Emission Band Shape Research Articles

Symmetry of Vibrationally Assisted Electronic Absorption and Emission Band Shapes of Impurities in Solids

For impurities that couple to the lattice through an interaction containing both linear and quadratic terms, mirror symmetry, at and above $T=0$, of the resulting absorption and emission spectra about the zero-phonon line is highly dependent on the strength of the quadratic interaction parameter. Exact symmetry at all temperatures occurs only for the linear interaction. Overlap of the linear-plus-quadratic interaction spectra, although always in the vicinity of the zero-phonon line, is exactly coincident with the zero-phonon line at $T=0$. Values of the linear and quadratic interaction parameters can be determined from the shapes of experimental broad-band spectra, the spacings of "hot lines," and the intensities of the zero-phonon lines.

Symmetry of Quadratic-Mode Absorption and Emission Band Shapes of Impurities in Solids

Symmetry of Quadratic-Mode Absorption and Emission Band Shapes of Impurities in Solids

The temperature dependence of vibronic spectra in irradiated silicon

Crucible grown and high purity floating zone silicon specimens have been irradiated at room temperature with gamma rays. High resolution cathodoluminescence spectra of the vibronic bands with zero phonon lines at 0.717, 0.724, 0.790, 0.795, 0.898 and 0.969 eV are given. The widths and energies of these lines, and the intensities of the 0.790, 0.795 and 0.969 eV lines relative to the total intensities of their associated bands, are given in the temperature range 25 to 75 K. These data are discussed in terms of the theory of linear and quadratic electron-phonon coupling in the adiabatic approximation, using Debye spectra of coupled phonons and, for the 0.790, 0.795 and 0.969 eV lines, densities of coupled phonons derived from the emission band shapes. The theory gives a selfconsistency between the spectral band shapes and the widths and energies of the zero phonon lines, but does not satisfactorily fit the temperature dependence of the intensities of the zero phonon lines. It is not known whether the failure of the theory is due to a Jahn-Teller effect (no absorption at the centres has been detected), or to the coupling by the electron-phonon interaction of electronic states whose energies are not greatly separated compared with the phonon energies involved.

Broad band absorption and luminescence of defects in ionic crystals

Studies of the optical absorption and emission band shapes of simple defects in ionic crystals have been carried out within the framework of a single configuration coordinate model and its elementary extentions. The semi-classical and linear coupling approximations have been tested against the fully quantum-mechanical treatment with differing ground and excited state force constants. As might be expected, for strong electron-lattice coupling the semi-classical approximation appears to be more useful than the linear coupling model. Using previously computed configuration coordinate curves and the measured effective frequencies, the absorption and emission band shapes of the F center in KCl, KBr, and KI have been computed in various approximations. The Condon approximation has been tested and found to be fairly adequate. The frequency factor multiplying the sums over states in the usual expression for the absorption coefficient is found to be of minor importance. The calculations show that the temperature shift of the maxima of the absorption bands comes about in part because of the temperature dependence of the lattice constant.

Soft X-Ray Valance State Effects in Conductors

Investigation of the NiLIIIband at various excitation voltages and the use of Ni and Ti thin films in the path of the NiLII,IIIand TiLII,IIIradiation, has shown that with proper electrode geometry and using target voltages of about 4 kv the effect of self absorption on emission edge shift and band shape is negligible for these elements. However, the LII/LIIIintensity ratio of titanium is strongly effected by self absorption, but it is not effected for nickel and iron by self absorption.A survey of several investigators’ soft x-ray band spectra from alloys and conducting compounds showed that the observed band changes were related to the electronegativity difference of the combining elements or the ionic character of the band. Peaks appearing on the low energy side of transition metal bands when combined with 2nd period elements appear to be a cross transition between the 2s or 2p band of the non-metal and inner level of the metal.

Shape of Emission Bands of Fluorescent Complex Molecules

The shape of emission bands is determined by the vibrational-energy distribution of the luminescence centers and by the form of the potential curves. A new potential is introduced, permitting us to establish the intensity distribution within a band as a function of frequency and temperature. The calculations are based on a semiclassical approximation of the Franck—Condon factors and are worked out with the aid of the Mehler formula. The obtained expression is in close agreement with the experimental results.

Determination of Electron Distribution and Bonding from Soft X-Ray Emission Spectroscopy

AbstractFrom measurements of the shift in wavelength with chemical combination, it appears there is very little transfer of charge between the metal and nonmetal atoms for the Group-IV transition-metal carbides and diborides. For Group-VI transition-metal carbides and beyond, there appears to be a definite transfer of charge from the carbon to the metal atom. The bonding in the Group-IV transitionmetal carbides is largely co valent, and metallic for the Group-IV transition-metal diborides. A comparison of shapes of emission bands with crystal structure indicates that, in general, NaCl-type structures have symmetrical peaks with narrow bandwidths compared to hexagonal structure, which have asymmetrical peaks, broad bandwidths, and long tails to the bottom of the bands.

The shape of the emission bands of luminescent solids

Single emission bands of luminescent solids generally exhibit a bell-shaped form. Closer examination of emission curves of a simple nature demonstrates a very close Gaussian form, but only when energy is plotted as a function of frequency. More complex emission curves can be very well described as a sum of Gaussian distribution functions. The experimentally determined shape of the emission profile is compared with theoretical emission curves, calculated from the configuration co-ordinate model.

On the Existence of “Sub-Bands” in the Luminescence Emission Spectrum of Manganese Activated Zinc Silicate*

Failure of the luminescence decay of manganese-activated zinc silicate to follow an exact exponential curve has been cited as evidence for the presence of two different emitting centers with different decay rates. However, the shapes of the spectral emission curves taken at different times after excitation are identical, showing that if two centers exist their emissions are the same. The luminescence emission of this phosphor has also been measured at 4°K and shows a single peak a few hundred angstroms wide. There is little observed change in the emission between 90°K and 4°K. The width of the room temperature emission is therefore the result of the temperature-broadening of an already broad band. No experimental evidence is found for bands predicted on the assumption that emission spectra are formed by the superposition of bands having the shape of Gauss error curves. Theoretical work on the shape of emission and absorption bands is reviewed and does not support the Gauss error curve theory.