Showing papers on "Photoexcitation published in 1968"
TL;DR: In this paper, the authors investigated the electric field dependence of this process and showed that the only effect is a broadening of the spectrum at high fields which are comparable in magnitude with the binding field experienced by a hole in an excited state.
Abstract: Structure has been observed in the photoconductivity spectrum of semiconducting diamond at energies below the ionization threshold. Photoconductivity maxima in this spectral region have been found to coincide exactly with maxima in the absorption spectrum associated with transitions to excited states of the aluminum acceptor center (ionization energy=0.373 eV). In general, the acceptor spectrum is better resolved in these photoconductivity measurements than in absorption measurements on the same specimen, and this is particularly true for synthetic semiconducting diamonds in which the acceptor concentration is very high. Measurements in the temperature range 4 to 150\ifmmode^\circ\else\textdegree\fi{}K have shown that a two-stage process is predominantly responsible for the appearance of the excited-states spectrum in photoconductivity: optical excitation of holes from the ground state to these excited states, followed by thermal excitation into the valence band. This process is referred to as "photothermal ionization." An investigation of the electric field dependence of this process has shown that the only effect is a broadening of the spectrum at high fields which are comparable in magnitude with the binding field experienced by a hole in an excited state. At temperatures close to 4\ifmmode^\circ\else\textdegree\fi{}K, where the thermal contribution is negligible, residual structure can still be observed which is specimen-dependent, and the features may appear as either minima or maxima superimposed on the low-energy tail of the photo-ionization continuum. This low-energy tail is presumably due to direct photoexcitation of slightly perturbed acceptor centers, and minima can appear on this tail because of the strong competitive absorption to the excited states. However, in many specimens, tunneling from the excited states appears to be possible, and the features are then observed as maxima. This picture is further substantiated by the fact that in those diamonds for which tunneling does not normally occur, it can be induced by simultaneously illuminating the crystal with radiation of energy lying within the photoconductivity continuum. This increases the number of ionized centers present, and so increases the probability of tunneling.
75 citations
TL;DR: The photoelectric effects observed by Baessler and Vaubel1 are caused by photoexcitation of electrons from traps into the conduction band as discussed by the authors, and triplet excitons play a role in this process.
18 citations
24 Jun 1968
TL;DR: In this paper, a chemical kinetic model describing photochemical reactions that are likely to be important in "cold" argon ahead of a strong shock wave is examined on a quantitative basis, including the propagation of resonance radiation far from the nucleus, the shock front in the wings of the resonance absorption line, partial trapping of the absorbed resonance, subsequent photoionization of excited atoms, photoionisation of ground state argon, and certain recombination and deexcitation processes.
Abstract: A chemical kinetic model describing photochemical
reactions that are likely to be important
in "cold" argon ahead of a strong shock wave is examined
on a quantitative basis. The model includes
the propagation of resonance radiation far from the
shock front in the wings of the resonance absorption
line, partial trapping of the absorbed resonance
radiation, subsequent photoionization of excited
atoms, photoionization of ground state argon,
and certain recombination and deexcitation processes.
Specific consideration is given to shock
tube geometry, the occurrence of both nonequilibrium
and equilibrium regions of variable lengths
behind the pressure discontinuity, and the (experimentally)
known shock tube wall reflectivity. Theoretical
predictions of electron and excited atom
concentrations ahead of the shock wave are presented
for typical shock tube operating conditions.
8 citations
6 citations
TL;DR: The photo-induced electron spin resonance of ZnS:Cu,Ga has been observed in this article, where the optical absorption and photoconductivity spectra were measured, and with these spectra, combined with the photoexcitation spectra for spin resonance, they verified the identification of the donor-acceptor pair responsible for the ESR signal.
Abstract: The photo-induced electron spin resonance of ${\mathrm{Ga}}^{+2}$ previously found with ZnS:Cu,Ga has been observed for ZnS:Ag,Ga. The photoexcitation spectra depend on the identities of both donor and acceptor, on the concentration of these impurities, and on crystal structure. The optical absorption and photoconductivity spectra were measured, and with these spectra, combined with the photoexcitation spectra for spin resonance, we verify the identification of the ${\mathrm{Ga}}^{+2}$ responsible for the ESR signal as part of a donor-acceptor pair. The lack of any observable dependence of the resonance spectrum on the acceptor is explained on the basis of the minimal overlap between electron and positive hole wave functions and the zero effective charges of the photoexcited donor and acceptor. From the dependence of the photoconductivity and ESR photoexcitation spectra on concentration of donor and acceptor, it is suggested that impurity-band photoconduction occurs at the higher concentrations.
2 citations