electrons. Dissociation from the BF 3 + states with one outer-valence hole produces only BF 2 + , while dissociation from the BF 3 + states with two outer-valence holes and one electron excited to the a 2 orbital produces predominantly B +
TL;DR: In this paper, a new phenomenon has been observed in solid-state photoemission with circularly polarized light, which is similar to the circular dichroism in photoabsorption.
Abstract: A new phenomenon has been observed in solid-state photoemission with circularly polarized light. Its outward appearance is similar to the circular dichroism in photoabsorption. For special observation geometries, large photoelectron intensity asymmetries arise upon reversal of the photon helicity. The effect is not based on spin-orbit interaction, as is demonstrated by the example of photoemission from graphite (Z = 6). Similar asymmetries are expected for all single-crystal solids thus opening up a new class of experiments providing new tests for photoemission theories. Detailed information on the dynamics of photoexcitation of a solid can be gained, as discussed in terms of a simple orbital model.
TL;DR: The ground and excited-state reactions of matrix-isolated gallium atoms with methane have been investigated by UV-visible and infrared spectroscopies as mentioned in this paper, but no evidence was found for a ground-state reaction.
Abstract: The ground- and excited-state reactions of matrix-isolated gallium atoms with methane have been investigated by UV-visible and infrared spectroscopies. Gallium atoms were co-condensed with methane-doped argon (1:10 CH 4 /Ar) at 12 K. No evidence was found for a ground-state reaction. Gallium atom photoexcitation at 354 nm ( 2 S→ 2 P) resulted in rapid disappearance of all Ga UV absorptions, and the appearance of IR absorptions, which are attributed to methylgallium hydride
TL;DR: In this paper, the green-emission at 520 nm-wavelength has been observed in photoluminescence of Ga2S3 crystals prepared by the iodine-transport CVD method.