Showing papers on "Absorption (logic) published in 1984"

Valence-band photoemission and optical absorption in nickel compounds

Abstract: Photoemission, optical-absorption, and isochromat spectra of NiO and ${\mathrm{NiCl}}_{2}$ are studied theoretically by the consideration of configuration interactions within the metal-ligand cluster. It is shown that the satellites in the valence-band photoemission spectra contain significant ${d}^{7}$ final-state components produced by photoemission of a $d$ electron from the largely ${d}^{8}$-like ground state and that final states giving the main lines are predominantly ${d}^{8}$-like resulting from ligand $\ensuremath{\rightarrow}3d$ charge-transfer transitions following the $d$-electron emission. This identification differs markedly from the traditional one, according to which the main lines are due to ${d}^{7}$ final states and the satellites are produced by ligand $\ensuremath{\rightarrow}3d$ shakeup transitions. The crystal-field splitting and the apparent reduction of Racah parameters are shown to be due to hybridization between different configurations. The resonance enhancement of the satellites rather than the main lines at the $3p\ensuremath{\rightarrow}3d$ photoabsorption threshold is attributed partly to covalency and partly to the small number of $3d$ holes in the nickel compounds as compared to other $3d$ transition-metal compounds. Excitation energies for ligand $p\ensuremath{\rightarrow}\mathrm{Ni} 3d$ charge-transfer optical absorption are calculated and it is shown that the fundamental absorption edge of NiO at ~4 eV is not due to the $p\ensuremath{\rightarrow}d$ charge-transfer transitions. Instead, $d\ensuremath{\rightarrow}d$ charge-transfer transitions are proposed as the origin of the NiO fundamental edge. Energy levels involved in the intra-atomic $d\ensuremath{\rightarrow}d$ optical absorption are also calculated by the configurationinteraction approach and good agreement with experiment and energy levels calculated by the ligand-field theory is obtained. Finally the isochromat spectrum of NiO is discussed, based on the same approach.

Optical properties of pyrolytic boron nitride in the energy range 0.05—10 eV

Abstract: Optical reflectance studies of the $c$ face of pyrolytic boron nitride were carried out at room temperature in the photon energy range 0.45---10 eV. A Kramers-Kronig analysis was performed to determine the in-plane real (${\ensuremath{\epsilon}}_{1}$) and imaginary (${\ensuremath{\epsilon}}_{2}$) dielectric functions. Infrared-active transverseoptic phonons were observed at 770 and 1383 ${\mathrm{cm}}^{\ensuremath{-}1}$. Strong impurity- and defect-associated absorption was observed throughout the visible and near-ultraviolet region (1---5 eV) which masks, to some extent, the threshold for absorption across the electronic energy band gap. From ${\ensuremath{\epsilon}}_{2}(\ensuremath{\omega})$ and the absorption coefficient we estimate the direct band gap between $\ensuremath{\pi}$ bands to be 5.2\ifmmode\pm\else\textpm\fi{}0.2 eV. Interband transitions associated with high joint density of states were observed at 6.10 and 6.85 eV. The structure in ${\ensuremath{\epsilon}}_{2}(\ensuremath{\omega})$ at energies $\ensuremath{\hbar}\ensuremath{\omega}g5$ eV is identified with specific transitions predicted by previous energy-band calculations.

Ultrasonic-attenuation in upt3

Abstract: We report measurements of the ultrasonic absorption in UP${\mathrm{t}}_{3}$ through the superconducting transition. The attenuation varies as ${T}^{2}$ at low temperature and is inconsistent with the identification of UP${\mathrm{t}}_{3}$ as a singlet superconductor. Our results can be well explained by assuming that UP${\mathrm{t}}_{3}$ is an anisotropic (triplet) superconductor in a polarlike state, where the gap vanishes along a line on the Fermi surface.

Breathers and Photoinduced Absorption in Polyacetylene

Abstract: The dynamics of trans-polyacetylene is studied within the adiabatic model of Su, Schrieffer, and Heeger. A single soliton has a maximum velocity, ${v}_{m}$, which can be higher than the sound velocity. A long-time simulation shows that, from an initial electron-hole pair, a soliton and antisoliton emerge with velocities $\ifmmode\pm\else\textpm\fi{}{v}_{m}$ and that a persistent breather is left behind. This breather may account for the high-energy intragap structure observed in photoinduced absorption, including isotope dependence.

Photoluminescence and excitation spectroscopy in heavily doped n- and p-type silicon

Abstract: Low-temperature photoluminescence and excitation spectroscopy measurements on heavily doped (up to 4\ifmmode\times\else\texttimes\fi{}${10}^{20}$ ${\mathrm{cm}}^{\ensuremath{-}3}$) $n$- and $p$-type silicon are reported. From the luminescence spectra values for the optical and the reduced band gap are deduced and compared with theoretical calculations. The shrinkage of the reduced band gap follows an ${n}^{\frac{1}{3}}$ law for carrier concentrations $n$ above the critical Mott density. Both $n$- and $p$-type samples show an identical shift of the reduced gap, whereas the shift of the optical gap is different due to the different density-of-states masses for electrons and holes. From photoluminescence excitation spectra the position of the optical gap is determined independently. A good agreement of the data obtained by these selective absorption measurements with the results from conventional luminescence spectra is found.

Deformation potentials and electron-phonon scattering: Two new theorems

Abstract: This paper gives two new theorems about the band deformation potential ${D}_{\ensuremath{\alpha}\ensuremath{\beta}}^{b}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}},n)$ which expresses the shift of electron energies under external strains, $\frac{\ensuremath{\partial}{\ensuremath{\epsilon}}_{\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}n}}{\ensuremath{\partial}{S}_{\ensuremath{\alpha}\ensuremath{\beta}}}$, and the electron-phonon deformation potential ${D}_{\ensuremath{\alpha}\ensuremath{\beta}}^{e\ensuremath{-}\mathrm{p}\mathrm{h}}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}},n)$ which gives the matrix element for an electron to scatter from state ($\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}},n$) to a nearby state ($\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}+\stackrel{\ensuremath{\rightarrow}}{\mathrm{Q}},n$) by absorption of an acoustic phonon of branch $j$ and amplitude $\stackrel{\ensuremath{\rightarrow}}{\mathrm{u}}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{Q}}j)$, namely $〈\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}+\stackrel{\ensuremath{\rightarrow}}{\mathrm{Q}},n|{\mathcal{H}}_{e\ensuremath{-}\mathrm{p}\mathrm{h}}|\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}},n〉=i{u}_{\ensuremath{\alpha}}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{Q}}j){Q}_{\ensuremath{\beta}}{D}_{\ensuremath{\alpha}\ensuremath{\beta}}^{e\ensuremath{-}\mathrm{p}\mathrm{h}}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}},n)$. First, it is shown for a rigid ion model that ${D}_{\ensuremath{\alpha}\ensuremath{\beta}}^{e\ensuremath{-}\mathrm{p}\mathrm{h}}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}},n)$ equals ${D}_{\ensuremath{\alpha}\ensuremath{\beta}}^{b}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}},n)+m{v}_{\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}n\ensuremath{\alpha}}{v}_{\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}n\ensuremath{\beta}}$, where ${\stackrel{\ensuremath{\rightarrow}}{\mathrm{v}}}_{\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}n}$ is the electron group velocity. Second, it is shown that the Fermi surface average of ${D}_{\ensuremath{\alpha}\ensuremath{\beta}}^{e\ensuremath{-}\mathrm{p}\mathrm{h}}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}},n)$ equals the Fermi surface average of $m{v}_{\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}n\ensuremath{\alpha}}{v}_{\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}n\ensuremath{\beta}}$. From the second theorem, it is deduced that the first theorem is probably generally valid in a metal [i.e., not restricted to a rigid-ion model) provided the band deformation potential ${D}_{\ensuremath{\alpha}\ensuremath{\beta}}^{b}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}},n)$ is defined relative to an energy which moves with the Fermi energy under strains. The first theorem contradicts the common belief that the two deformation potentials are always the same, but preserves the usual form of the deformation-potential theorem at band edges where ${v}_{\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}n\ensuremath{\alpha}}$ vanishes.

Energy Transfer during Silicon Irradiation by Femtosecond Laser Pulse

Abstract: Measurements of the total energy reflected from crystalline and amorphous silicon illuminated by a femtosecond laser pulse are reported, for wavelengths of 0.62 and 0.31 \ensuremath{\mu}m. The results are interpreted by use of a highly nonlinear regime of light propagation in the dense plasma of electrons and holes ($e\ensuremath{-}h$). Densities higher than ${10}^{22}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ are reached. We demonstrate that free-carrier absorption is dominated by $e\ensuremath{-}h$ collisions.

Two-photon laser spectroscopy of xenon collision pairs

Abstract: A computer-controlled, frequency-doubled, pulsed dye laser has been used in conjunction with a sensitive and linear photodetection scheme to measure the two-photon absorption profiles of the ground to $6p{[\frac{1}{2}]}_{0}$, $6p{[\frac{3}{2}]}_{2}$, and $6p{[\frac{5}{2}]}_{2}$ two-photon transitions in xenon over a pressure range of a few Torr to 10 000 Torr. Absolute two-photon coefficients are reported. Line-shape measurements of the shift and width of the Lorentzian core allow the deduction of the long-range dispersion terms of the Xe-${\mathrm{Xe}}^{*}$ interaction potentials. An analysis of the line wings under the quasistatic approximation has permitted the extraction of the difference potentials for $3.75\ensuremath{\le}R\ensuremath{\le}8.0$ \AA{}. These short-and long-range difference potentials have been combined and used in an Anderson-Talman phase-shift calculation to reconstruct the absorption profiles. Measurements of the fluorescence following selective laser excitation have been taken and branching fractions for collisional excitation to other members of the $6p$ manifold at a xenon pressure of 100 Torr are reported.

Eu 2 + -sensitized Mn 2 + luminescence in RbMg F 3 :Eu,Mn

Abstract: Energy transfer from ${\mathrm{Eu}}^{2+}$ to ${\mathrm{Mn}}^{2+}$ ions is shown to occur in RbMg${\mathrm{F}}_{3}$:Eu,Mn crystals. Emission and excitation spectra indicate that the energy transfer is a resonant process between the ${\mathrm{Eu}}^{2+}$ $^{6}P_{\frac{7}{2}}\ensuremath{\rightarrow}^{8}S_{\frac{7}{2}}$ emission and the ${\mathrm{Mn}}^{2+}^{6}A_{1g}\ensuremath{\rightarrow}^{4}T_{2g}^{2}$ absorption levels. The energy transfer is so efficient, even at low dopant concentrations of \ensuremath{\sim}2000 ppm, that at these low concentrations energy transfer must occur between copious ${\mathrm{Eu}}^{2+}$-${\mathrm{Mn}}^{2+}$ close pairs. These ${\mathrm{Eu}}^{2+}$-${\mathrm{Mn}}^{2+}$ pairs from preferentially in the lattice. For higher ${\mathrm{Mn}}^{2+}$ concentrations the energy transfer occurs not only among ${\mathrm{Eu}}^{2+}$-${\mathrm{Mn}}^{2+}$ pairs, but also between ${\mathrm{Eu}}^{2+}$and ${\mathrm{Mn}}^{2+}$ ions.

High-pressure x-ray diffraction study of solid xenon and its equation of state in relation to metallization transition

Abstract: A high-pressure x-ray diffraction study of solid xenon was performed at pressures up to 32 GPa at room temperature using a diamond-anvil cell. The fcc structure was found to remain stable up to the maximum pressure used in the present study. An equation of state of Birch-Murnaghan type was fitted to the compression data reduced to 0 K to obtain the pressure-volume relation of solid xenon under an extremely high pressure. On the basis of this Birch-Murnaghan equation of state at 0 K and the optical absorption data reported previously, the pressure-induced metallization transition in solid xenon is expected to take place when the volume is compressed to about 10 ${\mathrm{cm}}^{3}$/mol and the corresponding pressure is about 200 GPa. In the present work, the molar volume was reduced to 16.3 ${\mathrm{cm}}^{3}$/mol at 32 GPa (${V}_{0}=34.74$ ${\mathrm{cm}}^{3}$/mol at 0 K).

Effective-medium theory of a heterogeneous medium with individual grains having a nonlocal dielectric function

Abstract: A formulation is given to obtain the effective dielectric function ${\ensuremath{\epsilon}}^{*}(\ensuremath{\omega})$ of a heterogeneous medium in which nonlocality due to the spatial dispersion of the individual grains is important. The formulation is then applied to the calculation of the ${\ensuremath{\epsilon}}^{*}(\ensuremath{\omega})$ of a medium consisting of spherical metallic grains. A very general method of solving the boundary-value problems involving a nonlocal $\ensuremath{\epsilon}$ is presented. The results are presented in both the average---$T$-matrix approximation and the coherent-potential approximation. Numerical results are obtained with a hydrodynamic model of the metallic dielectric function although in principle other nonlocal dielectric functions could also be used. Finally the optical absorption by dye molecules adsorbed on metallic spheres is calculated, and the results are compared to those obtained by using the local-dielectric-function model for the metal. The important effects of the nonlocality manifest themselves in the characteristic shifts of the resonances and in the decrease in the peak heights.

Optical constants of arc-evaporated amorphous carbon in the far-infrared spectral region

Abstract: The optical constants $n$ and $k$ of arc-evaporated amorphous carbon films between 400 and 4800 ${\mathrm{cm}}^{\ensuremath{-}1}$ were determined by measuring the transmittance and the reflectance of the films. Two broad bands with maxima at 703 and 1233 ${\mathrm{cm}}^{\ensuremath{-}1}$ in the spectral dependence of the absorption index $k$ are interpreted as a disorder-induced absorption due to tetrahedrally coordinated carbon atoms. A shoulder extending to about 1650 ${\mathrm{cm}}^{\ensuremath{-}1}$ gives reason to believe that there is also a considerable amount of trigonally coordinated carbon atoms in the film.

Interaction-Induced Phenomena

Abstract: As earlier lectures have shown, an absorption or light scattering lineshape (I(ω)) is related to molecular properties via a correlation function $$ I(w) \equiv I\left[ {\int\limits_{0}^{\infty } {dt{\text{ }}{{{\text{e}}}^{{iwt}}} {\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\thicksim}$}}{M}}(t) \bullet {\underset{\raise0.3em\hbox{$\smash{\scriptscriptstyle\thicksim}$}}{M}}(0) > } } \right] $$ (1.1) where I[…] indicates that I(ω) is some functional of the correlation function, M is the total dipole moment of the sample.

Photoionization of rare-gas-dimer excited states: Experimental measurements of absolute cross sections for Xe 2 * ( 0 u + , 1 g , 2 g ) in the ultraviolet ( 248 ≤ λ ≤ 351 nm)

Abstract: Photoionization of excited states of the xenon dimer (${\mathrm{Xe}}_{2}$) has been observed and absolute ionization cross sections have been measured for several laser wavelengths between 248 and 351 nm. Production of the excimers is accomplished by two-photon ionization of ground-state Xe atoms at 193 nm, followed by formation and the subsequent dissociative recombination of $\mathrm{Xe}_{2}^{}{}_{}{}^{+}$ $1{(\frac{1}{2})}_{u}$ ions, collisional and radiative relaxation of the Xe $6p$ and $6{s}^{\ensuremath{'}}$ manifolds, and formation of low-lying excimer states by three-body collisions. Absolute photoionization cross sections are subsequently determined at 351.1, 337.1, 307.9, 277.0, and 248.4 nm by combining a second rare-gas---halide excimer (or ${\mathrm{N}}_{2}$) laser pulse with a microwave-absorption technique to monitor the absolute photoelectron density (in real time) as a function of the intensity of the second laser pulse. The use of microwave absorption allows for the detection of photoelectrons in the presence of a high-pressure background gas. Laser-induced fluorescence and spontaneous emission studies of the temporal behavior of the populations of all of the Xe $6p$ states as well as the $6{s}^{\ensuremath{'}}{(\frac{1}{2})}_{0}$ and $6s{(\frac{3}{2})}_{1}$ levels confirm that a molecule is being photoionized. The molecular states involved are ${0}_{u}^{+},{1}_{g}$, and ${2}_{g}$ which correlate with the $6s{(\frac{3}{2})}_{1}$ excited level and a $^{1}S_{0}$ (ground-state) atom. The optical transitions associated with each laser wavelength $\ensuremath{\lambda}$ studied appear to be ${\mathrm{Xe}}_{2}^{*}{0}_{u}^{+}\ensuremath{\rightarrow}{\mathrm{Xe}}_{2}^{+} 1{(\frac{3}{2})}_{g} (\ensuremath{\lambda}=248.4 \mathrm{nm})$; ${1}_{g} (or {2}_{g})\ensuremath{\rightarrow}1{(\frac{3}{2})}_{u} (\ensuremath{\lambda}=277.0 \mathrm{nm})$; and ${1}_{g} (or {2}_{g})\ensuremath{\rightarrow}1{(\frac{1}{2})}_{u}$ for $\ensuremath{\lambda}=307.9,337.1, \mathrm{and} 351.1$ nm. The discrepancy between the measured cross sections and the values calculated previously (Lorents, Eckstrom, and Huestis, 1973) from a quantum-defect approach is roughly a factor of 2 at 308 nm but increases rapidly at shorter wavelengths. Also, in contrast to theoretical predictions for the $6s$ atomic Xe excited states in which the photoionization cross section $\ensuremath{\sigma}$ falls monotonically with photon energy, the measured $\ensuremath{\sigma}$ peaks at 7\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}18}$ ${\mathrm{cm}}^{2}$ for $\ensuremath{\lambda}=308$ nm ($\ensuremath{\hbar}\ensuremath{\omega}=4$ eV). The profile of the photoionization cross-section spectrum that is reported here for $\mathrm{Xe}_{2}^{}{}_{}{}^{*}$ is, however, remarkably similar to that calculated by Rescigno et al. [J. Chem. Phys. 68, 5283 (1978)] for the $^{1}\ensuremath{\Sigma}$ excited state of the ${\mathrm{Ar}}_{2}$ excimer ($\mathrm{Ar}_{2}^{}{}_{}{}^{*}$).

Correlation between Fe, Cr, and the broad red emission band in γ-ray-irradiated MgO

Abstract: Absorption and luminescence spectra for MgO have been obtained during $\ensuremath{\gamma}$-ray irradiation and subsequent thermal annealing. The growth and annealing of the ${\mathrm{Fe}}^{3+}$ band is attributed to the existence of various electron-trapping centers. A clear correlation is observed between Fe and Cr, with ${\mathrm{Cr}}^{3+}$ acting as an electron trap during irradiation. In addition, the results indicate that the broadband emission at 700 nm is most likely due to ${\mathrm{Fe}}^{2+}$, arising from reactions of the type ${\mathrm{Fe}}^{3+}+{e}^{\ensuremath{-}}$ and $V+{\mathrm{Fe}}^{1+}$.

Electronic structure of black phosphorus studied by polarized soft-x-ray emission and absorption spectroscopy

Abstract: Soft-x-ray emission ($K\ensuremath{\beta}$ emission) and absorption ($K$ absorption) spectra of a black P single crystal polarized along each crystalline axis are presented for the first time. The experimental results indicate (1) the upper part of the valence band near the Fermi level is largely contributed by the $3{p}_{z}$ states, (2) the $3{p}_{y}$ orbital forms the middle to lower part of the $3p$-derived (or the first) valence band, (3) the $3{p}_{x}$ states spread over a whole range in the band, and (4) the conduction band has a clear but less drastic dependence on the nature of the constituent atomic orbitals. The emission and absorption curves were numerically calculated with the results of the band calculation of black P based on the self-consistent pseudopotential method and were compared with the experimental results. Excellent agreement has been found in the features of each spectrum, although a relative displacement of the emission and absorption spectra was observed between the experiment and the calculation. This discrepancy may be attributed to the overestimation of the $3p$-band width in the band calculation, although a many-body effect in the emission or absorption process cannot be entirely excluded.

Far-infrared study of bis(tetramethyltetraselenafulvalene) hexafluoroantimonate [(TMTSF) 2 SbF 6 ]: Coexistence of metallic and semiconducting states

Abstract: Detailed reflectance measurements of ${(\mathrm{TMTSF})}_{2}$${\mathrm{SbF}}_{6}$ in the far-infrared region, above and below the metal-insulator transition temperature, are presented. The compound undergoes a phase transition at 12-14 K, and below this temperature a series of sharp lines appear in the $a$-axis polarization. We interpret these lines as "phase phonons" of the theory of Rice for the electron-phonon interaction of a one-dimensional conductor with an energy gap. Below the transition temperature a background absorption due to phonons generated by the Holstein mechanism can also be seen, suggesting that the semiconducting state coexists with a metallic state. We propose that the semiconducting state is related to the formation of a spin-density wave with an energy gap of 180 ${\mathrm{cm}}^{\ensuremath{-}1}$ (22 meV).

Symmetry-adapted linear combination of atomic orbitals bases and band-structure computation for quasi-one-dimensional solids

Abstract: A method to derive LCAO bases adapted to the symmetry of a quasi-one-dimensional crystal is presented. The complete solution is given for chains of $L{(2q)}_{q}\mathrm{mc}$ line-group symmetry in general and for the beryllium hydride polymer and tetracyanoplatinate chain in particular. We also give a quantitative account of computational reductions and discuss band touchings and slopes, crystal-field splitting, and selection rules for direct optical absorption.

Theory of a zone-boundary collective state in A1: A model calculation

Abstract: A two-band model, which previously was used successfully to evaluate the optical absorption in A1, is applied to derive the $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}$- and $\ensuremath{\omega}$-dependent dielectric function ${\ensuremath{\epsilon}}_{M}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}},\ensuremath{\omega})$ for $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}$ parallel to the [100] direction with use of degenerate perturbation theory. Within the nearly-free-electron approximation, it is shown that a pair of (200) Bragg planes gives rise to another pole in the energy-loss function $\mathrm{Im}[\frac{\ensuremath{-}1}{\ensuremath{\epsilon}}]$ and hence to a collective mode. Both the dispersion of the mode throughout the first Brillouin zone and the strength of the mode are evaluated and are found to agree very well with electron-energy-loss spectroscopy data. A detailed discussion of the nature of this mode is given. The mode is of the same origin as the so-called zone-boundary collective state (ZBCS) first proposed by Foo and Hopfield in Na. Comparison is made with a numerical calculation of ${\ensuremath{\epsilon}}_{M}(\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}},\ensuremath{\omega})$ by Singhal for some discrete $\stackrel{\ensuremath{\rightarrow}}{\mathrm{k}}$ values. The general importance of the ZBCS for the understanding of the energy-loss spectrum and for more complicated systems is pointed out.

Electron Heating due to S-Polarized Microwave (Laser) Driven Parametric Instabilities

Abstract: Microwave experiments and simulation calculations for laser parameters are reported of electron heating and microwave (laser) absorption due to parametric instabilities excited in the strongly steepened density profile near the critical surface. An approximate theory is developed that shows that $\frac{{T}_{H}}{{T}_{e}}$ should be a function of only the variable $\frac{{f}_{H}I{{\ensuremath{\lambda}}_{0}}^{2}}{{T}_{e}}$. The microwave measurements and simulation calculation results agree (\ifmmode\pm\else\textpm\fi{}20%) with this theory.

Measurement of the three-photon absorption cross section in potassium iodide at 532 nm

Abstract: The intrinsic recombination luminescence of the self-trapped exciton was utilized in the measurement of the three-photon absorption cross section in KI at 532 nm. By means of a comparative two-and three-photon absorption experiment, the value was found to be ${\ensuremath{\sigma}}^{(3)}=6\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}81}$ ${\mathrm{cm}}^{6}$${\mathrm{s}}^{2}$. Proper interpretation of the experimental observations required careful analysis of the kinetics describing the excitation and relaxation processes.

First Observation of an Identity-Forbidden Transition in Two-Photon Absorption Spectroscopy: I 1 Σ - -X 1 Σ + Transition in CO

Abstract: An identity-forbidden two-photon transition has been observed for the first time, between the ${I}^{1}{\ensuremath{\Sigma}}^{\ensuremath{-}}$ and ${X}^{1}{\ensuremath{\Sigma}}^{+}$ states in carbon monoxide. Such transitions are dipole allowed only for the absorption of two photons with different frequencies and polarization states. Only $P$ and $R$ rotational branches of the (7-0) band are observed, and the line intensities are maximized for perpendicular photon polarizations. This is the first experimental demonstration of the two-photon electric-dipole selection rule ${\ensuremath{\Sigma}}^{+}\ensuremath{\rightarrow}{\ensuremath{\Sigma}}^{\ensuremath{-}}$.

Far-infrared absorption in Cd 1-x Mn x Se: New impurity-band effects

Abstract: Far-infrared magnetotransmission measurements on $n$-type ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Se}$ imply the disappearance of optical excitation of ${D}^{+}{D}^{\ensuremath{-}}$ complexes as well as a red shift of the absorption associated with photon-induced hopping in magnetic field $B\ensuremath{\approx}1$ T. In these relatively high donor-concentration samples ($a{N}_{D}^{\frac{1}{3}}\ensuremath{\approx}0.15$), these effects can be consistently understood within a model of hydrogen moleculelike pairing of electrons localized on neighboring donors. At $B=0$ the pair ground state is the spin singlet. The Zeeman effect of the triplet level (a few meV/T) causes the ${S}_{z}=\ensuremath{-}1$ state to cross the singlet level and become the ground state. The model also explains large negative magnetoresistance observed in this material.

Presence of the D 2 h -symmetry ( T 1 + ) 2 center in alkali halides: Magnetic circular dichroism study

Abstract: Magnetic circular dichroism measurements show that the ${A}_{1}$, ${A}_{2}$, ${C}_{1}$, and ${C}_{3}$ absorption bands of ${({\mathrm{T}1}^{+})}_{2}$ center in KI and KBr have the same excited states as the ${A}_{1}$, ${A}_{2}$, ${C}_{1}$, and ${C}_{3}$ bands due to the ${D}_{2h}$-symmetry ${({\mathrm{Au}}^{\ensuremath{-}})}_{2}$ center in KCl, respectively. This confirms the presence of the ${D}_{2h}$-symmetry ${({\mathrm{T}1}^{+})}_{2}$ center in alkali halide crystals.

Picosecond coherent Raman measurements of optical-phonon relaxation in LaF 3 Ce 3 +

Abstract: An investigation of dephasing rates of some optical phonons in ${\mathrm{LaF}}_{3}$: ${\mathrm{Ce}}^{3+}$ (2 at.% ${\mathrm{Ce}}^{3+}$) crystals by picosecond time-resolved coherent anti-Stokes Raman spectroscopy is presented. At low temperature the lifetime of the 78-${\mathrm{cm}}^{\ensuremath{-}1}$ phonon is 380 ps, while those of five other phonons of g200 ${\mathrm{cm}}^{\ensuremath{-}1}$ energy are in the range $2l{T}_{1}l10$ ps. The decay of the 78-${\mathrm{cm}}^{\ensuremath{-}1}$ phonon is due to anharmonic spontaneous emission of two 39-${\mathrm{cm}}^{\ensuremath{-}1}$ optical phonons with a cubic anharmonic matrix element $〈{{B}^{(3)}}_{78 39 39}^{0 k \ensuremath{-}k}〉=0.15\ifmmode\pm\else\textpm\fi{}0.05$ ${\mathrm{cm}}^{\ensuremath{-}1}$. The decay of the other phonons studied likely occurs by an analogous process. The matrix elements for these phonons appear to be in the range 0.2-0.6 ${\mathrm{cm}}^{\ensuremath{-}1}$. The dephasing rate of the 78-${\mathrm{cm}}^{\ensuremath{-}1}$ phonon is followed as a function of temperature to ~100 K, and the rates are well fitted by an energy relaxation model involving emission to 39-${\mathrm{cm}}^{\ensuremath{-}1}$ phonons and absorption of ~40 and ~118-${\mathrm{cm}}^{\ensuremath{-}1}$ thermally excited phonons. The matrix elements for absorption processes are (0.3\ifmmode\pm\else\textpm\fi{}0.1) and (0.5\ifmmode\pm\else\textpm\fi{}0.2) ${\mathrm{cm}}^{\ensuremath{-}1}$, respectively. The rate of absorption of thermally excited 78-${\mathrm{cm}}^{\ensuremath{-}1}$ phonons is 10% of the rate of the other up-conversion processes, and it is suggested that this is due to a type of summetry selection rule for phonon-phonon scattering arising from small distortions away from a bimolecular unit cell to a hexamolecular cell, which creates two distinct types of phonons.

High Frequency Instabilities in Underdense Plasmas Produced by a 0.35 µm Laser Beam

Abstract: High frequency parametric instabilities excited when an intense electromagnetic pulse (ω0, k 0) propagates through an underdense plasma are currently under active investigation.1,2 Energy and momentum conservation give $$ {\omega _{o}} = {\omega _{1}} + {\omega _{2}}$$ and, $$ {\omega _{o}} = {\omega _{1}} + {\omega _{2}} $$ where at least one of the decay waves is an electron plasma wave (epw) with the dispersion relation $$ {\underline k _{o}} = {\underline k _{1}} + {\underline k _{2}} $$ In the underdense corona of a laser fusion pellet there are two such instabilities of particular concern; the two-plasmon decay (2ωp) instability and the stimulated Raman scattering (SRS) instability. This is because the epw’s associated with these instabilities can have phase velocities on the order of the speed of light and therefore produce very energetic electrons when they damp.3 Since such electrons can preheat the fuel in a laser fusion pellet before significant compression the 2ωp and SRS instabilities are of particular concern. In the 2ωp decay instability, the two decay waves are both epw’s so that frequency matching can occur only if ω0 ≃ 2ωp. This instability can therefore be expected to occur only near the quarter-critical layer of an inhomogeneous plasma.

Optical absorption of Mo-based alloys

Abstract: A study of the optical absorption of Mo-based dilute alloys ($\mathrm{Mo}\mathrm{Re}$, $\mathrm{Mo}\mathrm{Os}$, $\mathrm{Mo}\mathrm{Pt}$) by a sensitive differential technique is presented. The experiment is analyzed for the difference in the optical conductivity between the alloy and pure Mo. The structures observed in the differential optical conductivity are classified into two groups: One is sensitive to impurity species and the other is not. The former structures, which appear in the region of photon energy below 2 eV, are found to behave consistently with the prediction of the rigid-band approximation in the framework of the indirect optical transition model. The latter structures, which lie above 2 eV, are shown to be well described in terms of the change in optical conductivity arising from impurity-induced wave-vector-nonconserving optical absorption processes, on the basis of the electronic band-structure calculation of pure molybdenum.

Microscopic calculation of the imaginary optical potential for /sup 208/Pb(p,p) at 14 MeV

Abstract: A microscopic calculation of the second-order imaginary optical potential is made for $^{208}\mathrm{Pb}$(p,p) at 14 MeV incident energy using random phase approximation transition densities for intermediate excited states. The sensitivity of the surface absorption on details of the nuclear structure wave functions is discussed.

Giant ESR Line in One-Dimensional ( C H 3 ) 4 NMn Cl 3 (TMMC) at Very Low Frequencies

Abstract: We report on a new electron-spin-resonance (ESR) effect occurring in one-dimensional systems at very low frequencies. The ESR line is narrowed and enhanced when the chain axis is directed along the linearly oscillating field (perpendicular to the static one). In the ideal case the line becomes singular for $\ensuremath{\omega}\ensuremath{\rightarrow}0$. The phenomenon has been observed by measuring ESR absorption in a quasi-one-dimensional ${(\mathrm{C}{\mathrm{H}}_{3})}_{4}$NMn${\mathrm{Cl}}_{3}$ sample at frequencies 25 and 225 MHz. This effect is described by Bloch equations with damping on a single spin component, in perfect agreement with a microscopic theory.

Low-temperature free-carrier absorption of radiation near the absorption edge of Eu-rich EuO

Abstract: In this work we investigate the nature of the absorption lines at 1.8\ensuremath{\mu}m (0.68 eV) and 2.2 \ensuremath{\mu} m (0.56 eV) in the low-temperature absorption spectra of radiation of Eu-rich EuO. We interpret these lines as due to electronic spin-flipping transitions between spin bands split by the exchange field proportional to the magnetization of the crystal. Under simplified assumptions, we calculate the magnonassisted contribution to the free-carrier absorption of radiation. We show that this absorption is peaked at $\ensuremath{\hbar}\ensuremath{\omega}\ensuremath{\sim}{J}^{\ensuremath{'}}S$, where $\ensuremath{\omega}$ is the frequency of the radiation and ${J}^{\ensuremath{'}}S$ is the split of the spin band. Together with direct spin interband transitions (magnetic dipole transitions), our estimates qualitatively explain the observed absorption spectra below the insulator-metal transition temperature at 50 K.