Showing papers on "Photoexcitation published in 1986"
TL;DR: In this paper, the photoluminescence and photoexcitation spectra of ultrasmall structures, referred to as quantum ribbons and quantum disks, were studied and compared.
Abstract: We have studied the photoluminescence and photoexcitation spectra of ultrasmall structures, of approximately 500 A in dimension, which we refer to as quantum ribbons and quantum disks. These are fabricated from GaAs‐AlGaAs quantum wells grown by molecular beam epitaxy and patterned by electron beam lithography. Contrary to our expectation, photoluminescence from these structures is extremely efficient. The excitation spectra of the two types of small structures differ greatly from each other and from that of the as‐grown quantum wells. These differences may be a result of the confinement of the carriers in these small structures.
252 citations
TL;DR: In this article, the vacuum ultraviolet absorption cross sections of SiH4, GeH4 and Si2H6 were reported for the wavelength region 107-220 nm using synchrotron radiation as a light source.
Abstract: The vacuum ultraviolet absorption cross sections of SiH4, GeH4, Si2H6, and Si3H8 are reported for the wavelength region 107–220 nm using synchrotron radiation as a light source. Absorption maxima of these compounds were found at the exciting wavelengths of 115–119 nm. Broad peaks observed were mostly assigned as primarily Rydberg transitions of the σSiH and σSiSi bonding electrons to the 4s, 4p, and 4d orbitals. The absorption features of germane resemble those of monosilane. In the photoexcitation of monosilane, the emission of the SiH(A 2Δ→X 2Π) transition was observed and its onset was found to be 132±2nm. The absorption spectrum of disilane showed five peaks. They were mostly assigned as 2a1g→4s, 2a1g→np(n=4−6) transitions and the strongest band was overlapped by 1eg→4d and 1eu→4p Rydberg transitions. In trisilane molecules three very weak and broad peaks were recognized and assigned as 3b2→4s, 4p and 4a1→4s, 4d Rydberg transitions. The strongest band was tentatively assigned as the superposition of...
247 citations
TL;DR: Absorption en temps resolu de la picoseconde de l'absorption midgap associee a des solitons charges photogeneres dans le trans-PA.
Abstract: Piscosecond time-resolved absorption studies of the midgap absorption associated with photogenerated charged solitons in trans-polyacetylene are reported. Both direct photogeneration of charged-soliton pairs and neutral- to charged-soliton conversion are observed. The former are created by intrachain absorption and decay on a subpicosecond time scale. The latter are formed following interchain photoexcitation and are the dominant population of charged solitons at times greater than 20 ps.
91 citations
TL;DR: In this paper, the photoexcitation of solitons, polarons, and bipolarons leads to relatively large third-order nonlinear optical processes (χ (3) ) on time scales of order 10 −13 s.
65 citations
TL;DR: In this article, photoabsorption and fluorescence cross sections of SiH4 were measured in the 106-160 nm region using synchrotron radiation as a light source, and two states with long vibrational progressions were observed and assigned to the 5s and 6s Rydberg series converging to the first ionization state (2t2)−1.
Abstract: The photoabsorption and fluorescence cross sections of SiH4 were measured in the 106–160 nm region using synchrotron radiation as a light source. Two states with long vibrational progressions in the 106–123 nm region were observed and assigned to the 5s and 6s Rydberg series converging to the first ionization state (2t2)−1. The wavelength thresholds for producing fluorescences from the SiH ( A 2Δ→X 2Π) and Si (4s 1P0→3P2 1D2) transitions were observed at 131.5 and 120.0 nm, respectively. The quantum yields for both fluorescence systems were measured. The photodissociation processes for producing the excited fragments of SiH* ( A ) and Si* are discussed.
58 citations
TL;DR: In this article, the fluorescence quantum yield has been determined from the absorption and fluorescence cross sections measured simultaneously, in which the emitters are attributed to CF+4, CF2, and CF3, respectively.
Abstract: Photoabsorption and fluorescence cross sections of CF4 were measured in the 50–130 nm region using synchrotron radiation as a light source. Oscillator strengths for Rydberg states were determined from the absorption cross sections measured. The fluorescence spectra were dispersed to identify the emission species. Fluorescences appear in the excitation wavelength regions of 50–57.4, 75–80, and 85–95 nm, in which the emitters are attributed to CF+4, CF2, and CF3, respectively. The fluorescence quantum yield has been determined from the absorption and fluorescence cross sections measured simultaneously. The yield has a maximum of 26% at 55 nm.
53 citations
41 citations
TL;DR: In this article, the photodissociation dynamics of (SO2)−2 to form SO2/SO−2 products is investigated over the wavelength range 656 (1.89 eV) to 458 nm (2.71 eV).
Abstract: The photodissociation dynamics of (SO2)−2 to form SO2/SO−2 products is investigated over the wavelength range 656 (1.89 eV) to 458 nm (2.71 eV). Product angular distributions are obtained. An asymmetry parameter analysis indicates the lifetime of the (SO2)−2 photoexcited state is much less than a rotational period. Product kinetic energy distributions are obtained at all wavelengths. Both the overall shape of these distributions and comparison with statistical phase space theory calculations indicate the excited state assessed by the photon is repulsive consistent with the asymmetry parameter analysis. An impulsive model analysis suggests the bond between the two SO2 moeities in (SO2)−2 is probably between one oxygen atom on each moeity and the structure is quasilinear. Structure is also observed in the kinetic energy distributions. It is suggested this is due to selective photoexcitation of vibrational states of either the SO2 or SO−2 moeity in (SO2)−2. Hodges and Vanderhoff have reported a bimodel photo...
35 citations
TL;DR: In this paper, a three-beam signal was observed from the intersection of crossed molecular beams of K and NaCl irradiated by a cw dye laser at wavelengths from 590 and 735 nm.
Abstract: Emission at the Na D lines has been observed from the intersection of crossed molecular beams of K and NaCl irradiated by a cw dye laser at wavelengths from 590 and 735 nm. The three‐beam signal exhibits a threshold near 735 nm and is linearly dependent on the laser and molecular beam intensities. This three‐beam signal has been observed under experimental conditions in which all two‐beam signals are accounted for, and is attributed to the formation of Na* by photoexcitation of the KNaCl reaction complex. The effective two‐body cross section for the process is approximately 10−21 cm2 in a laser field of 1.5 kW/cm2.
33 citations
TL;DR: In this article, a detailed study of the transport in GaAs/Al x Ga 1-x As modulation-doped structures in the low field and high magnetic field quantum limit for varying amounts of parallel conduction in the AlGaAs region is presented.
Abstract: We present a detailed study of the transport in GaAs/ Al x Ga 1-x As modulation-doped structures in the low field and high magnetic field quantum limit for varying amounts of parallel conduction in the AlGaAs region. We observe the apparent breakdown of quantum Hall effect behavior due to low mobility carriers in the parallel channel. The onset of conduction through the parallel channel by quantum transport measurments has been observed, along with a non-linear dose dependence due to photoexcitation.
32 citations
TL;DR: In this paper, it was shown that below-band-gap photoexcitation produces large bleaching of the exciton absorption in GaAs quantum well heterostructures, and this effect was used to perform the first investigation of room-temperature bandtail absorption in these structures.
Abstract: We have discovered that below‐band‐gap photoexcitation produces large bleaching of the exciton absorption in GaAs quantum well heterostructures. We have used this effect to perform the first investigation of room‐temperature bandtail absorption in these structures. We find that the below‐band‐gap absorption follows a spectral Urbach’s rule. In addition, proton‐bombarded samples show an Urbach energy correlated with bombardment‐induced defects. This sensitive technique has enabled us to study samples as thin as 1 μm at energies where the absorption coefficient is ∼10 cm−1.
TL;DR: In this paper, the symmetry breakdown between excitation and S1 origin excited fluorescence is quantified for transitions involving planar vibrational modes, for modes which are totally symmetric, model calculations are performed based on the formalism of Craig and Small for the interference of Condon and Herzberg-Teller derived transition moments.
TL;DR: In this article, the absolute values of the photoionization cross sections of Cr2+ in InP were measured for the first time by deep-level optical spectroscopy, and it was shown that the σ0n cross section exhibits both a resonant and a nonresonant character.
Abstract: We have studied by deep‐level transient spectroscopy and deep‐level optical spectroscopy n‐type chromium‐doped InP. We definitively confirm that the Cr2+/Cr3+ acceptor state is positioned at EC −0.4 eV in InP. We have measured for the first time the absolute values of the photoionization cross sections of Cr2+ in InP by deep‐level optical spectroscopy. The σ0n cross section exhibits both a resonant and a nonresonant character. The former corresponds to the internal transition 5T2→5E of Cr2+, while the latter is attributed to the photoexcitation from the Cr2+ to the conduction band. The threshold of this transition at EC −0.41 eV indicates a very small Franck–Condon shift. The scales of the absolute photoionization values (σ0n) and photoneutralization (σ0p) cross sections towards the conduction and valence bands are the same, which seems to indicate no selection rules.
TL;DR: In this paper, the photoresponse of the passive film formed on iron in neutral borate solution was studied, and a conversion efficiency from the flux of photons absorbed in the film to the photocurrent was calculated as a function of photon energy.
Abstract: Spectroscopic photoresponse of the passive film formed on iron in neutral borate solution was studied. The spectroscopic three parameter reflectometry and the photoelectrochemical technique were employed to obtain the spectra of absorption coefficient and photocurrent. From these spectra, a conversion efficiency, η, from the flux of photons absorbed in the film to the photocurrent was calculated as a function of photon energy. The result shows that the photoexcitation process of electron‐hole‐pair formation includes two transition mechanisms; a direct allowed transition and an indirect allowed transition. The direct transition, which occurs in the photon energy higher than 2.6 eV, is attributed to the excitation between the valence (O‐2p) and the conduction (Fe‐3d) bands of iron oxide, whereas the indirect transition, which occurs in the photon energy higher than 0.8 eV, is associated with electronic levels of impurities or d‐d transition in the passive film. The maximum value of η observed at a photon energy of 3.1 eV is about 20%, and this high efficiency suggests that the high electric field exists in the passive film of iron.
TL;DR: In this paper, a static-exchange approximation of photoexcitation and partial-channel photoionization cross sections of the outer-valence-shell 2π, 1π, 5σ, and 4σ electrons in nitric oxide is presented.
TL;DR: In this paper, the shape of acoustic pulses excited in crystalline silicon during absorption of Nd3+ YAG laser radiation was studied as a function of incident light intensity, and it was revealed that the amplitude of the dilatation wave is saturated while the duration of the compression pulse is shortened.
Abstract: The shape of acoustic pulses excited in crystalline silicon during absorption of Nd3+ YAG laser radiation was studied as a function of incident light intensity. It was revealed that the amplitude of the dilatation wave is saturated while the duration of the compression pulse is shortened. A theoretical model is suggested which explains the above experimental facts by a decrease in the time of Auger recombination for nonequilibrium carriers with an increase in their concentration for higher intensity of the optical excitation. The value of the Auger constant obtained from the experiment is ∼5×10−31 cm6s−1.
TL;DR: In this paper, the S2←← S0 photoexcitation spectrum of jet-cooled acetophenone is presented, and the observed homogeneously broadened linewidths indicate S2 lifetimes ≤ 0.26 ps while the measured emission lifetimes range from 540±30 ns for S2 000 excitation to 130±50 ns for s2 1210 excitation.
Abstract: The S2 ← S0 photoexcitation spectrum of jet‐cooled acetophenone is presented. The observed homogeneously broadened linewidths indicate S2 lifetimes ≤0.26 ps while the measured emission lifetimes range from 540±30 ns for S2 000 excitation to 130±50 ns for S2 1210 excitation. The dynamics revealed by the spectrum are discussed in terms of the known photochemical and photophysical rates of the excited states of acetophenone. Arguments are presented which identify the emitting state as a known triplet state of acetophenone.
TL;DR: In this paper, a photofield emission spectrometer was used to observe departures from free electron behaviour in the total energy distributions of photof-ield emission from the (100, (110), (111), (211), and (510) facets of a tungsten field emitter.
TL;DR: In this article, an In0.16Ga0.84As−GaAs pseudomorphic quantum well heterostructure laser was used for photoexcited electron and hole collection.
Abstract: Efficient collection of photoexcited electrons and holes by thin, strained quantum wells is demonstrated in an In0.16Ga0.84As‐GaAs pseudomorphic quantum well heterostructure laser. The undoped laser structure, which was grown by molecular beam epitaxy, consists of five 50 A pseudomorphic In0.16Ga0.84As quantum wells separated by thick (700 A), unstrained GaAs confining layers. Despite the fact that the quantum wells are undoped, decoupled, and of dimensions which are known to be too small to provide efficient carrier collection in unstrained AlxGa1−xAs‐GaAs structures, 77 K photopumped laser operation is achieved on quantum well transitions (λ∼870 nm) at a threshold excitation intensity of 9.3×103 W/cm2. At photoexcitation intensities even as high as five times the threshold value, spontaneous emission from the quantum well is more than 700 times as intense as the confining layer luminescence. Based upon these photoluminescence results and some simple physical arguments, we suggest that carrier collection...
TL;DR: In this article, a method for determining compensation densities in p-type material by measuring the mobility of photoexcited electrons at low temperatures where most of the holes are frozen out and the total impurity concentration is equal to 2ND was proposed.
Abstract: Due to mixed conduction effects, conventional temperature‐dependent Hall measurements do not yield an accurate characterization of doping levels in high‐purity p‐type HgCdTe (NA≲5×1015 cm−3). In the present work, we report a new and highly promising technique for determining compensation densities in p‐type material by measuring the mobility of photoexcited electrons at low temperatures where most of the holes are frozen out and the total impurity concentration is equal to 2ND. As long as the photoexcitation is sufficiently strong that the electrons dominate the conductivity yet sufficiently weak that scattering by photoexcited holes is negligible, the compensation may be determined directly by fitting a comprehensive theory for transport in a photoexcited narrow gap semiconductor to the measured mobilities. Experimental photo‐Hall results are reported for p‐type Hg1−xCdxTe samples (0.215
TL;DR: X-ray photoabsorption measurements were carried out on krypton near the $1s$ ionization threshold, by use of synchrotron radiation, and support in detail the $Z+1$ approximation for both two- and three-electron excitations.
Abstract: X-ray photoabsorption measurements were carried out on krypton near the $1s$ ionization threshold, by use of synchrotron radiation. A rich, well resolved spectrum of simultaneous two- and three-electron excitations is detected. Feature identification is supported by both relativistic and nonrelativistic calculations. The data support in detail the $Z+1$ approximation for both two- and three-electron excitations. No indication of a preferred Coster-Kronig decay channel for $1s4s$ hole states is found, in contrast with argon. The partial cross sections are in good accord with shakeup theory and photoelectron data on other noble gases.
TL;DR: In this paper, the authors describe ultrashort-cavity semiconductor film lasers made with InP, InGaAsP, and inGaAs have generated picosecond pulses over the wavelength range from 0.77 μm to 1.65 μm after optical pumping by an intense, 0.5 ps pulse at 0.625 μm.
Abstract: Ultrashort-cavity semiconductor film lasers made with InP, InGaAsP, and InGaAs have generated picosecond pulses over the wavelength range from 0.77 μm to 1.65 μm after optical pumping by an intense, 0.5 ps pulse at 0.625 μm. Each material composition operates over an energy range from just below the bandgap to more than 20 percent above the bandgap. For a given composition, pulse duration increases as wavelength increases; for different compositions, pulse durations increase as the bandgap energy decreases. Pulse durations range from 1.4 ps (for InP) to about 30 ps. Peak power is in the range of 1- 10 W. These unique properties of ultrashort-cavity film lasers are due to the combination of bandfilling caused by the intense photoexcitation and the ultrashort cavity, which provides widely spaced resonances to ensure single mode operation and also produces a short photon lifetime necessary for optical gain switching. The film lasers have a uniform, nearly Gaussian, spatial mode.
TL;DR: In this paper, the process of two-photon excitation of atoms is explained by a simple conceptual model which takes into account virtual states of the atom as well as photon statistics.
Abstract: The process of two‐photon excitation of atoms is explained by a simple conceptual model which takes into account virtual states of the atom as well as photon statistics. In addition to predicting the correct dependence of the two‐photon excitation probability on the intensity of incident light, the model also provides an explanation for a number of experimentally observed effects.
TL;DR: In this article, the authors postulate that the precise energy deposition of the incident photon places ions in an energy region dominated by the 100 → 57 and 100 → 71 → 43 fragmentation channels.
TL;DR: In this paper, a two-photon process was used to produce hydrated electrons from aqueous solutions of a binuclear platinum(II) complex, Pt2(POP)4−4.
01 Jan 1986-International Journal of Radiation Applications and Instrumentation. Part A. Applied Radiation and Isotopes
TL;DR: In this paper, the photoexcitation cross section of 115mIn has been measured in the photon energy range of 5 −11 MeV, induced by γ-rays produced from the neutron capture reaction in a tangential beam port in the University of Virginia reactor.
TL;DR: In this article, a set of semi-insulating bulk GaAs samples of various specifications and origins was studied under illumination conditions proper to reveal quenchable paramagnetic signals behaving like the metastable electrical defect EL2.
Abstract: A set of semi‐insulating bulk GaAs samples of various specifications and origins has been studied under illumination conditions proper to reveal quenchable paramagnetic signals behaving like the metastable electrical defect EL2. Whereas In‐alloyed crystals are almost free of any defect, all other materials display photosensitive signals which are either a quenchable quadruplet constrainable to the As4+Ga configuration or two enhanceable singlets not yet fully identified; the preferred occurrence of either type of spectrum is seemingly linked to the growth method.
TL;DR: In this article, the neglect of orbital relaxation in average-atom opacity calculations was investigated for the case of K-shell photoexcitation and photoionization in a chlorine plasma.
Abstract: The neglect of orbital relaxation in average-atom opacity calculations is investigated for the case of K-shell photoexcitation and photoionization in a chlorine plasma.