Showing papers on "Photoexcitation published in 1988"

Photoexcitation of polarons and molecular excitons in emeraldine base.

Abstract: We report the first observation of the photoinduced absorption spectrum of the emeraldine base form of polyaniline. The spectrum contains both subgap photoinduced bleaching at 1.8 eV and photoinduced absorptions at 0.9, 1.4, and 3.0 eV, as well as band-edge bleaching above 3.5 eV. The existence of the subgap bleaching peak is consistent with a model of optically excited localized molecular excitons. The energies of the three photoinduced absorption peaks and the dependence of the peak amplitudes on pump power give evidence for the photoexcitation of polaron pairs that recombine bimolecularly.

Pressure effects on kinetics and decay processes in xenon after selective photoexcitation

Abstract: Kinetics associated with the decay of the low‐lying excited states of xenon is investigated in a density range 2.5×1018 to 2.5×1020 atoms cm−3 (pressure range between 104 and 106 Pa). Three different experiments are performed, i.e., two‐photon excitation of 6p atomic states or 0+g molecular states, VUV one‐photon excitation of the 6s[3/2]1 and 6s’[1/2]1 atomic states, and laser probing of intermediate transient species. The basic experimental data are the time‐resolved second continuum excimer fluorescence at ∼173 nm and the photoionization current. It is found that all excitation decays through 6s[3/2]1 or 6s[3/2]2 with a branching ratio 9/1. Part of the 6s[3/2]1 species (70%) decay subsequently via the 6s[3/2]2 level. The effectiveness of these atomic intermediate steps is directly demonstrated by the transient laser probing. This result, the time analysis, and the discussion allow to set a clear basis to the involved processes and to rule out part of the models proposed in the literature. In particular...

Shape‐resonant and many‐electron effects in the S 2p photoionization of SF6

Abstract: The core‐level photoexcitation and photoionization of SF6 were studied in the vicinity of the resonances below and above the S 2p threshold. The decay channels of the S 2p→6a1g discrete excitation were characterized, with decay leading mostly to valence‐shell satellites. The S 2p continuum data show an oscillatory asymmetry parameter β(S 2p) near threshold that is virtually identical to β(Si 2p) in SiF4. It also resembles—but differs from—theoretical curves for β(S 2p) in atomic sulfur and in SF6. Data at the feature assigned as an eg shape resonance indicate strong multielectron properties for this state, because a resonance in the S 2p satellite is observed at the same photon energy as the main‐line resonance. We propose a unified model which generally includes configuration interaction both in the continuum‐state manifold and between discrete doubly excited states and the continua, to explain this unexpected satellite behavior. Finally, the S(L2,3VV) Auger electron asymmetry parameter shows no signific...

Fluorescence yields from photodissociative excitation of HCOOH, HCOOCH3, and CH3COOH in the vacuum-ultraviolet region

Abstract: The photoexcitation processes of HCOOH, HCOOCH3, and CH3COOH were studied in the vacuum-ultraviolet region by using synchroton radiation and a pulsed discharge lamp as light sources. The absorption and fluorescence cross sections of these molecules were measured in the 106-250-nm region. Fluorescences were detected from photoexcitation of HCOOH and HCOOCH3, but not from CH3COOH. Fluorescence produced at 123.9 nm was dispersed and identified as the excited OH and HCOO radicals. Fluorescence quantum yields of HCOOH and HCOOCH3 increase with decreasing excitation wavelengths with maxima of 5 and 0.3 percent at 106 nm, respectively.

Infrared absorption studies of the divacancy in silicon: New properties of the singly negative charge state.

Abstract: The infrared optical absorption peak at 0.34 eV in silicon, which is usually associated with the singly negative charge state of the divacancy, has been investigated in electron-irradiated samples with use of different optical excitation conditions. It is proposed that a strong Jahn-Teller distortion makes it possible to populate this charge state, when the defect initially is in the neutral charge state, either by the capture of a photoexcited free electron from the conduction band, or by the direct photoexcitation of an electron from the valence band to a defect orbital. Experimental evidence for the existence of these reactions is presented. A defect level at ${E}_{c}$-0.54 eV, frequently associated with the singly negative charge state of the divacancy, is identified as one of the levels from which these photoexcited free electrons originate. The 0.34-eV peak is attributed to an internal transition in the singly negative charge state of the divacancy center, implying the existence of a shallow defect state at approximately ${E}_{c}$-0.07 eV for this charge state. Experimental support is given for the existence of this shallow state. A tentative explanation, based on the strong Jahn-Teller distortion of the singly negative charge state, is suggested for the fact that the doubly negative charge state is not observed at temperatures below 90 K.

Influence of photoexcitation on hopping conduction in neutron- transmutation-doped GaAs

Abstract: The nature of the tunneling‐assisted hopping conduction in neutron‐transmutation‐doped GaAs has been studied under photoexcitation with a photon energy of 1.32 eV. It is found that the dopants activated by annealing around 400 °C provide the electrons to the defect levels originating the hopping conduction even when under photoexcitation. The hopping conduction under photoexcitation is affected by quenching in photoconductance below 120 K concerned with the main electron trap (EL2) and/or the As antisite defect (AsGa) induced by the neutron irradiation. The photoconductance of the samples with a lower radiation damage, AsGa≤1×1018 cm−3, consists of the coexistence of the hopping and band conductions.

Magneto-photogalvanic effects in noncentrosymmetric crystals

Abstract: The theory of magneto-photogalvanic effects in noncentrosymmetric crystals is presented. For linearly-polarized radiation the magnetophotocurrent is caused not only by the photo-Hall mechanism but also by the magnetic-field-induced asymmetry of the photoexcitation rate. The circular magnetophotocurrent can be induced both in optically active and inactive cristals without a symmetry centre. Time-dependent magnetophotocurrents are discussed in connection with the light-induced renormalization of free carrier energy. Finally the results of experimental studies are briefly reviewed.

Gap states in undoped amorphous silicon studied by below-gap modulated photocurrent spectroscopy

Abstract: The energetic distribution and nature of dangling-bond states in undoped a-Si: H have been investigated by a newly developed gap-state spectroscopy which is based on the frequency-resolved spectrum of modulated photocurrent with below-gap photoexcitation. The energy scale is directly specified by the energy of the below-gap light populating the particular gap states. The result has led to the conclusion that the peak of the gap-state distribution associated with doubly occupied dangling bonds (D-) is located about 0•5 eV below the conduction band edge. The analysis for the pre-exponential factor of the thermal emission rate of electron from the D- centre suggests that the emission occurs in two stages through excited states.

Rydberg atoms in parallel magnetic and electric fields. III. Experimental investigations of the Stark structure of the diamagnetic manifold of lithium

Abstract: For pt. II see ibid., vol.21, no.21, p.3499-521 (1988). A high-resolution laser spectroscopy experiment performed on an atomic beam of lithium has allowed the authors to analyse systematically the photoexcitation spectra from the ground state in the presence of parallel electric and magnetic fields. For the strengths studied, the Stark and diamagnetic energies are small compared with the energy separation between states with different principal quantum numbers n. Complete n manifolds have been experimentally investigated and the structure of the energy spectrum has been investigated as a function of the intensity of one of the fields, the other one being kept constant. This experiment provides a confirmation for the existence of the three classes of states predicted from an analysis of the properties of the Hamiltonian of atomic hydrogen.

Photophysical properties of matrix‐isolated Mg2: Evidence for efficient predissociation

Abstract: Optical absorption and excitation profiles recorded for emission of magnesium dimer in argon and krypton matrices allow identification of the A 1Σ+u and B 1Πu states. The radiative lifetime measured for the lowest vibrational levels of the A 1Σ+u state is 1.0 ± 0.25 ns in Ar consistent with a strongly allowed A 1Σ+u–X 1Σ+g transition. Excitation into the higher vibrational levels produces a rise time of 0.5 ± 0.25 ns in the A→X emission, indicating the rate of vibrational relaxation under solid state conditions. Photoexcitation into the higher vibrational levels of the A 1Σ+u state also produces atomic triplet emission by a mechanism of homogeneous predissociation occuring via coupling of this bound, optically active state with the repulsive, optically inactive 3Πu state. The enhancement of predissociation in solid krypton compared with argon is interpreted in terms of the greater overlap of the coupling states in the former solid which is facilitated by the differential shifting of the bound and repulsive dimer states in the two solids. From the observations that the bound A 1Σ+u state is stabilized while predissociation occurs for lower vibrational levels in Kr relative to Ar, it is concluded that the repulsive 3Πu state crosses the bound state on its inner limb.

Orientation‐dependent phase modulation in InGaAs/GaAs multiquantum well waveguides

Abstract: The electro‐optic effect and phase modulation in In0.2 Ga0.8 As/GaAs multiple quantum wells have been experimentally studied for the first time. The experiments were done with 1.06 and 1.15 μm photoexcitation which are, respectively, 25 and 115 meV below the electron–heavy hole excitonic resonance. Strong quadratic electro‐optic effect was observed near the excitonic edge in addition to the linear effect. These are characterized by r63 =−1.85×10−19 m/V and (R33 −R13 )=2.9×10−19 m2 /V2 . In addition, we observe a dispersion in the value of r63 . The relative phase shifts are higher in the strained system at 1.06 μm than in lattice‐matched GaAs/AlGaAs.

Picosecond solvation dynamics of S1trans‐stilbene by nanosecond transient Raman spectroscopy with ‘‘optical depletion timing’’

Abstract: The laser‐power dependent Raman spectral changes observed for S1 trans‐stilbene in hexane are analyzed in terms of the competition between a picosecond solvation process occurring immediately after the S1←S0 photoexcitation (by the pump laser) and the optical depletion of the S1 state caused by the Sn←S1 photoexcitation (by the probe laser). Such spectral changes were not observed in acetonitrile, indicating that the solvation process is absent in a polar solvent. The scheme of ‘‘optical depletion timing’’ is demonstrated to be useful as an effective means to study picosecond phenomena by nanosecond spectroscopic measurements.

Optical nonlinearities in Al(x)Ga(1-x)As/GaAs asymmetric coupled quantum wells.

Abstract: Optical nonlinearties of AlxGa1−xAs/GaAs asymmetric coupled quantum wells within a p-i-n structure have been observed by using time-resolved spectroscopy. Under photoexcitation, exciton absorption lines exhibit spectral shifts of as much as ≈1 meV. The recovery times of these spectral shifts are of the order of hundreds of picoseconds when the excitation photon is above the lowest exciton state but become less than 10 psec when excitation is below the lowest exciton state, indicating a virtual process. The behavior of these spectral shifts is consistent with the presence of a polarization induced by optical pumping. The polarization is believed to be due to the excitons that have nonvanishing electric dipole moments along the axis of the asymmetric coupled quantum wells.

Long-lived cation radical formation of aromatic amines in poly(methyl methacrylate) matrix at room temperature by two-photon excitation

Abstract: Very stable cation radicals of N, N, N′, N′-tetramethyl-p-phenylenediamine and N,N,N′,N′-tetramethylbenzidine were produced in poly(methyl methacrylate) matrix by the intense photoexcitation of pulsed UV lasers or by a focused Xenon lamp. The color of the ion radicals could be recognized even a year after the photoexcitation at room temperature, although the rate of fading was accelerated by raising the temperature to the glass-transition temperature of the polymer (ca. 120 °C). The light intensity dependence of the produced cation radicals proved the two-photonic nature of this photoionization.

Photophysics of atomic magnesium isolated in solid methane and perdeuteromethane. II. Temperature dependence of steady state and time‐resolved luminescence

Abstract: The visible emission resulting from photoexcitation of the first allowed singlet resonance transition of atomic magnesium isolated in the solid methanes is analyzed with spectral and time resolution. Two emission bands are observed in both solid methane (CH4) and solid perdeuteromethane (CD4) at 516 and 560 nm. The measured radiative lifetimes of the two bands are 12 and 19 ms, respectively, and independent of isotopic variation of the host. On the basis of their spectral position and radiative lifetimes, the emission features are assigned to the spin‐forbidden 3P–1S intercombination band of atomic magnesium. Emission spectra recorded at various temperatures in the range 12–33 K were found to show reversible temperature dependence whereby the intensity of the higher energy 516 nm band decreased monotonically with a concomitant increase in that of the lower energy 560 nm band. Time‐resolved measurements of the two bands in Mg/CH4 and Mg/CD4 show that the radiative lifetime of the 516 nm band decreases with...

Nonequilibrium Phonon Effects In The Energy Relaxation Of Hot Carriers In Quantum Wells

Abstract: The energy relaxation of photoexcited hot electrons and holes in quantum well structures has been studied extensively using time resolved photoluminescence techniques. The energy loss rates (ELR) for both types of carrier have been systematically measured using undoped and modulation doped structures for a wide range of well widths. It is now well established that the ELR is reduced in low dimensional structures, especially for intense photoexcitation, and this effect has been explained, at least in part, by invoking the presence of nonequilibrium phonons which are generated in the relaxation process. Our measurements show that the ELR is not a strong function of well width for either low or high excitation densities, although in the former case the electron rate is substantially lower than that for holes. For intermediate excitation densities we find a substantial increase in the ELR both in GaAs and GaInAs structures for decreasing well width. Theoretical calculations of the ELR of electrons and holes have been made using a model in which carriers confined to a single subband interact with bulk optical phonons. In wide wells the wavevector to which the carriers couple lies within the plane of confinement. For narrower wells there is an increased coupling to out of plane modes due to the relaxation of momentum conservation. Under these conditions the carriers then couple to a larger number of phonon modes which reduces the nonequilibrium phonon population.

Electron-electron scattering during femtosecond photoexcitation in quantum wells

Abstract: We model the dynamics of carriers injected into a semiconductor quantum well during femtosecond photoexcitation using an ensemble Monte Carlo simulation which includes two-dimensional electron-electron scattering. The time evolution of the nonequilibrium electron distribution is used to calculate the time dependence of the multi-subband dielectric matrix during the simulation so that transient effects in the screening are incorporated into the inter-carrier scattering rate. Our results show that band filling occurs within 200fs for small injection energies, the rate of which increases with increasing injection density.

X-Ray and Vacuum Ultraviolet Interaction Data Bases, Calculations, and Measurements

Abstract: The papers presented in this volume include: X-ray optical properties: A review of the constraints and the data base; Measurements of X-ray fluorescence yields of plastics by photoexcitation; and The components of the photoionization cross section in the VUV and soft X-ray region.

Bicolor laser photoexcitation of NO in SF6 sensitized system

Abstract: The fluorescence from NO vibronic states exhibited when a pulsed UV laser irradiation is combined with a cw IR laser excitation, has been investigated in an energy range close to the dissociation threshold. The observed formation of the NO D2Σ+ (ν′= 0) Rydberg state in high rotational levels requires the absorption of one UV quantum at λ = 193.3 nm by molecules of NO which previously had an internal energy equivalent to four CO2 laser quanta. Moreover the UV laser beam is used to probe the temperature of the sample pumped by the cw CO2 laser. The population of NO B 2Π(ν′ = 7) valence state is strongly enhanced under bicolor irradiation compared to what occurs with single UV irradiation. From a Boltzmann rotational energy distribution, a temperature of 450 K has been determined in the reactional zone.