Showing papers on "Photoexcitation published in 1970"
TL;DR: In this paper, a relative cross section for simultaneous excitation and ionization of the helium atom is derived from the sudden approximation, which is shown to agree with that obtained from the velocity form of the cross section in the Coulomb approximation.
Abstract: A relative cross section for simultaneous excitation and ionization of the helium atom is derived from the sudden approximation. It is shown to agree with that obtained from the velocity form of the cross section in the Coulomb approximation. It is connected to the asymptotic cross section of Kabir and Salpeter through the cusp condition at the nucleus. An analysis of the influence of ground-state correlations on the asymptotic double-excitation cross section is carried out.
105 citations
TL;DR: Radiative recombination in single crystals and glasses of arsenic sulphide and selenide under photoexcitation was observed in this article, where the recombination was performed in a single crystal and a single glass.
Abstract: Radiative recombination in single crystals and glasses of arsenic sulphide and selenide under photoexcitation is observed
85 citations
TL;DR: In this article, the ability of thermoelectric power measurements to permit a description of carrier density and phonon-drag variations caused by photoexcitation was tested in 100−Ω·cm n and p-type silicon.
Abstract: The ability of thermoelectric power measurements to permit a description of carrier‐density and phonon‐drag variations caused by photoexcitation was tested in 100‐Ω·cm n‐ and p‐type silicon. At low temperatures the major effect of photexcitation is to decrease the phonon‐drag contribution to the thermoelectric power by increasing the phonon density in the crystal. At higher temperatures the thermoelectric effect can be used to investigate changes in the electronic contribution due to photoexcitation. An apparently anomalous increase in thermoelectric power with photoexcitation was consistently found in p‐type silicon over an intermediate temperature range.
41 citations
TL;DR: In this article, a correlation between high plasma resonance energy and low cathode-ray efficiency is pointed out, and it is shown that photoexcitation efficiencies give agreement in some cases and substantial lack of agreement in others.
Abstract: In cathode‐ray excitation of a phosphor the energy of the fast electrons appears, in an intermediate stage, as excitation of the valence shell electrons. We have used photons of energy up to 25 eV to simulate cathode‐ray excitation. Attempts to account for cathode‐ray efficiencies on the basis of the photoexcitation efficiencies give agreement in some cases and substantial lack of agreement in others. A correlation between high plasma resonance energy and low cathode‐ray efficiency is pointed out.
37 citations
TL;DR: In this paper, a general method was presented for calculating the $f$ values for all permitted dipole transitions involving simultaneous photoionization and photoexcitation of helium, and the cross section for ions excited into the $n=2$ state was compared with experiment and with a similar calculation by Salpeter and Zaidi; their result that the ejected electron is in a $p$ state is confirmed.
Abstract: A general method is presented for calculating the $f$ values for all permitted dipole transitions involving simultaneous photoionization and photoexcitation of helium. The cross section for ions excited into the $n=2$ state is compared with experiment and with a similar calculation by Salpeter and Zaidi; their result, that the ejected electron is in a $p$ state is confirmed. The probability for the residual ${\mathrm{He}}^{+}$ ion to be excited to a state of principle quantum number $n$ is evaluated for $n=2\ensuremath{-}10$, and the probability for the photoelectron to be in a $p$ state is compared with the probability for it to be emitted in an $s$ state. It is found that the inclusion of contributions from simultaneous photoionization and photoexcitation, and from double ionization in the total helium photoionization cross section, improves the agreement between theory and the experimental data at high energies.
33 citations
TL;DR: In this paper, the dependence of trap filling on the intensity, temperature, and wavelength of the photoexcitation has been investigated for trapping centers in S-Se-annealed CdS-CdSe crystals and in crystals of cdS subjected previously to electron radiation damage and photoexcited annealing.
Abstract: The dependence of trap filling on the intensity, temperature, and wavelength of the photoexcitation has been investigated for trapping centers in S–Se‐annealed CdS–CdSe crystals and in crystals of CdS subjected previously to electron radiation damage and photoexcited annealing. For certain traps in both these kinds of crystals, it was found that trap filling was not only critically dependent on the temperature of excitation but also on the wavelength of excitation. In fact, for each process observed, a band of wavelengths is most effective, the maximum wavelength of this band being larger than the wavelength for maximum photoconductivity. This result makes previous attempts to explain this kind of phenomenon in terms of a Coulomb‐repulsive trap less acceptable and provides additional evidence for the presence of photochemical changes at and below room temperature in CdS‐like materials.
21 citations
TL;DR: In this article, a room temperature study of the red emission from GaP (Zn, O) as a function of 4880 and 5145 photoexcitation intensity was initiated to determine the importance of nonradiative bulk recombination centers shunting the radiative path through the Zn-O complexes.
Abstract: This paper describes a room‐temperature study of the red emission from GaP (Zn, O) as a function of 4880‐ and 5145‐A photoexcitation intensity, and was initiated to determine the importance of nonradiative bulk recombination centers shunting the radiative path through the Zn–O complexes. The technique used is the investigation of the saturation behavior of the emitted red light as a function of excitation intensity. An analysis is given in which the functional dependence of the red‐light emission on such parameters as the excitation absorption coefficient, diffusion length, surface recombination velocity, and excitation beam diameter is considered. The diffusion lengths and surface recombination velocities were obtained by means of low‐intensity excitation spectral measurements. Diffusion lengths covering the range 0.5–5.0 μ were measured and surface recombination veloclties from 5 to 100 times the diffusion velocity were extracted. The samples saturated at intensities ranging from 3×1019 photons/sec cm2 ...
20 citations
TL;DR: In this article, a spectrophotometer has been built using commercial units which can measure photoinduced changes in absorption of 5 parts in 106 and lifetimes of photo-induced transitions greater than 10−6 S.
Abstract: A spectrophotometer has been built using commercial units which can measure photo-induced changes in absorption of 5 parts in 106 and lifetimes of photo-induced transitions greater than 10−6 S. Results are presented of the triplet-triplet absorption spectra and lifetimes of some polycyclic aromatic hydrocarbons and of a shortlived species occurring during photoexcitation of lumiflavine.
13 citations
TL;DR: Simultaneous photon absorption and energy transfer from a previously excited impurity atom are treated for some simple cases in this article, where it is shown that in suitable systems the absorption coefficient for this cooperative process is orders of magnitude greater than for two-photon absorption under comparable excitation.
13 citations
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 in a shock tube is examined on a quantitative basis.
Abstract: A chemical kinetic model describing photochemical reactions that are likely to be important in "cold" argon ahead of a strong shock wave in a shock tube 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, imprisonment of the absorbed resonance radiation, subsequent photoionization of excited atoms, photoionization of ground state argon, and certain recombination and de-excitation processes. Specific consideration is given to shock tube geometry, the finite extent of the equilibrium region and the (experimentally) known shock tube wall reflectivity. Theoretical predictions of excited atom and argon ion concentrations in the precursor region are presented for typical shock tube operating conditions. The regimes favorable to the production of argon ions by photoionization of ground state argon and by photoionizati on of photoexcited argon, respectively, are delineated.
10 citations
TL;DR: In this paper, the wavelength dependence of the photoionization cross-section (PCS) of the Zn acceptor level at Ev + 0.31 eV in silicon has been determined for temperatures of 85 and 290 °K from an investigation of photoexcitation of holes from this level into the valence band.
Abstract: The wavelength dependence of the photoionization cross-section (PCS) of the Zn acceptor level at Ev + 0.31 eV in silicon has been determined for temperatures of 85 and 290 °K from an investigation of the photoexcitation of holes from this level into the valence band. The experimental results are compared with the theoretical predictions based on a deep-centre model using a short-range potential. The agreement between theory and experiment is reasonable.
[Russian Text Ignored]
TL;DR: In this paper, the total curve of the dipole photoexcitation of the 6 Li nucleus was described in a single way, and the partial cross-sections of photoreaction and the decay spectra were compared to experimental data.
TL;DR: In this paper, the authors used pulsed photoexcitation from argon and He-Ne lasers to understand the kinetics of the radiative emission in (Zn, O)−doped GaP, primarily at room temperature.
Abstract: To understand the kinetics of the radiative emission in (Zn, O)‐doped GaP, primarily at room temperature, we have measured luminescence lifetimes using pulsed photoexcitation from argon and He–Ne lasers. Measurements of time decay and temperature dependences of the radiative emission have enabled us to identify the emission in the near infrared as resulting from transitions involving a bound electron at an oxygen site to a free hole in the valence band. Measurements of the temperature dependence of the decay times and the luminescence intensities of the exciton (red) and bound‐electron to free‐hole (infrared) emissions also substantiate the conclusions that nonradiative recombination paths exist out of the Zn–O complex and provide evidence for the existence of a nonradiative ``shunt path'' out of the conduction band. The bound electron to free‐hole recombination out of oxygen is believed to be primarily radiative. For an annealed sample, the decay times of the exciton and bound‐to‐free transitions are mea...
TL;DR: The present work establishes the presence of aggregates in doped single crystals of CDD (cyclododecane) by direct observation.
Abstract: THE photoexcitation properties of organic crystals have generally been studied for pure or mixed aromatic single crystals. In these cases the host is photoactive1–3. The substructures of such molecular crystals are not as well known as in the cases of ionic4 and metallic crystals5, although such structures may well influence the properties involved in photoexcitation. D. J. M. and Irons6 used cyclododecane (CDD) single crystals as the photoinert host for phosphorescence lifetime studies of the photoexcited triplet state of dissolved aromatic impurities. Lifetime variations were found and these were attributed to various states of aggregation of the photoactive solute species in the single crystal host. The tendency to aggregate was correlated to lattice mismatch between host and dopant molecular dimensions. The present work establishes the presence of aggregates in doped single crystals of CDD (cyclododecane) by direct observation.
TL;DR: In this article, the authors measured the spin-forbidden singlet-triplet absorption spectra of aromatic molecules and molecular crystals using photoexcitation techniques and showed how the absorption coefficient as a function of energy (and hence the oscillator strength of the singlet triplet transition) can be determined from the excitation s...
Abstract: The measurement of spin-forbidden singlet-triplet absorption spectra of aromatic molecules and molecular crystals has been facilitated recently by the use of photoexcitation techniques. In direct absorption spectroscopy a long path length of sample is necessary to observe such weak (f < 10−4) transitions. Such a problem is overcome by monitoring the concentration of triplet state molecules indirectly (via the phosphorescence(1,2) or delayed fluorescence(3) of the sample) as a function of excitation energy. The increased sensitivity of this technique over direct absorption spectroscopy has enabled high resolution T 1 ← S 0 absorption spectra to be obtained from pure(4) or doped crystals(2) of thickness less than 1 mm. Previously, transition energies and polarizations have been measured by this method. We wish, in this note, to indicate how the absorption coefficient as a function of energy (and hence the oscillator strength of the singlet-triplet transition) can be determined from the excitation s...
TL;DR: In this paper, the optical absorption of free carriers in cadmium selenide was investigated by photoexcitation of carriers and, in the case of a strongly n -type sample, for equilibrium electrons as well.
TL;DR: In the case of strong photoexcitation, the maximum open-circuit voltage approaches the internal built-in potential difference due to the impurity gradient between the ohmic contact probes on the semiconductor slice as mentioned in this paper.
Abstract: Photoelectric fields induced in semiconductors with nonuniform impurity profiles cancel the built-in electric field. In the case of strong photoexcitation, the maximum open-circuit voltage approaches the internal built-in potential difference due to the impurity gradient between the ohmic contact probes on the semiconductor slice.