Showing papers on "Photoexcitation published in 1978"

Hot electrons and phonons under high intensity photoexcitation of semiconductors

Abstract: It has become well established during the last few years that intense photoexcitation of a semiconductor leads to the heating of the carriers and the generation of nonequilibrium phonons. These phenomena which result from the relaxation of photoexcited carriers to the band extrema by interaction with other carriers and by emission of phonons, are reviewed in this paper. At relatively low intensities ( 5 W/cm 2 for GaAs) the photoexcited carrier distribution is Maxwellian with a carrier temperature T e different from the lattice temperature. T e as high as 150K and effective phonon temperatures as high as 3700K have been observed in GaAs. The observed variation of T e with excitation intensity leads to the conclusion that in semiconductors like GaAs the polar optical mode scattering is the dominant energy loss mechanism from the electron gas to the lattice. Similar results are obtained in CdSe and CdS. At higher intensities (>10 5 W/cm 2 for GaAs), the carrier dist0ribution becomes non-Maxwellian for reasons not well understood at present. We will also discuss some recent measurements of variation of T e with excitation wavelength and of the transmission spectra of photoexcited GaAs.

The Electron-Hole Liquid in Semiconductors: Theoretical Aspects

Abstract: Publisher Summary The chapter presents a discussion on the theoretical aspects of the electron-hole liquid in semiconductors. In an intrinsic semiconductor such as germanium or silicon, there are essentially no free carriers at very low temperatures. However, high densities of positive (hole) and negative (electron) charge carriers can be generated in these crystals by photoexcitation with a photon energy greater than the forbidden gap. In recent years, experiments in Ge and Si have revealed that optically pumped electrons and holes at high densities can undergo a phase transition at liquid helium temperatures into a metallic, liquid state. This condensate represents a state of matter particularly unique in nature because the electrons and holes, by reason of their small masses, are simultaneously in the quantum limit. This chapter presents a discussion on the collective behavior of electrons and holes at low temperatures and high densities. The chapter presents an historical introduction, mentions a few of the important early experiments performed, and presents a comparison of theory and experiment. Semiconductors can be divided into two classes. Those in which the conduction band minimum and valence band maximum occur at the same value of the wavevector are known as direct gap semiconductors, while those in which they are separated in k space are known as indirect gap semiconductors. The chapter discusses the latter.

Photoabsorption in molecular nitrogen: A moment analysis of discrete‐basis‐set calculations in the static‐exchange approximation

Abstract: Theoretical investigations of photoexcitation and ionization cross sections in molecular nitrogen are reported employing the recently devised Stieltjes–Tchebycheff moment-theory technique in the static-exchange approximation. The coupled-channel equations for photoabsorption are separated approximately by identifying the important physically distinct excitation processes associated with formation of the three lowest electronic states of the parent molecular ion. Approximate Rydberg series and pseudospectra of transition frequencies and oscillator strengths are constructed for the seven individual channel components identified using Hartree–Fock ionic core functions and normalizable Gaussian orbitals to describe the photoexcited and ejected electrons. Detailed comparisons of the theoretically determined discrete excitation series with available spectral data indicate general accord between the calculated and observed excitation frequencies and oscillator strengths, although there are some discrepancies and certain Rydberg series have apparently not yet been identified in the measured spectra. The total Stieltjes–Tchebycheff vertical photoionization cross section obtained from the discrete pseudospectra is in excellent agreement with recent electron–ion coincidence measurement of the cross section for parent–ion production from threshold to 50 eV excitation energy. Similarly, the calculated vertical partial cross sections for the production of the three lowest electronic states in the parent molecular ion are in excellent accord with the results of recent electron–electron coincidence and synchrotron–radiation branching ratio measurements. The origins of particularly intense resonancelike features in the discrete and continuum portions of the photoabsorption cross sections are discussed in terms of excitations into valencelike molecular orbitals. Small discrepancies between theory and experiment are attributed to specific autoionization processes and channel couplings not included in the calculations. In contrast to previously reported model or local potential studies, the present results employ the full nonlocal and nonspherical molecular Fock potential in ab initio photoabsorption calculations. The excellent agreement obtained between theory and experiment in molecular nitrogen suggests that highly reliable photoabsorption cross sections for diatomic molecules can be obtained from Hilbert space calculations and the Stieltjes–Tchebycheff method in the static-exchange approximation under appropriate conditions.

Low density photoexcitation phenomena in semiconductors: Aspects of theory and experiment

Abstract: The basic mechanisms related to the photoexcitation of electron-hole pairs in semiconductors under conditions of low excitation density, low temperature and high crystal purity are reviewed. The use of high-resolution emission spectroscopy of band-to-impurity optical transitions in GaAs to measure the energy distribution functions ƒ(E) of electrons and holes in optically excited carrier plasmas of well defined densities (1010 cm−3≤n≤ 1013 cm−3) is described. With this experimental method (i) the energy relaxation of initially hot carrier distributions after pulsed photoexcitation ( h ω ⪢ E g ), (ii) stationary non-equilibrium distributions of electrons in the conduction band under cw photoexcitation ( h ω ≳ E g ) and (iii) the transport properties of resonantly excited carrier plasmas in low electric fields ( 0≤| F |≤20 V/cm ) are investigated. The observed distribution functions are compared with theoretical results on the basis of the known band structure data of GaAs, taking into account polar optic and acoustic phonon scattering, the interaction among the carriers, ionized impurity scattering, and using approximate solutions of the appropriate transport equation.

EXAFS studies of disordered solids

Abstract: The availability of intense sources of continuous radiation in the X-ray region has stimulated the development of a technique for structure determination which is particularly suited to studies of local environment in complex systems. The absorption cross section for the photoexcitation of an electron from a deep core state to a continuum state exhibits oscillations as a function of photon energy known as the extended X-ray absorption fine structure, or EXAFS. These oscillations result from the interference between the outgoing photoelectron wavefunction and that portion of it scattered back from neighboring atoms, and therefore contain considerable information about the local environment of the excited atom species. Analysis of the EXAFS can yield not only the distance but also the type and number of the nearest neighbors of the excited atom. The purpose of this paper is to discuss briefly the theoretical background and the practical aspects of EXAFS data analysis. The discussion is then illustrated through application of these techniques to EXAFS studies of arsenic dopants in amorphous silicon and of PdGe metallic glass.

Excitation in ion–molecule collisions. A study of protonated aromatic molecules by ion kinetic energy spectrometry

Abstract: Collision induced dissociation of protonated aromatic ions formed in a chemical ionization source has been studied using the technique of mass analyzed ion kinetic energy spectrometry. The observed fragmentations are concordant with those seen in the photodissociation spectra of the same ions with one significant exception. Loss of H is always a major collision induced dissociation process but is not observed in photodissociation spectra. This differences is not due to differences in the total amount of energy transferred in ionic excitation but to differences in the form in which the energy is supplied. Collision induced dissociation can lead to processes forbidden in photoexcitation, particularly those associated with electron unpairing in violation of the eveneven+even rule for fragmentations.

Two-photon excitation of atomic oxygen

Abstract: A standard perturbation expansion in the atom-radiation field interaction is used to calculate the two photon excitation cross section for 1s(2) 2s(2) 2p(4) p3 to 1s(2) 2s(2) 2p(3) (s4) 3p p3 transition in atomic oxygen. The summation over bound and continuum intermediate states is handled by solving the equivalent inhomogeneous differential equation. Exact summation results differ by a factor of 2 from a rough estimate obtained by limiting the intermediate state summation to one bound state. Higher order electron correlation effects are also examined.

Optical Excitation of the Surface Photoelectric Effect of Metals Using Synchrotron Radiation

Abstract: By fully exploiting the properties of synchrotron radiation, a surface photoyield spectrum of a metal, namely aluminum, was obtained for the first time. This type of spectrum has been theoretically discussed since 1928. The extreme surface sensitivity, which is implicit in the transition matrix element for the surface photoexcitation process, was experimentally confirmed. The applied photoemission technique, therefore, provides one of the most powerful tools for the investigation of charge distribution and dielectric response at metal surfaces.

Infrared absorption in phosphorus doped silicon and the D- band

Abstract: Measurements are reported of the optical absorption in phosphorus doped silicon in the wavelength range 25 mu m to 700 mu m, together with the concentration dependence of refractive index. Sample doping levels varied from Nd=5*1015 Pcm-3, where the donors are virtually isolated, to Nd=2.7*1018 Pcm-3 where they are strongly interacting and just on the insulator side of the metal-insulator transition (critical concentration Nc=3*1018 Pcm-3). Specimens doped to 6*1016 Pcm-3 exhibited two new absorption lines at 294 cm-1 and 250 cm-1, and it is proposed that these lines lead to the formation of an absorption edge which moves towards smaller wavenumber as Nd to Nc. The absorption line at 294 cm-1 is identified as being the lower limit of a D2- band, and its position is in good agreement with Norton's measurement of a band edge using the temperature dependence of a photoexcitation technique.

Experimental evidence for soft x‐ray population inversion by resonant photoexcitation in multicomponent laser plasmas

Abstract: High‐intensity laser‐plasma experiments conducted on targets consisting of carbon and magnesium indicate population inversion in Mg+10 and Mg+11 ions at wavelengths of ∼100 A. The results indicate that soft x‐ray population inversion has been achieved, for the first time, by resonant photoexcitation. By this process, intensity inversions of the hydrogenic resonance lines from n=3 and n=4 are observed at electron densities as high as 1020 cm−3.

The Importance and Use of Energy Transfer in the Liquid-Phase Chemiluminescence

Abstract: Nonradiative electronic energy transfer (ET) is of great importance and of wide use in studies of chemiluminescence (CL) in solution. Some components of a chemiluminescent system, e. g. reactants, intermediates or products may serve as efficient energy acceptors. In some cases fluorescent energy acceptors, the activators of CL, are necessary components of a CL system. In contrast to the conventional (non-laser) photoexcitation the chemical excitation may produce a triplet state directly, rather than only via the excited singlet state.

Optically induced birefringence change in la-doped lead zirconate-titanate ferroelectric ceramics

Abstract: Optically induced birefringence change (OIBC) in PLZT is considered in view of space charge formed by photoexcitation of carriers. On the basis of optical measurements it is shown that, in general, complete screening of the applied field by space-charge field does not occur. Experimental results confirm theoretical calculations of electric fields and currents related to OIBC on the basis of a one-dimensional model.

Use of multiple photoexcitation in an optically thick silicon-aluminium plasma to obtain lasing at 44 AA

Abstract: By means of a new analytic formalism, which is developed to describe the average effect of photoexcitation processes in optically thick media it is shown that gain can be achieved in the Al XII 1s2p1P-1s3d1D, 1s2s1S-1s3p1P and 1s2p1P-1s3s1S transitions at 44 AA at densities well below, and over spatial extents well beyond, those proposed previously. Photoexcitation of the n=3 level in Al XII by the 1s2 1S-1s2p1P transition in Si XIII is used to generate the population inversion between the n=2 and n=3 states in Al XII. The gain is enhanced in magnitude and spatial extent by photoexcitation of the 1s2p1P and 1s3p1P states of the Si and Al plasmas respectively. An estimate of the inherent experimental limitations of this inversion technique is also made by comparing the calculated gains with upper limits obtained by pumping with a strict black-body photon flux in the pumping line.

Analysis of generation in space charge regions of solar cells

Abstract: A mathematical formulation of carrier transport in space charge regions in the presence of photoexcitation is presented. The analysis is only useful if bulk recombination can be neglected or is an a priori known function of position. The physical insight offered by this formulation is illustrated for the case of carrier transport in the space charge region of a heterojunction, and in a surface field region.

Forbidden lines in quasars and active galaxies

Abstract: We consider the effects of photoexcitation and photoionization from the metastable levels of ions in the high radiation environments expected in the nuclei of Seyfert galaxies, broad-line radio galaxies, and quasars. Anomalous line ratios in the narrow-line components and the absence of forbidden lines in the broad-line components of these objects may result from proximity of the emitting region to the central UV source, and do not necessarily imply high electron densities. Detailed calculations are presented for the case of (O III).

Use of multiple photoexcitation in an optically thick silicon-aluminum plasma to obtain lasing at 44 A. Interim report

Abstract: It is a well established phenomenon of radiative transfer in stellarlike plasmas that as the optical depth of a line increases, the mean radiative energy flux in the line correspondingly increases to approach the Planck blackbody value. Recently, the possibility of achieving stimulated emission in the x-ray or soft x-ray region by utilizing the interaction of strong lines from different elements or neighboring ionization stages has been considered in the context of resonance line pumping in high density, albeit optically thin, plasmas. Proper utilization of photon trapping effects in the pumped and pumping plasmas can yield comparable gains at soft x-ray frequencies to those proposed. However, these are at plasma densities that are significantly below and over spatial extents that are larger than those that would be required were one to be restricted to optically thin plasmas in which an exponentially decaying inversion density is generated. The demonstration is made by treating the physical situation of a non-collisionally dominated aluminum plasma with the aid of a new global rate equation formalism developed to include photoexcitation processes, on the average, into ionization equilibrium calculations involving optically thick laboratory plasmas.

Photoexcitation of the 7310-keV Level in 120 Sn and the 6517-keV Level in 117 Sn

Abstract: An experiment has been made on the nuclear resonant scattering from natural tin using thermal-neutron-capture γ rays in vanadium. Two levels were found to be simultaneously excited; one is the 7310-keV ( J =1) level in 120 Sn and the other the 6517-keV ( J =3/2) level in 117 Sn. Decay schemes of the resonant levels are proposed, Level parameters, such as effective cross sections, total radiative and ground-state transition widths, branching ratios and separations between energies of the incident line and the resonance, are deduced.

Study of photoisomerization of polymethine dyes at low temperatures employing pulse laser photolysis

Abstract: Establishing the photostationary equilibrium of the isomers of polymethine dyes under the influence of constant light radiation from a probe lamp at low temperatures makes it possible, by varying the wave-length of the photoexcitation, to record the differential absorption spectra of both the cis- and trans-photoisomers, and to study the kinetic characteristics of the di-cis-photoisomers, which are not observed under ordinary conditions.

Computer calculation of band scheme for luminescence of ZnS:Ag,Al crystal

Abstract: This paper makes a theoretical calculation of the temperature dependence of luminesrcence and photoconductivity for a ZnS:Ag,Al single crystal on the basis of a five-level zone model with two recombination centers and three traps. Experimental values of the level parameters were used in the calculations which were performed on an M-222 computer using the standard programs of the TA-1M translator. The calculated TDL and TDP curves are in satisfactory agreement with the experimental curves, measured with photoexcitation and cathodoexcitation. The calculations serve as a basis for analyzing the different shapes of TDL curves and a treatment is presented for them. Inferences about the characteristic features of luminescence are drawn by using the recombination model of M. Schon and H. A. Klasens in the calculations.