Showing papers on "Photoexcitation published in 1995"

Coherent Laser Control of the Product Distribution Obtained in the Photoexcitation of HI

Abstract: Active control of the distribution of products of a chemical reaction was demonstrated by using a method based on the principle of quantum mechanical interference. Hydrogen iodide (HI) molecules were simultaneously excited above their ionization threshold by two competing pathways. These paths were absorption of three ultraviolet photons of frequency ω 1 and one vacuum ultraviolet photon of frequency ω 3 = 3ω 1 . The HI + and I + signals were modulated as the phase between the lasers was varied, with the HI + signal lagging by 150° ± 15°. A mechanism consisting of autoionization and predissociation is proposed.

Dipole‐bound excited states of the I−⋅CH3CN and I−⋅(CH3CN)2 ion–molecule complexes: Evidence for asymmetric solvation

Abstract: Dipole‐bound excited states are reported for the I−⋅CH3CN and I−⋅(CH3CN)2 cluster ions, located just below their vertical electron detachment energies (determined using negative ion photoelectron spectroscopy). The absorption cross sections for excitation to these states are observed to increase with increasing dipole moments of the solvent molecules in the I−⋅M series (M=methyl iodide, acetone, acetonitrile). Photoexcitation at the peak of the transition to the dipole‐bound state results exclusively in the dipole‐bound fragment ion, M−. The photoelectron spectrum of the CH3CN− fragment was also recorded by sequential two‐photon absorption in the I−⋅CH3CN parent, indicating that the excess electron is indeed weakly bound (≤10 meV) with very little intramolecular distortion evident upon electron detachment. The I−⋅(CH3CN)2 cluster displays two absorption bands, one below each of the two features in the photoelectron spectrum. The most intense band correlates with the weaker, lower binding energy photoelectron band. Excitation of this strong absorption band results in production of both CH3CN− and (CH3CN)−2 fragments, while excitation of the weaker absorption band only results in electron detachment. We interpret these results in the context of two structural isomers for I−⋅(CH3CN)2: one with the solvent molecules surrounding the I− and another with both solvent molecules on the same side, asymmetrically solvating I− in a configuration with a large electric dipole moment in the neutral cluster over this anionic geometry.

Photodesorption of neutrals from metal surfaces: a wave packet study

Abstract: By solving the time-dependent Schrodinger equation for representative wave packets, the desorption dynamics of neutrals from metal surfaces following photoexcitation is studied. Computational parameters are chosen to resemble NO/Pt(111). Three different one-dimensional models of increasing complexity are employed to elucidate (a) basic dynamical features, (b) the role of dissipative boundary conditions and (c) non-phenomenological quenching rates. Among the computed quantities are desorption probabilities, density and flux time-of-flight spectra, kinetic energies of the desorbing particles, and “snapshots” of the wave packets in configuration and momentum space. Besides others, effects of the initial vibrational excitation of the molecule-surface bond by temperature or by IR laser pulses, and of coordinate-dependent resonance lifetimes are discussed.

Use of flavin photochemistry to probe intraprotein and interprotein electron transfer mechanisms

Abstract: Photoexcitation of flavin analogs generates the lowest triplet state (via intersystem crossing from the first excited singlet state) in the nanosecond time domain and with high quantum efficiency. The triplet, being a strong oxidant, can abstract a hydrogen atom (or an electron) from a reduced donor in a diffusion-controlled reaction. If the donor is a redox protein, the oxidation process can be used to initiate an electron transfer sequence involving either intramolecular or intermolecular reactions. If the donor is an organic compound such as EDTA, the neutral flavin semiquinone will be produced by H atom abstraction; this is a strong reductant and can subsequently transfer a hydrogen atom (or an electron) to an oxidized redox protein, thereby again initiating a sequence of intramolecular or intermolecular processes. If flavin photoexcitation is accomplished using a pulsed laser light source, the initiation of these protein electron transfer reactions can be made to occur in the nanosecond to microsecond time domain, and the sequence of events can be followed by time-resolved spectrophotometry to obtain rate constants and thus mechanistic information. The present paper describes this technology, and selected examples of its use in the investigation of redox protein mechanisms are given.

Photoinduced electron transfer processes in oligothiophene/C60 composite films

Abstract: We present near steady‐state photoinduced absorption (PIA) spectroscopy and steady‐state light‐induced electron spin resonance (LESR) studies on photoinduced electron transfer reactions in composite films of well defined α‐oligothiophenes (Tn, n=6, 7, 9, and 11) as electron donor with buckminsterfullerene (C60) and a methanofullerene derivative (1‐(3‐cholestanoxycarbonyl)‐ propyl‐1‐phenyl‐[6.6]C61) as electron acceptors. Dispersion on a molecular level of these fullerenes in oligothiophene films causes quenching of both the photoluminescence and intersystem crossing after photoexcitation across the π–π* energy gap as a result of a fast electron transfer reaction. The PIA spectra exhibit bands due to T+•n radical cations, fullerene radical anions and electroabsorption oscillations of the π–π* band edge resulting from strong local electric fields set up by the photoinduced charges. LESR spectra give additional unambiguous evidence of the photoinduced electron transfer reaction. Using PIA and LESR, we do not...

In situ fabricated ga2o3-gaas structures with low interface recombination velocity

Abstract: Amorphous Ga2O3 films have been deposited in situ on (100) GaAs layers grown by molecular beam epitaxy in ultrahigh vacuum. The Ga2O3–GaAs interface is stable during photoexcitation and the photoluminescence (PL) intensity, measured at 514.5 nm excitation wavelength, is enhanced drastically by a factor of 420 as compared to a corresponding bare GaAs surface. The Ga2O3–GaAs interface recombination velocity derived from a modified dead layer model is below 104 cm/s. Furthermore, the PL intensity of Ga2O3–GaAs structures approaches that of a very low interface state density (2×109 eV−1 cm−2) AlGaAs–GaAs reference structure.

Photorefractive Quantum Wells and Thin Films

Abstract: The photorefractive effect consists of three roughly separate physical processes: photoexcitation of charge carriers; electronic transport and trapping; and an electrooptic effect. Of these three processes, the first and the last explicitly involve the interaction of light with matter. One way to improve photorefractive performance is to enhance the optical interactions, i.e., by operating near an optical resonance of the material. This enhances both the photogeneration of charge carriers and the electrooptic effect. The electro-optic effect is one limit of an optical nonlinearity, and nonlinearities are strongest near a resonance.

The neutral oxygen spectrum. 2: Pumping by hydrogen Lyman-beta under the optically thin condition: A first application to the classical novae

Abstract: A calculation, employing a detailed model of neutral oxygen, is carried out to give fluorescent line intensities expected in a long-proposed photoexcitation by accidental resonance (PAR) process in which hydrogen Lyman-beta photoexcites the oxygen spectrum. The results pertain to the optically thin case but provide an upper limit to the fluorescent intensities which can be attained. They are applied to analyze line ratios involving the strong 8446 A line observed in classical novae during the diffusion-enhanced and Orion phases. Operation of the PAR process in the novae is verified. It is found that photoexcitation rates in the ejecta reach values greater than 0.1/sec, corresponding to hydrogen Lyman-beta radiation field intensities greater than 1250 ergs/cm/sec/sr.

Time‐delayed luminescence from oxidized porous silicon after ultraviolet excitation

Abstract: Oxidized porous silicon is known to luminesce efficiently and with a ns‐fast response in the green‐blue region of the spectrum (2.3–2.7 eV) at room temperature. For microporous Si processed by rapid thermal oxidation at 1100–1200 °C in a dry oxygen ambient, we observe that the blue light has a time‐delayed component that indicates carrier trapping. In addition to the well‐established ultrafast light (∼1 ns) a signal with a time delay of order 1 s is present under UV photoexcitation. The time‐delayed blue light exists at low temperature and has an excitation onset edge at 4.3±0.1 eV, an energy that is usually associated with a band discontinuity at the Si–SiO2 interface.

Photoluminescence analysis of Er‐doped GaAs under host photoexcitation and direct intra‐4f‐shell photoexcitation

Abstract: Er‐doped GaAs shows a sharp and simple intra‐4f‐shell luminescence spectrum of Er3+ ions under host photoexcitation, when the crystal is grown with deliberate oxygen codoping by metalorganic chemical‐vapor deposition. Photoluminescence‐excitation measurements by direct intra‐4f‐shell excitation, however, revealed that there are many kinds of Er3+ centers in the same crystal. To clarify the differences between these Er centers, photoluminescence was measured under both host photoexcitation and direct intra‐4f‐shell photoexcitation. It was found that there are three types of Er centers with distinctly different characteristics. The first type of Er center shows a sharp and simple photoluminescence spectrum with a high intensity under host photoexcitation. This center had been assigned as an Er atom coupled with two oxygen atoms (ErGa–2O center). The second type of Er center show sharp and simple spectra but only under direct intra‐4f‐shell photoexcitation and not under host photoexcitation. In a sample with Er concentration of 1.4×1018 cm−3, several such centers with distinctly different atomic configurations were found at substantial concentrations, probably exceeding that of the ErGa–2O center. As there is no intra‐4f‐shell luminescence for this type of center under host photoexcitation, no energy transfer path exists between the host and the 4f shells of this type of Er center. The third type of Er center shows complicated spectra even under intra‐4f‐shell photoexcitation with a specific photon energy. Analysis of photoluminescence spectra from this type of Er center revealed evidence of energy migration among Er3+ ions, suggesting that Er‐rich aggregates are the origin of this type of PL spectra. Under host photoexcitation, this type of Er center shows luminescence but with a much lower intensity than the ErGa–2O center. The luminescence from Er centers similar to the third type probably is dominant in the PL spectrum of Er‐doped GaAs grown without deliberate oxygen codoping, which does not contain an appreciable concentration of the ErGa–2O centers.Er‐doped GaAs shows a sharp and simple intra‐4f‐shell luminescence spectrum of Er3+ ions under host photoexcitation, when the crystal is grown with deliberate oxygen codoping by metalorganic chemical‐vapor deposition. Photoluminescence‐excitation measurements by direct intra‐4f‐shell excitation, however, revealed that there are many kinds of Er3+ centers in the same crystal. To clarify the differences between these Er centers, photoluminescence was measured under both host photoexcitation and direct intra‐4f‐shell photoexcitation. It was found that there are three types of Er centers with distinctly different characteristics. The first type of Er center shows a sharp and simple photoluminescence spectrum with a high intensity under host photoexcitation. This center had been assigned as an Er atom coupled with two oxygen atoms (ErGa–2O center). The second type of Er center show sharp and simple spectra but only under direct intra‐4f‐shell photoexcitation and not under host photoexcitation. In a sample with...

Dark current mechanisms and conditions of background radiation limitation of n‐doped AlGaAs/GaAs quantum‐well infrared detectors

Abstract: The most important mechanism of dark current in n‐doped quantum‐well infrared photodetectors (QWIPs) is due to interaction of electrons with longitudinal optical phonons. Theoretical expressions are derived for the carrier lifetime, and for generation currents originating from both photoexcitation as well as from thermal excitation in a single quantum well. Detector gain is discussed briefly. Calculated values of thermal generation currents and the ratio of photocurrent to thermal current are found to accord well with experimental data. Finally the conditions of background radiation limitation of QWIPs are investigated and the theory gives a temperature of background radiation limitation TBLIP=81 K for a 9 μm cutoff detector with a two‐dimensional grating and optical cavity, for 300 K background temperature, optics f number=1 with 100% optical transmission, provided that a photocurrent to dark current ratio of 1 criterion is used.

Excited states and electron transfer reactions of fullerenes

Abstract: Since the discovery of carbon clusters in the laser ablated graphite vapours, fullerenes C60 and C70 have provided a constant source of excitement for the experimental and theoretical scientists alike. Their brilliant colours in solutions and three dimensional aromaticity has fascinated the photochemists. Our group has recently characterised singlet and triplet states of fullerenes using complementary twin techniques of laser fAash phQtolysis and electron pulse radiolysis. Singlet - singlet (S l--->S ) absorption spectra were studied in picosecond time domain and the rates of intersystem crossing were established. Triplet states were produced directly by photoexcitation in laser flash photolysis experimenp and *by energy transfer in pulse radiolysis. Triplet-triplet (T l---->T ) n spectra were characterised and rate constants for various triplet decay channels were measured. Fullerenes have low reduction potentials and form charge transfer (CT) complexes with good donors like aliphatic and aromatic amines. Exciting photochemical behaviour was observed on photoexcitation of the CT complexes which undergo ultrafast charge separation (CS) and charge recombination (CR) reactions. The dynamics of CS and CR reactions was studied in picosecond time scales in Cs0-diphenylarnine and C60-triphenylamine systems. Using external heavy atom effect and solvent polarity variations, various steps in the CS and CR processes generating fullerene triplet on CR were elucidated. Recent results of complexing fullerenes with suitable hydrophilic cage like r-cyclodextrin (CD) making C60 water soluble will also be presented. Excited state behaviour of fullerene complex in CD cage would be discussed. n

Site‐specific fragmentation following Si:2p core‐level photoexcitation of F3SiCH2Si(CH3)3 in the vapor phase

Abstract: Site‐specific fragmentation following Si:2p photoexcitation of (trifluorosilyl) (trimethylsilyl)methane [F3SiCH2Si(CH3)3 (FSMSM)] has been studied by means of photoelectron‐photoion and photoion–photoion coincidence techniques. The total photoionization efficiency curve of FSMSM has only one broad peak near the Si:2p core‐ionization threshold and no evidence is obtained for the occurrence of selective excitation of each of the two Si atoms. From the results of ab initio calculations of FSMSM, it is found that the magnitude of the Coulomb interaction between the valence electrons and the Si:2p core electrons is very similar at the two Si sites. As a result, the difference in the chemical shift of the 2p core level between the two Si sites is very small. However, the evidence for site‐specific fragmentation of the molecule following the Si:2p core‐level photoexcitation is obtained by analyzing the photoion–photoion coincidence spectra; the relative yields of H+–SiF+ and CH+n–SiF+3 (n=1–3) ion pairs are enha...

Trapping of photocarriers in Ga‐doped Bi12GeO20 at 80 K

Abstract: Data from measurements of optical absorption, photoconductivity, dark conductivity, thermally stimulated conductivity (TSC), and thermoluminescence (TL) on samples of undoped and Ga‐doped, Czochralski‐grown Bi12GeO20 single crystals are reported. The photoconductivity is n type, and the dark conductivity is p type. Undoped BGO exhibits a broad, band‐edge absorption due to the optical excitation of electrons to the conduction band which gives the samples a yellow coloration. This absorption is reduced by the addition of Ga which acts as a compensating acceptor. When illuminated with light into this absorption band, but with photons of energy less than the band gap, photoexcitation of electrons occurs. These become trapped, inducing additional absorption and photoconductivity bands and TSC signals, but not TL. Excitation with photons of energy greater than the band gap induces both TSC and TL. Examination of the TSC and TL signals as a function of excitation wavelength allows the distinction between electro...

Calculations of femtosecond differential optical transmission in germanium

Abstract: Nonequilibrium electron and hole carrier dynamics are calculated for femtosecond photoexcitation in Ge using an ensemble Monte Carlo method. From the carrier distributions and k⋅p band‐structure calculations, the time‐dependent differential optical transmission that corresponds to pump‐probe experiments is determined. It is found that electrons quickly scatter out of the optically coupled region and that the primary electron relaxation channel is from the Γ valley to the X valleys, then to the L valleys. Holes dominate the nonlinear absorption spectra in Ge. It is also found that light holes make a substantial contribution to the initial transient of the differential transmission, particularly for high‐energy photoexcitation.

Electron Spin Dynamics of Photoexcited Corannulene-Lithium Complexes in Tetrahydrofuran. Fourier Transform Electron Paramagnetic Resonance

Abstract: Tetrahydrofuran (THF) solutions of corannulene (Cor) reduced by lithium metal exhibit electron paramagnetic resonance (EPR) features that depend on the reduction stage of Cor, the temperature, and the nuclear spin (isotope effect). Photoexcitation of these solutions results in EPR emissive spectra, attributed to the trianion, Cop-, and to the photoelectron, e-photo. These electron spin polarized (ESP) effects are discussed within the framework of intramolecular electron transfer reactions of ion complexes of highly charged constituents, in conjunction with ESP mechanisms that take into account radical-triplet interactions.

Photoinduced localized magnetic polaron and luminescence in eu chalcogenides, especially in eute

Abstract: There has been a controversy over the photoexcited electron state in Eu chalcogenides. This paper examines whether the photoexcited electron in Eu chalcogenides, especially in EuTe, is the self-trapped magnetic polaron (STMP) or the photoinduced localized magnetic polaron (PILMP) bound by the 4f hole produced in the photoexcitation process. The theoretical analysis has been performed for the recent systematic spectroscopic study of Eu chalcogenides [J. Phys. Soc. Jpn. 63, 4616 (1994)]. This paper shows that the photoexcited electron in EuTe, associated with the photoluminescence band at 1.47 eV, forms the PILMP state, which brings about the large lattice displacement around the 4f hole through the 4f-hole--optical-phonon interaction, producing a large Stokes shift. The observed value of the Stokes shift is never explained by the STMP model. It is revealed that the attractive Coulomb interaction between the photoexcited electron and the 4f hole is not weak, which supports the magnetic exciton model in the absorption process. The luminescence band at 1.47 eV in EuSe is understood by the same mechanism. The magnetic polaron effect in EuSe is quite large compared with that in EuTe.

Photoluminescence peak energy evolution for porous silicon during photo-oxidation and gamma -ray oxidation

Abstract: The evolution of photoluminescence (PL) from a series of as-anodized porous silicon (PS) samples with peaks in a wide energy range has been investigated systematically during oxidation enhanced by either laser illumination or gamma -ray irradiation. Under both oxidation conditions, PL spectra initially located in the infrared-red region undergo a blue shift, while those initially located in the orange-yellow region experience a red shift. Eventually, all the peaks are pinned at fixed energies which are in an energy range narrower than that for as-anodized PS samples. These experimental results can be explained by the quantum confinement-luminescence centres model, which argues that photoexcitation proceeds in nanoscale silicon and photoemission through the luminescence centres outside nanoscale silicon, and also by assuming that during oxidation the main luminescence centres change from one kind to another and/or the relative contributions to the PL intensity from various kinds of luminescence centres vary.