A low-lying resonance in the spectrum of H

It was proved by Pekeris⁽¹⁾ that the singly excited states of H^- lie exactly at, or slightly above, the ground state of hydrogen. Using a theory of Fano,⁽²⁾ these fictitious states will have a configuration interaction with the H^- continuum. The strength of this configuration interaction is computed for the mixing of a 1s2p¹P⁰ state with the H^- continuum for different values of the fictitious binding energy of the 2p valence electron. In every case, the effect of the configuration interaction is to induce a rapid change of the phase shift of the continuum wave function by a quantity of π/2 over an energy range of a few times 0.01 eV, at an energy somewhat above the hydrogen ground state. The variation from π/2 to π is much slower. Such a swift change of the phase shift may be identified with the occurrence of a low-lying shape resonance.     

The shape of the 11S0 − 21P1 Fano resonance in the H- photo-ionization continuum

The interaction between the discrete autoionized state 1s2p of negative hydrogen and the overlapping split-configuration continuum is described in terms of stationary perturbations of nonorthogonal states that are eigenstates of two different first-order approximations to the Hamilton operator for the system.The eigenfunctions for the split-configuration states are taken to be those of a free p electron outside a neutral hydrogen atom in the ground state. The eigenfunction adopted for the short-lived, closed 1s2p configuration is the product of two functions that belong to symmetric Hartree-Fock eigenfunctions for the 1s²¹S and 2p²¹D states, respectively. This eigenfunction defines that Hamilton-operator approximation, of which the 1s2p state is an eigenstate.An improved Hamilton operator defines two different, but not independent, perturbation functions, one of which is a measure of the instability of the 1s2p configuration. The width of the resonance is found, correspondingly, to be determined by the product of two different perturbation terms. They are calculated in a Hartree-type approximation and found to be small, in part due to an effect of the nonorthogonality of the unperturbed states.The mean position of the singlet and triplet P Fano resonances in the ground state absorption continuum of H^- is found to be close to that of the diffuse interstellar bands at λ4430 and λ4760, but the calculated distance is about four times that of the bands. With exchange neglected and the position of maximum absorption fixed at λ4430 the singlet P resonance is found to have a width of 43 Å at half maximum, a minimum on the short-wave-length side, and a top absorption coefficient of 7 × 10^-16 cm²/ion, corresponding to an f value equal to 0.27.Tables of Hartree-Fock functions for the 2s²¹S, and the 2p²¹S, ¹D and ³P states of H^-, calculated by Claus Ingemann-Hilberg, are given in an appendix.     

Resonance in the infra-red spectrum of N−

The free-free continuous absorption coefficient of N^- is calculated using multichannel theory and Hartree-Fock target functions. The ³P^e shape resonance due to e-N interactions gives rise to a resonance feature in the absorption coefficient.     

Franck-Condon analysis of predissociations in the lower valence states of the NH radical

The origins of the predissociations observed by Smith, Brzozowski, and Erman for the A³Πand c¹Π states of NH have been investigated via a Franck-Condon analysis. It is shown that the measured variation of the predissociation probability is is proportional to the Franck-Condon densities between the bound rotational levels of the A and c states and the vibrational continuum of a ⁵Σ^- state arising from the ground state asympote. Similar calculations for the X³Σ^- ground state vibrational continuum, the only other state with a lower dissociation limit, showed that state to be ineffective in producing the observed predissociation.     

Thermal activation measurement of positron binding energies at surfaces

The formation of positronium by low-energy positrons incident on “clean” metal surfaces is thermally activated by increasing temperature. The activation energy E_a has been measured for a number of surfaces. E_a is understood as the energy required to form positronium (binding energy 1/2Ry) from positrons bound at the surface by an energy E_b, E_a = E_b+φ_-? 1/2Ry, where φ_- is the electron work function. Representative values of E_b derived are Al(100):3.03(5) eV; Al(110):2.92(4) eV; Cu(111):2.80(5) eV.     

Far-infrared magneto-absorption of donor-bound excitons in germanium: Pseudo-acceptor model

The magneto-oscillatory absorption spectrum of the arsenic-bound excitons in germanium observed at 118.6 μm reveals a series of absorption lines similar to the Zeeman spectrum of the acceptor impurity. This fact indicates that the bound excitons have the excited states associated with the light-hole Landau ladders and these excited states can be described by the model of a hole bound to the D^- state, i.e. the pseudo-acceptor model. The hole binding energy of the ground state of the bound excitons has been obtained to be 4.7 meV, which is smaller compared with the binding energy of the acceptor impurity.     

Optical losses in YVO4: RE (RE = Nd3+, Er3+, Tm3+) laser crystals

The relevance of processes contributing to depletion of pump and upper laser levels has been assessed based on experimental data obtained during measurement of excited state absorption, steady state emission and dynamics of excited states as a function of excitation power and activator concentration. It has been concluded that the excited state absorption in YVO₄: Nd and YVO₄: Er is not significant except for that from the ⁴ I _11/2 level of Er³⁺. In these systems, the interionic processes are dominant. In particular, the reported decrease of the YVO₄: Er laser slope efficiency when the Er³⁺ concentration increased from 0.5 to 1 at % is due mainly to the up-conversion by energy transfer from the pump level and upper laser level. Excited state absorption cannot contribute to depletion of excited states involved in the ³ F ₄-³ H ₆ laser operation near 1800 nm in the YVO₄: Tm crystal. However, the heavy doping required to enhance the cross-relaxation process which feeds the upper laser level brings about the migration-accelerated energy transfer to energy sinks.     

Resonant Raman scattering at the indirect exciton in silver bromide

By excitation in the indirect exciton absorption of AgBr at low temperatures selectively enhanced two-phonon Raman scattering is observed. Using different excitation wavelengths the resonance enhancement is found to be associated with the Г-L exciton as intermediate state for the resonant scattering process. The resonant phonons involved are pairs of LA and TA phonons with opposite wave vectors near L. Measurements in the temperature range 2 K ? T ? 40 K show a decrease of the scattering intensity with increasing temperature. The origin of this temperature dependence is due to lifetime broadening of the scattering state. Several features of the indirect exciton absorption of AgBr are discussed.     

The 1P shape resonance in electron-hydrogen scattering

The shape resonance in the ¹P channel in electron-hydrogen scattering which occurs just above the n = 2 threshold has been studied by variational calculations employing a pseudostate basis. Its parameters are E_R = 0.7512 (Ry), Γ = 0.0015 (Ry).     

Large amplitude bending motion in CsOH, studied through ab initio-based three-dimensional potential energy functions

The large-amplitude bending motion in CsOH, a ‘classical’ molecule whose microwave spectrum was first recorded in 1967, has been studied ab initio. The three-dimensional potential energy surface has been calculated at the RCCSD(T)_DK3/[QZP + g ANO-RCC (Cs, O, H)] level of theory and employed in MORBID calculations of the rotation-vibration energies and intensities. The ground electronic state is ¹Σ⁺ with the equilibrium structure r_e(Cs-O) = 2.3930 Å, r_e(O-H) = 0.9587 Å, and ∠_e(Cs-O-H) = 180.0°. The O-H moiety is bound to Cs by an ionic bond and the molecule can be described as Cs^δ+(OH)^δ-. Hence, the bending potential is shallow and gives rise to large-amplitude bending motion. The ro-vibrationally averaged structural parameters, determined as expectation values over MORBID wavefunctions, are 〈r(Cs-O)〉₀ = 2.3987 Å, 〈r(O-H)〉₀ = 0.9754 Å, and 〈∠(Cs-O-H)〉₀ = 163°. Although the averaged structure in the vibrational ground state is far from being linear, the Yamada-Winnewissi-linearity parameter for CsOH is γ₀≈-1.0, the value characteristic for a linear molecule.     

Optical absorption of small polarons bound in octahedral symmetry: V− type centers in alkaline earth oxides

The optical absorption of V^–-type centers in alkaline-earth oxides is explained as a light induced transfer of holes between equivalent O^2- sites near cation vacancy type defects, the excitation acting against the self-induced trapping potentials. The interplay between symmetry-breaking hole-phonon- and symmetry-restoring resonance-interaction is formulated as a Pseudo-Jahn-Teller effect. The model explains the width, oscillator strength, position and resonance splitting of the bands. The parameters explaining these features are consistent with the value of the ground state resonance interaction, known for MgO:V^–. The model furthermore predicts the existence of a relaxed stable excited state lying close to that determined experimentally for MgO:V^–. Transitions between nonequivalent orbitals at the equivalent O^2- sites lead to bands at higher energies than the previous ones, observed for all V^–-type centers. Generalisations of the treatment of these simple systems to optical absorption of free small polarons are given.     

The CO dimer: new light on a mysterious molecule

The present state of research on the CO dimer is reviewed. In recent years, both infrared and millimeter-wave spectra have been measured and partially assigned by the use of combination differences. A microwave-millimeter wave double resonance experiment, reported here for the first time, provides independent confirmation of these assignments and the resulting (CO)₂ energy level scheme. In the double resonance experiment, the OROTRON spectrometer functions as a supersensitive intra-cavity millimeter-wave detector. We update the continuing, but difficult, experimental efforts in recording the spectra, the quest for secure assignments, and the construction of a consistent and reliable energy level scheme. Although at present we have only limited knowledge of some aspects of the CO dimer, such as its geometrical structure, we have succeeded in characterizing unambiguously nine “stacks” of ground state energy levels with “microwave accuracy” (∼0.1 MHz). Every energy level within a given stack exhibits the same symmetry: either A⁻ or A⁺. Only transitions between A⁺ and A⁻ levels are allowed, and consequently ordinary pure rotational transitions within a stack are forbidden. Transitions between stacks can be thought of as tunneling transitions, and the separation of the lowest energy A⁺ and A⁻ states corresponds to a value of for the effective “tunneling splitting” of the CO dimer. The stacks tend to fall into two groups, corresponding to “isomers” with effective inter-molecular separations of either 4.0 or 4.4 Å. The larger inter-molecular separation of the true ground state (4.4 Å) likely corresponds to a C-bonded configuration, while the low-lying excited state with the smaller separation (4.0 Å) likely displays an O-bonded geometry.     

Investigation of negative ion states in HCl and HF by configuration interaction methods

Multi-reference configuration interaction calculations are employed for the study of Born-Oppenheimer potential energy curves in HF/HF^- and HCl/HCl^-. Large gaussian basis sets including negative ion functions as well as diffuse s, p and d AOs are employed thereby. In HCl^- a repulsive 2Σ⁺ state emerges from the calculations approximately 4·2 eV above the HCl X ¹Σ⁺ ground state; no such entity could be observed in HF^- in the energy range treated. All other CI roots which produce potential curves parallel to and above the X ¹Σ⁺ curve are found to possess quite diffuse charge distributions in the basis set variations undertaken and can therefore not be considered resonant states but rather as discrete representations of free-electron species in the HX + e^- continuum. For large internuclear distances the HF^- and HCl^- curves lie below those of the neutral species, whereby the crossing between the X ²Σ⁺ ionic and X ¹Σ⁺ curves are calculated to occur at 3·2 a ₀ in HCl/HCl^- and 2·6 a ₀ in HF/HF^-. Finally it is argued that non-adiabatic effects involving the low energy HX^- continuum states in the Born-Oppenheimer approximation and the bound HX^- species at large internuclear separations (with continuation inside the HX potential well) are ultimately responsible for observed electron scattering resonances, in accordance with recent work of Domcke and Cederbaum and of Nesbet.     

Thermal annealing effect of on optical constants of vacuum evaporated Se75S25−xCdx chalcogenide thin films

Chalcogenide glasses are interesting materials due to their infrared transmitting properties and photo induced effects exhibited by them. Thin films with thickness of 3000 Å of the glasses Se₇₅S_25−xCd_x with x=6, 8 and 10 at% prepared by melt quench technique were evaporated by thermal evaporation onto glass substrates under a vacuum of 10⁻⁶ Torr. The optical constants (absorption coefficient, refractive index and extinction coefficient) of as-prepared and annealed films have been studied as a function of photon energy in the wave length region 400-1000 nm. Analysis of the optical absorption data shows that the rule of non-direct transitions predominates. It has been found that the absorption coefficient and optical band gap increase with increasing annealing temperatures. The refractive index (n) and the extinction coefficient (k) were observed to decrease with increasing annealing temperature.     

Kernresonanzabsorption nicht Doppler-verbreiterter Gammastrahlung in Re187

Nuclear resonant absorption of gamma-rays has been observed in nuclei of Re¹⁸⁷ bound in a crystal lattice. At a temperature of 20° K a small fraction of the gammaquanta of the 134 keV transition to the ground state is emitted with essentially no energy lost to recoil, the recoil momentum being taken up by the entire crystal, not by the individual nucleus. Nuclei of Re¹⁸⁷ similarly bound in a crystal lattice have been irradiated with such gamma-rays emitted without loss of energy, resulting in an oberservable resonance absorption. Using a relative velocity of the order of 10 cm/sec the line emitted with the natural line width has been shifted away from the absorption line, resulting in the destruction of the resonance phenomena. Analysis of the variation in transmission as a function of the Doppler shift of the emitted gamma-ray yields a value ofτ=(1,5±0,2) · 10^?11 sec for the lifetime of the 134 keV excited state in Re¹⁸⁷.     

Luminescence spectroscopy of rare earth-doped oxychloride lead borate glasses

Ln-doped oxychloride lead borate glasses were studied using luminescence spectroscopy. Rare earth ions were limited to trivalent Pr³⁺, Tm³⁺, Eu³⁺ and Er³⁺. Luminescence spectra were registered, which correspond to ³P₀-³H₄ and ¹D₂-³H₄ transitions of Pr³⁺, ¹G₄-³H₅ and ¹G₄-³F₄ transitions of Tm³⁺, ⁵D₀-⁷F_J (J=0, 1, 2, 3, 4) transitions of Eu³⁺ and ⁴S_3/2,²H_11/2-⁴I_15/2 and ⁴I_13/2-⁴I_15/2 transitions of Er³⁺. Luminescence decays from the excited states of Ln³⁺ ions were analyzed in detail. The experimental results indicate that relatively high phonon energy of the host gives important contribution to the excited state relaxation of rare earth ions.     

The influence of crystal field split impurities (Tb) on the superconducting properties of LaAl2

The superconducting transition temperatures of the system La_1-xTbxAl₂ have been measured. By making use of the theory of Keller and Fulde the energy splitting between the ground state and the first excited state of Tb³⁺ is found to be 5 ± 1 K. Furthermore, we have determined the upper critical field H_c2(T) of these alloys and compared the results with those found for La_1-xGd_xAl₂. In this way the influence of the crystal field splitting of Tb³⁺ on the critical field data is clearly demonstrated.     

The nature of the predominant acceptors in high quality zinc telluride

We report very sharp bound exciton luminescence spectra in high quality melt-grown very lightly compensated ZnTe, p-type with N_A-N_D in the low 10⁺¹⁵ cm^-3. Bound exciton localisation energies at seven shallow neutral acceptors with E_A between ~55 and ~150 meV are very insensitive to E_A. Optical absorption and dye laser luminescence excitation spectroscopy were necessary to obtain a full separation of the transitions due to different acceptors, together with a study of certain ‘two-hole’ luminescence satellites in which the acceptor is left in a series of orbital states after bound exciton decay. Two shallow acceptors are P_Te and As_Te, a third possibly Li_Zn while a fourth, relatively prominent in our best undoped crystals, may be a complex. A deeper, 150 meV acceptor, frequently reported in the ZnTe literature and electrically dominant in most of our undoped crystals has the Zeeman character of a point defect. We present clear evidence from our spectra that this energy does not represent the binding of a single hole at a doubly ionized cation vacancy, a popular attribution since 1963. This acceptor may be covered by another impurity, possibly Cu_Zn. We also report bound phonon effects, lifetime broadening of excited bound exciton states and observe a single unidentified donor with E_D ~18.5 meV. This energy is determined using selective dye laser excitation at the weak neutral donor bound exciton line and from the onset of valence band to ionized donor photo-absorption.