Showing papers on "Photoexcitation published in 1981"

Theory of the bulk photovoltaic effect in pure crystals

Abstract: A theory is presented for the intrinisic anomalous bulk photovoltaic effect observed in noncentrosymmetric crystals, e.g., BaTi${\mathrm{O}}_{3}$. An exact formula is derived for the calculation of the short-circuit photovoltaic current in a pure crystal in terms of its Bloch states and energy bands. Unlike a conventional field or diffusion current, the photovoltaic current is essentially determined by the change of wave functions upon photoexcitation of an electron from the valence to the conduction band. Our theory also reveals that the bulk photovoltaic effect can occur even in pure nonpyroelectric piezoelectric crystals, e.g., Te and GaP, which have no polar axis and therefore no a priori direction for the photovoltaic current.

Photoexcitation and ionization in carbon dioxide: Theoretical studies in the separated-channel static-exchange approximation

Abstract: Theoretical studies are reported of total and partial-channel photoexcitation and ionization cross sections in carbon dioxide. As in previously reported studies of discrete and continuum dipole spectra in diatomic (N2, CO, O2, F2) and polyatomic (H2O, H2CO, O3) molecules in this series, separated-channel static-exchange calculations of vertical-electronic transition energies and oscillator strengths and Stieltjes-Tchebycheff moment methods are employed in the development. Detailed comparisons are made of the static-exchange excitation and ionization spectra with photoabsorption, electron-impact excitation, and quantum-defect estimates of discrete transition energies and intensities, and with partial-channel photoionization cross sections obtained from fluorescence measurements and from tunable-source and (e,2e) photoelectron spectroscopy. The spectral characteristics of the various discrete series and continua are interpreted in terms of contributions from compact 2πu(π*), 5σg(σ*), and 4σu(σ*) virtual valence orbitals, and from more diffuse discrete and continuum Rydberg orbitals. The 2πu(π*) orbital is found to contribute to discrete excitation series, whereas the 5σg (σ*) and 4σu (σ*) orbitals generally appear in the photoionization continua as resonance-like diabatic valence features. Good agreement obtains between the calculated discrete excitation series and the results of a recent analysis of the available spectroscopic data. The calculated outer-valence-shell (1πg-1)X 2Πg, (1πu-1)A 2Πu, (3σu-1)B 2Σu+, and (4σg-1)C 2Σg+ partial-channel photoionization cross sections are in good accord with measured values, and clarify completely the origins of the various structures in the observed spectra. There is evidence, however, of coupling among scattering states associated with 1πg-1 and 1πu-1 ionic channels, giving rise to moderate disagreement with tunable-source photoelectron and fluorescence measurements over a portion of the spectrum. In the inner-valence-shell region, the calculated 2σu-1 and 3σg-1 cross sections are in qualitative accord with the observed many-electron spectral intensities, and provide a basis for quantitative interpretation when combined with appropriate intensity-borrowing calculations. The calculated carbon and oxygen K-edge cross sections are in good agreement with available cross sections obtained from electron-impact and photoabsorption measurements. It is of particular interest to find the oxygen K-edge (1σg-1, 1σu-1) cross section exhibits both the expected 5σg(σ*) and 4σu(σ*) resonance-like features. Finally, comparisons are made throughout of the discrete and continuum spectra in carbon dioxide with the results of previously reported studies in CO and O2, and the origins of the similarities and differences in the cross sections in these cases are clarified.

Photoion–fluorescence photon coincidence study of radiative and dissociative relaxation processes in VUV photoexcited SO2. Fluorescence of SO+2, SO+, and SO

Abstract: Line photoexcitation sources in the range 102–212 eV are used to produce excited electronic states of SO2 and/or SO+2 Photoion–fluorescence photon coincidence measurements show that SO+2 states in the 16 eV region fluoresce at λ<370 nm, with a quantum yield φF(? 2B2+? 2A1)∼6×10−5, in competition with predissociation The products of predissociation of excited states of SO+2 are identified by their TOF mass spectra and/or dispersed optical emission SO+(A 2Π–X 2Π), SO(B 3Σ−–X 3Σ−), and SO(A 3Π–X 3Σ−) emissions are observed when the excited products are formed with ionic or (super) excited states of SO2 The measured lifetime of the SO+A 2Π state is 24±04 μs The results are discussed in relation to photoion–photoelectron measurements and in the context of unimolecular dissociation models

Photoelectrochemical Properties of Polycrystalline TiO2 Doped with 3d Transition Metals

Abstract: The photoelectrochemical properties of the polycrystalline doped with 3d transition metals (Cr, Mn, V, Cu, Ni, and Zn) have been investigated. The electrodes doped with Cr, Mn, V, and Cu show the visible light photoresponses. It is concluded that the visible light photoresponse is based on the impurity band formed near the π* conduction band of the in energy position, in analogy with the Co‐doped electrode reported previously. The cathodic photocurrent of the reduction was also observed for the doped and undoped electrodes. From the good agreement of the spectral dependences of the anodic and cathodic photocurrents, it is judged that both type photocurrents are brought about by the same photoexcitation process. The observed large cathodic photocurrents for the electrodes that give the visible light photoresponse are attributable to the d‐orbitals of the doped metal as the active site, by which the impurity band is formed in the bulk. A mechanism for the photosynthesis is also suggested from the simultaneous appearance of the photo‐oxidation and photoreduction currents on an electrode.

The surface photoeffect

Abstract: Angle-resolved photoelectron spectroscopy coupled with the polarized continuum of synchrotron radiation has been used to identify unambiguously a large enhancement in the photoexcitation from A(100) due to the surface photoeffect. This large enhancement in the cross section occurs only when the excitation field has a component perpendicular to the surface and is due to the dielectric response in the surface region, i.e., to the spatially varying electromagnetic field at the metal surface. An absolute determination of the differential photoionization cross section for excitation of Fermi-energy electrons shows quantitative agreement with Feibelman's self-consistent calculation. A simple picture of the surface photoeffect will be presented.

VUV fluorescence following photodissociation of N2O at 193 nm

Abstract: Photolytic studies performed at 193 nm demonstrate that NO in the highly excited D(v = 1,5) and E(v = 0) states is generated from N2O during irradiation in three sequential steps involving photodissociation, chemical reaction, and photoexcitation. The resulting NO fluorescence (160–230 nm) was analyzed with a system of rate equations, and the temporal behavior, intensity dependence, and pressure dependence were found to be consistent with a simple kinetic model. The quenching coefficient of NO by N2, Ar, and N2O were determined in this analysis to be qN2 = (2.7±0.8)×10−11 cm3 sec−1, qAr = (6.6±1.4)×10−11 cm3 sec−1, and qN2O = (1.5±0.4)×10−10 cm3 sec−1. Finally, dramatic changes in the spectral distribution of the ultraviolet NO fluorescence due to collisions with He were observed, which contrasts with the absence of spectral redistribution in collisions involving N2, Ar, and N2O.

Nuclear photoexcitation and delbrück scattering studied in the energy range 2 8 MeV

Abstract: Elastic scattering by nuclei in the range of mass numbers between 64 and 238 has been studied with monochromatic photons in the energy range between 2 and 8 MeV. These photons were provided either by a Ti(n,γ) source installed in the tangential through channel of the Grenoble high flux reactor, or by24Na and56Co sources produced by deuteron bombardment of Al or Fe at the Gottingen cyclotron. The photoexcitation of 23 nuclear levels has been observed and the decay properties and groundstate widths of the majority of these levels have been determined. For the lead scattering target the coherent elastic differential cross section has been studied in detail. There is evidence that below the photo-neutron threshold the elastic scattering via virtual photoexcitation of the nucleus can be approximated by extrapolating the real part of the Giant Dipole Resonance amplitude along a Lorentzian curve. Coulomb corrections to Delbruck scattering seem to play a small role at 6.5 MeV.

Raman spectroscopy as a surface sensitive technique on semiconductors

Abstract: It is demonstrated that the measurement of a bulk phonon property, i.e., the Raman intensity of the resonantly excited symmetry forbidden LO‐phonon via its dependence on the surface electric field, is a sensitive method to measure surface band bending in polar semiconductors. The observation of coupled phonon‐plasmon modes shows the existence of flat band conditions on clean, UHV‐cleaved (110)GaAs. This condition can also be reached on pinned surfaces by photoexcitation with laser power densities in excess of 500 W/cm2. A curve fitting of the observed L+/L−‐spectrum with the Lindhard–Mermin dielectric function yields information on the wave‐vector smearing due to strong light absorption and impurity scattering.

Frenkel defect formation and intersystem crossing at highly-excited states of the triplet self-trapped exciton in NaCl

Abstract: Measurements were made of the absorption change at the F band and of the singlet luminescence in NaCl, induced by photoexcitation of the lowest state of the triplet self-trapped excitons into excited states higher than the lowest state by 1.7 eV ∼2.8 eV. It is found that excitation with photons below 2.5 eV causes both the F -center creation and the σ-luminescence, while excitation with photons above 2.5 eV causes only the σ-luminescence. These results indicate that non-radiative transition from the higher excited states to the lowest state of the self-trapped exciton is selective. The cause of the selectiveness is discussed. The yield of the F center creation at the higher excited state of the self-trapped exciton was found to be about 10 -2 . The stability of the created F centers in NaCl is found to be lower than in other alkali chlorides.

GaAs–oxide interface states: Gigantic photoionization via Auger‐like process

Abstract: Spectral and transient responses of photostimulated current in MOS structures were employed for the study of GaAs-anodic oxide interface states. Discrete deep traps at 0.7 and 0.85 eV below the conduction band were found with concentrations of 5 x 10 to the 12th/sq cm and 7 x 10 to the 11th/sq cm, respectively. These traps coincide with interface states induced on clean GaAs surfaces by oxygen and/or metal adatoms (submonolayer coverage). In contrast to surfaces with low oxygen coverage, the GaAs-thick oxide interfaces exhibited a high density (about 10 to the 14th/sq cm) of shallow donors and acceptors. Photoexcitation of these donor-acceptor pairs led to a gigantic photoionization of deep interface states with rates 1000 times greater than direct transitions into the conduction band. The gigantic photoionization is explained on the basis of energy transfer from excited donor-acceptor pairs to deep states.

Stieltjes-Tchebycheff calculations in the static-exchange approximation of photoexcitation and ionisation in water

Abstract: Calculations of the variation with photon energy of the photoionisation cross sections to the first three ionic states of water are described, using the Stieltjes-Tchebycheff imaging technique and the static-exchange approximation. The results are compared with experimental data from (e, 2e) spectroscopy and from synchrotron radiation. Good agreement with experiment is found, and the calculations highlight the need for more experimental data, especially at low photon energies.

Photoexcitation and ionization in ozone: Stieltjes-Tchebycheff studies in the separated-channel static-exchange approximation

Abstract: Theoretical studies are reported of total and partial-channel photoexcitation/ionization cross sections in ozone employing Stieltjes–Tchebycheff (S–T) techniques and the separated-channel static-exchange approximation As in previously reported investigations of excitation and ionization spectra in diatomic and polyatomic molecules employing this approach, vertical electronic dipole transition spectra for the twelve occupied canonical Hartree–Fock symmetry orbitals in ozone are constructed using large Gaussian basis sets, appropriate computational methods, and noncentral static-exchange potentials of correct molecular symmetry Experimental rather than Koopmans ionization potentials are employed when available in construction of transition energies to avoid the incorrect ionic-state orderings predicted by Hartree–Fock theory, and to insure that the calculated series have the appropriate limits The spectral characteristics of the resulting improved-virtual-orbital discrete excitation series and corresponding static-exchange photoionization continua are interpreted in terms of contributions from valencelike 7a1(sigma*), 2b1(pix*), and 5b2(sigma*) virtual orbitals, and appropriate diffuse Rydberg functions The 2b1(pix*) valence orbital apparently contributes primarily to discrete or autoionizing spectra, whereas the 7a1(sigma*) and 5b2(sigma*) orbitals generally appear in the various photoionization continua Moreover, there is also evidence of strong 2p-->kd atomiclike contributions to ka2 final-state channels in the photoionization continua The calculated outer-valence-shell 6a1, 4b2, and 1a2 excitation series are compared with electron impact–excitation spectra in the 9 to 13 eV interval, and the corresponding partial-channel photoionization cross sections are contrasted and compared with the results of previously reported studies of photoionization in molecular oxygen The intermediate- and inner-valence-shell excitation series and corresponding photoionization cross sections are in general accord with quantum-defect estimates and with the measured electron-impact spectra, which are generally unstructured above ~22 eV Of particular interest in the intermediate-valence-shell spectra is the appearance of a strong sigma-->sigma* feature just above threshold in the 3b2-->kb2 photoionization cross section, in qualitative agreement with previously reported studies of the closely related 3sigmag-->ksigmau cross section in molecular oxygen Finally, qualitative comparisons are made of the calculated K-edge excitation and ionization spectra in ozone with recently reported photoabsorption studies in molecular oxygen

UV photoconduction in α-Al2O3 single crystals

Abstract: Photoconduction measurements have been made on single crystal α-Al2O3 in the spectral energy range 3·5eV−6·2eV. The extrapolation of the linear region of the spectral dependence curve gives 4·6eV as the band gap. Photocurrent starts at about 3·7eV and in the spectral region prior to the band gap, two peaks are obtained in the photocurrent. These peaks seem to be due to photoexcitation of charge carriers from the traps present in the sample. The results may be explained in terms of energy bands for solids.

Photocurrent Noise Observed by Resonant Photoexcitation at 4.2 K in n-GaAs

Abstract: Low-frequency ( f < 50 Hz) photocurrent noise caused by the breakdown of the low-temperature freeze-out of neutral shallow donors at 4.2 K has been investigated under resonant photoexcitation in n -GaAs. At the onset of the breakdown (∼4 V/cm), the photocurrent noise was selectively generated by the resonant photoexcitation at the photon energies of the ( D 0 , X ) n =1,2 lines and the ( D + , X ) line. The observed results are explained by the current-filament formation.

Use of autoionization transition absorption peaks in isotopically selective photoexcitation

Abstract: Apparatus and process for isotopically selective multi-step photoionization in which the final or ionizing step is tuned to produce a specific transition to an excited state above ionization for which the ionization cross-section is substantially greater than for ionization transitions in general. The autoionization transition to an ionized state is typically made from a highly excited bound state which is reached in one or two isotopically selective energy jumps from the ground state or other low-lying levels. The isotope shift for the ionization transition is typically small compared to the bandwidth of the ionization transition and relatively broad band photoionization radiation covering the entire absorption line can be employed. Broad band radiation is more economic and is preferable for use wherever possible. A technique is also shown for identifying the ionization transitions of augmented cross-section.

Intensity of astrophysical lines in the transition region.

Abstract: Intensities of 88 extreme-ultraviolet (EUV) lines emitted from the solar chromosphere-corona transition region and observed on the Earth are calculated. The electrons are supposed to excite through electron impact excitation only since the otl processes, for instance photoexcitation and dielectric recombination excitaion, are not significant, are not significant. The electron density of the region is taken such that the electron impactdexcitation rate is negligible in comparison with the spontaneous transition probability. Hence the electrons from the upper level deexcite spontaneously, emitting a photon of wavelength corresponding to the energy difference between the two levels. The results are reported in a tabular form.

Intensity of astrophysical lines in the transition region

Abstract: Intensities of 88 extreme-ultraviolet (EUV) lines emitted from the solar chromosphere-corona transition region and observed on the Earth are calculated. The electrons are supposed to excite through electron impact excitation only since the otl processes, for instance photoexcitation and dielectric recombination excitaion, are not significant, are not significant. The electron density of the region is taken such that the electron impactdexcitation rate is negligible in comparison with the spontaneous transition probability. Hence the electrons from the upper level deexcite spontaneously, emitting a photon of wavelength corresponding to the energy difference between the two levels. The results are reported in a tabular form.

THE EFFECT OF ELECTRON-ELECTRON SCATTERING ON HOT PHOTOEXCITED ELECTRONS IN GaAs

Abstract: The effect of electron-electron scattering on hot photoexcited electrons in GaAs is studied by performing numerical calculations of the electron distribution function. The calculations apply to conditions of continuous monochromatic photoexcitation and take account of the following effects : injection of electrons into the conduction band (the injection energy is taken to be less than the L.O. phonon energy), collisions between electrons of unlike spin, collisions between electrons of like spin, electron-phonon collisions (via the piezoelectric and deformation potential interactions) and recombination. It is shown that various departures from quasi-equilibrium occur if the electron density is sufficiently low (≤1012 cm-3) or sufficiently high (≥1018 cm-3).