bluuebunny/crossref_metadata_2025 huggingface dataset:
Renner Teller and Spin-orbit Perturbations in Triatomic Molecules
Authors: Alijah Alexander , Duxbury Geoffrey
Published: February 2023

Recent high resolution experimental studies of the electronic spectra of triatomic dihydrides and their ions have provided a severe test of current models of electronic orbital angular momentum-nuclear - vibrational angular momentum coupling. This coupling is associated with the breakdown of the Born-Oppenheimer separation of electronic orbital and nuclear motion, the Renner-Teller effect, and also with spin-orbit coupling.
Persistent spectral hole burning in organic materials
Authors: Jianzhao Li , Lingbing Chen , Youyuan Zhao , Fuming Li
Published: February 2023

As a technique for high resolution spectroscopy one-photon and photon—gated persistent spectral hole burning was realized in our organic materials. Hole shape, properties and quantum efficiency of hole formation were investicated. Hole growth dynamics and theory for it were given.
Molecular Spectroscopy at Ultracold Temperatures
Authors: Helmerson K. , Wagshul M. , Lett P. , Rolston S. , Phillips W.
Published: February 2023

Associative ionization (AI) in sodium (Na + Na + 2 hv → Na2+ + e−) has been a convenient reaction in which to study excited state collisions under "ultracold" conditions (≤1 mK) because the reaction product is easily identified and collected with high signal-to-noise. Previous studies [1] have revealed evidence of molecular intermediate-state resonances under high-intensity laser conditions. Theoretical analysis [2] has associated this structure with singly- and doubly-excited Na2 intermediate states. In this work, we present a new technique utilizing a switched magneto-optical trap (MOT) to make clean measurements of the spectrum of associative ionization in a regime where we can identify the peaks as being associated with states of the Na2 molecule. In addition, our technique reveals new structure in the spectrum of AI versus laser frequency, pointing out the previously unappreciated importance of atomic hyperfine structure in the reaction.
Bond Energies of Small Transition Metal Cation Clusters
Authors: Russon Larry M. , Heidecke Scott A. , Birke Michelle K. , Conceicao J. , Armentrout P. B. , Morse Michael D.
Published: February 2023

A new experimental apparatus has been constructed for photodissociation spectroscopy of transition metal-containing cations. redissociation thresholds for

Co

2
+

,

Co

3
+

, and

Ti

2
+

have been observed and values of 2.765 ± 0.001 eV, 2.086 ± 0.002 eV, and 2.435 ± 0.002 eV, respectively, have been determined for the bond energies for these species. These are in good agreement with results obtained by collision-induced dissociation (CID) experiments. Comparison of bond strengths obtained from the observation of predissociation thresholds with those obtained by non-optical methods, such as collision-induced dissociation and Knudsen cell mass spectrometry, have allowed criteria for the interpretation of a pre-dissociation threshold as a bond strength to be developed.
Spectroscopic Analysis of the Transition Metal Aluminides: AlNi, AlCu, and AlZn
Authors: Behm Jane M. , Arrington Caleb A. , Blume Thorsten , Langenberg Jon D. , Morse Michael D.
Published: February 2023

A systematic gas-phase spectroscopic investigation of the transition metal aluminides has recently been initiated in this laboratory, with the aim of providing fundamental experimental measurements relevant to the chemical bonding between the p-block and d-block metals. In this study we hope to determine in what cases the aluminum atom bonds to the transition metal through a pσ vs. a pπ approach, and to investigate the possibility of pσ-dσ and pπ-dπ bonds in these mixed main group-transition metal molecules. The fundamental results obtained through the study of the transition metal aluminides may well be relevant to the forces acting between atoms in the bulk alloys of these elements, and this may provide new insights into the metallurgy of aluminum-transition metal alloys.
Vibrational Transfer in Collisions of Li2 A 1 Σ u + ( v i , j i ) with Neon
Authors: Gorgone Peter , Davis Scott , Gao Yunxiao , McCall Eric , Rosenberry Mark , Stewart Brian
Published: February 2023

We have measured a large number of rate constants for the process where Vi,ji,Vf and jf indicate the initial and final vibrational and rotational quantum numbers of the Li2 molecule.
Ultra-Narrow Velocity Distributions of Slow Atoms Produced with the Zeeman Tuning Technique
Authors: Zilio S.C. , Bagnato V.S.
Published: February 2023

We report on the use of a stimulated Raman transition in a slowed atomic beam to produce a narrow velocity distribution of atoms in a selected electronic state, which could be used for atomic collisions studies in the low temperature regime(1). The velocity selection is part of the deceleration process and in this sense it is unique. The atomic beam is decelerated by the radiation pressure force exerted on the atoms by a counter- propagating laser beam. The resonance condition is maintained along the deceleration path because the changing Doppler shift is compensated by Zeeman tuning the electronic sublevels(2) with a spatialy inhomogneous magnetic field. At the end of the slowing process, the initial Maxwell-Boltzman distribution is compressed to a narrow velocity distribution (Δv ~ 50 m/s), centered close to v = 0. This velocity bunching(3) increases considerably the number of atoms in each velocity class, allowing the use of velocity selection techniques as a feasible way of studying low velocity collisions.
Laser Preparation and Probing of Initial and Final Angular Momentum States in the Collision-Induced Energy Transfer Ca (4s4p 3P1) + He → Ca (4s4p 3P2) + He
Authors: Spain Eileen M. , Smith Christopher J. , Dalberth Mark J. , Leone Stephen R.
Published: February 2023

The nature of atomic collisions can be revealed through the preparation and probing of aligned or oriented states. Intense interest in this research topic is evident by the numerous current experimental and theoretical investigations.1 The results of these studies provide a means of visualizing the important mechanisms and symmetries of the collision process with unprecedented detail.
High Resolution Electronic Spectroscopy of Hydroxyl-Rare Gas Complexes
Authors: Chang Bor-Chen , Williamson James M. , Dunlop James R. , Miller Terry A.
Published: February 2023

For many years, scientists have studied the spectroscopy of van der Waals complexes in past to understand intermolecular interactions and potential energy surfaces. There are abundant experiments and theoretical calculations on the closed-shell van der Waals complexes.1,2 Relatively speaking, the open-shell complexes have attracted much less attention until recently. The sparsity of information is due to both the difficulty of preparing the species and the complexity of their spectroscopy, which involves both unquenched electronic orbital and spin angular momenta. In the past four years, there have been several experimental and theoretical papers centered on Ar•OH/D and Ne•OH/D.3,4 However, except for one microwave experiment5 the previous works lack information about fine and hyperfine structure due to the resolution limit of dye laser used in the laser-induced fluorescence (LIF) studies.
Molecular collisions at very low temperature
Authors: De Lucia Frank C.
Published: February 2023

We have reported a number of investigations in which a newly developed collisional cooling method was used to investigate collisional processes between gas phase atoms and molecules at very low temperatures.1-7 In these studies the molecules are effectively in thermodynamic equilibrium with their collision partners, but at temperatures far below their freezing points. Used in conjunction with conventional equilibrium cells, measurements over the entire 1 - 1000 K region can be made. This range makes possible both the observation of new collision phenomena at low temperature and the study of collisions over a wide enough range to consider the collisions spectroscopically.
Stark quantum-beat spectroscopy: The electric dipole moment of NO (A2Σ+)
Authors: Gray J. A. , Farrow R. L. , Durant J. L. , Thorne L. R.
Published: February 2023

The γ-system (A2Σ+ X2Π) of nitric oxide (NO) is observed in many environments including combustion and atmospheric applications. Molecular properties of the A2Σ+ state are thus important in collision theories used to predict A-X radiative efficiencies. These theories often involve electric-multipole expansions,1,2 in which the permanent electric dipole moment of A2Σ+ (µA) is the leading term for long-range interactions. Moreover, µA is a basic property of the molecular charge distribution and has often been used to gauge the accuracy of ab initio quantum chemistry calculations.3-6 Here we describe measurements of Stark quantum beats in the fluorescence of NO from which we derive a precise value for µA in v'=0. This is the vibrational level most often excited in laser-based methods for detecting NO. Bergeman and Zare,7 using a radio frequency-optical double-resonance technique, provide the only other measurement of µA in a different vibrational level (v'=3). We thus also determine the change in µA with v', which provides a sensitive test of computed dipole moment functions.
Applications of High Resolution Coherent Raman Spectroscopy
Authors: Nibler Joseph W.
Published: February 2023

In recent years, the development of improved laser sources of high power and narrow linewidth has led to significant advances in high resolution Coherent Raman spectroscopies such as CARS (Coherent Anti-Stokes Raman Spectroscopy) and SRS (Stimulated Raman Gain/Loss Spectroscopy). The apparatus at Oregon State University is representative of current capabilities and consists of a cw ring dye laser whose output is increased in a three-stage dye amplifier pumped by the 532 nm output of a seeded, single frequency Nd:YAG pulsed laser. The resolution is thus limited by the Fourier transform of the amplifying pulse which is 5-10 ns for most commercial Nd:YAG lasers. Very recently, we have extended this using a custom long-pulse Nd:YAG laser of 30-50 ns 532 nm pulse duration.1
Optimization of Raman Signal Enhancement in Kretschmann Configuration
Authors: Primeau N. , Coutaz J.L. , Abello L.
Published: February 2023

Several experimental and theoretical studies [1,2,3] of Surface Plasmons Enhanced Raman Scattering (SPERS) in Kretschmann geometry (prism-metal-dielectric stacks) have demonstrated that it is possible to use surface plasmons (SP) to enhance Raman scattering from molecules adsorbed on a metal film. The enhancement process originates from two contributions: First, excitation of SP at pump frequency (ωi) and second, excitation of SP at Stokes frequency (ωs). To get the strongest Raman signal, scattered light must be detected on the prism side [3]. Indeed, the collected light comes from the outcoupling of SP at the Stokes frequency. In an experiment of SPERS, the Raman intensity depends on the distance of the active molecules to the metal surface [2]. Moreover, it has been suggested [3] that the thickness of the metal film deposited onto the prism should also be taken into account if one wants to improve the limit of detection.
New Spectroscopic Techniques on the Basis of Elliptically Polarized Pumping
Authors: Gancheryonok Igor I. , Kanematsu Yasuo , Kushida Takashi
Published: February 2023

When rather high-power polarized radiation (pump) propagates in an isotropic medium, an induced anisotropy may arise due to the nonlinear character of the interaction. This anisotropy can significantly affect the polarization of the probe field as well as its intensity. The cited effect lies in the basis of nonlinear polarization spectroscopy(NPS) and optically heterodyned polarization spectroscopy(OHPS). Usually the pump beam has linear or circular polarization and the probe one is linearly polarized. More general case occurs when pumping has elliptical polarization. In this paper we show how additional "degree of freedom"(pump ellipticity) of such a type of excitation can be advantageously applied to the investigation of initially isotropic substances.
Tutorial on Survey of Non-Medical Applications of Computed Tomography
Authors: Gordon Richard
Published: February 2023

Summary not available.
Industrial Application of Medical X-Ray Computed Tomography (CT) Scanners
Authors: Friddell K. D. , Lowrey A. R. , Lempriere B. M. , Cruikshank D. W.
Published: February 2023

The Engineering Technology Department of the Boeing Aerospace Company is conducting ongoing research into non-destructive evaluation (NDE) techniques for application to various carbon based composites. While each of the more well-established techniques such as ultrasonics, radiography, and thermography provide some qualitative information, they do not provide engineering data that can be directly applied to engineering analysis. The application of the medical x-ray CT scanner to carbon-carbon composites has opened the door to the establishment of accept/reject criteria based on engineering analysis and tests in a manner not possible in the past. Scanning procedures have been developed during the past year on General Electric (GE) medical scanners. A summary of the work accomplished to date is contained in the following sections as well as recommendations for future industrial applications.
Industrial Materials Inspection Using Computed Tomography with Few Views
Authors: Heffernan Patrick B. , Robb Richard A.
Published: February 2023

We address the problem of industrial materials inspection using computed tomography. Of particular interest is the possibility of checking for defects when there are only a few measured projections. This situation may arise because of physical restrictions on the geometry of the projection collection apparatus, or from a desire to provide rapid measurement and reconstruction in a routine inspection mode.
The use of computed tomography in the non-destructive testing of polymeric materials
Authors: Persson Sture , Östman Erik
Published: February 2023

Computed tomography was introduced in 1971 as a powerful tool in the sphere of medical X-ray diagnostics. The inventors, Houndsfield (1) and Cormack (2, 3), were awarded the 1979 Nobel Prize in Medicine for their achievement.
Application of Tomographic Imaging to Structural Ceramics: Green-State Monolithics and Ceramic-Ceramic Composites*
Authors: Ellingson W. A. , Vannier M.
Published: February 2023

Monolithic ceramic materials such as SiC, Si3N4, Al2O3, and ceramic-ceramic composite materials such as SiC-reinforced Al2O3, SiC-SiC, and SiC-reinforced glass are emerging as important engineering structural materials. A major problem with using ceramic materials in engineering structures is the lack of reliability. The low reliability is caused by unreliable mechanical properties which are usually caused by internal flaws such as cracks and pore clusters in monolithic materials and clustering of whiskers and whisker stratification in composite materials. Design methodology with brittle solids is in its infancy and nondestructive examination methods which can be used to detect the load limiting flaws are necessary before the ceramic materials can be reliably used.
Reconstruction System for Real-Time CT
Authors: Gmitro Arthur , Gindi Gene
Published: February 2023

Many applications of computed tomography either require or would benefit from "real-time" data acquisition and image reconstruction. Real-time is a rather nebulus concept that is specific to each application. However, for visual inspection of the image data, real-time may be equated with video-rate processing. There are many applications where projection data can be collected at this rate. Boyd1 has recently described an x-ray gantry that can collect all the projection data in a time on the order of a video frame time. Real-time reconstruction, on the other hand, is quite difficult and has not been demonstrated on data of any appreciable size. We describe a system for real-time image reconstruction. The system implements the filtered back-projection algorithm and is based on an electro-optic implementation of the filtering operation, an optical back-projection, and integration on a video detector. This system when coupled to a fast data acquisition system will enable on-line real-time inspection of a dynamic process (i.e., "CT video").
True 3-D Transmission Electron Microscopy
Authors: Nelson Alan Caril
Published: February 2023

A transmission electron microscope operating in the 100kV range has been designed to perform three dimensional analysis of relatively thick specimens. Specimens can be up to 8mm in diameter and over one micrometer thick, so that the third dimension is useful at sub-micron resolutions. We call our technique microtomography.
Narrow-Band Chemical-Shift Imaging
Authors: Feig Ephraim , Greenleaf Fred
Published: February 2023

Our objective is to use a priori knowledge of the NMR signal and minimal amount of data collection to obtain a chemical-shift image. Our method differs from the standard Fourier transform mapping of chemical shifts (1) in a way that allows us to reduce, often drastically, the number of observations.
Industrial Applications of Gradient Field NMR
Authors: King J. Derwin , Matzkanin George A. , Rollwitz William L.
Published: February 2023

Nuclear Magnetic Resonance (NMR) techniques and systems have been developed to provide spatial resolution, measurement localization and selective detection and measurement of solid and liquid materials having specific ranges and combinations of spin-lattice, T1, and spin-spin, T2, relaxation times. Spatial resolution and localization are achieved by use of gradient fields while multi-pulse methods have been developed to obtain T1, T2 selectivity. NMR systems utilizing Sensors based on the use of U-shaped magnets and flat, spiral-wound radiofrequency detection coils have been developed to make remote, spatially localized NMR measurements from a single surface. A schematic diagram of this approach is shown in Figure 1. The size, shape and location of the localized region from which NMR signals are obtained is determined by the magnitude and gradients of the magnetic fields. The localized region can be moved closer to, or farther away from, the sensor by varying the magnetic field strength. The NMR system incorporates an integral microcomputer for control of the data acquisition and signal processing, and includes a radiofrequency transmitter operating at 2 MHz capable of producing pulses of a controlled width and power up to 200 kW peak to insure optimum NMR detection over a specified remote region.
Maximum Entropy Tomography
Authors: Newton T.J.
Published: February 2023

Tomographic data are inevitably corrupted by noise, and the number of projections available is often small. Such data cannot define an image uniquely, but are consistent with a whole range of "feasible images". Recognising that our choice of a single image is not unique, the Maximum Entropy Method chooses the feasible image which has the greatest configurational entropy: Where pi is the proportion of intensity originating from pixel "i" and mi is the corresponding measure or initial estimate.
Maximum Bounded Entropy: a New Method of Tomographic Reconstruction
Authors: Frieden B. Roy , Zoltani Csaba K.
Published: February 2023

It is widely appreciated that the faithfulness in estimation of an unknown object depends strongly upon what is known a priori about the object. In particular, the knowledge that any object must be positive, called "positivity,” has been well exploited in both restoration schemes1-4 and reconstruction schemes.5-8 Positivity, specifically, produces the benefit of sharper edge gradients where the edge meets the known (or zero) background level,9 the extent where the gradient profile contains spatial frequencies that may exceed appreciably the cutoff frequency in the data.3,4
Image Restoration by Two-Dimensional Deconvolution in Limited-View Reconstruction
Authors: Dhawan A.P. , Rangayyan R.M. , Gordon R.
Published: February 2023

The use of a priori knowledge in restoring images reconstructed using limited data has been investigated by various authors [1-4] In these methods an attempt has been made to fill the missing spatial frequency information in the reconstructed image. The use of an. optical feedback system [2] and the use of a Fourier expansion minimization method [5] have been suggested to recover the lost information. The main disadvantge of these techniques is that they are very time consuming.
The Application of Gamma Ray Computer Tomography to Oil Recovery Studies
Authors: Nicholls C.I. , MacCuaig N. , Gilboy W.B.
Published: February 2023

During the last two years work has been published(1,2)in which medical X-ray CT Scanners were used to study the flow of fluids through rock cores in order to improve the understanding of flow processes occurring within oil reservoirs. Whilst such experiments provide a valuable qualitative insight we believe that the use of isotopic ɣ-rays, which are practically monoenergetic compared to the output of even filtered X-ray tubes, offers several advantages. In principle all measurements become absolute, in that results are repeatable and machine independent (unlike X-ray CT scanners) and since the results are quantitative, estimates of confidence limits are easily made. Monochromatic ɣ-rays do not suffer from beam hardening and radioactive sources also offer advantages in terms of cost, compactness and a well defined variation of output intensity with time - ie. exponential decay. However, the photon output may be nearly four orders of magnitude lower than that from an X-ray tube and so current scan times are ~3000 s for a single beam arrangement. It is unlikely that moving to multiple (ie fan) beams will reduce this by more than a factor of ~30 and hence it is improbable that very rapid measurements on dynamic systems with time scales below 100 s will ever be feasible.
Tutorial on Reconstruction on Families of Curves
Authors: Cormack Allan M.
Published: February 2023

Summary not available.
Time Resolved Spectroscopy of Excitons in Indirect Band Gap Superlattice Materials
Authors: Wilson B. A. , Logan R. A. , Derkits G. E. , Harbison J. P.
Published: February 2023

There is currently a great deal of interest in exciton states in semiconductor superlattice materials. Since these studies are driven primarily by possible applications in laser diode technology, this work has been restricted to direct band gap crystalline semiconductors, where large radiative matrix elements support laser action. It is likely, however, that materials with weaker radiative transitions provide a different, but equally interesting arena for study. Longer-lived excitons undergo more complete thermalization allowing studies of relaxation processes and ground state interactions. For example, in bulk materials it is in indirect band gap semi­conductors where the condensation of electron-hole droplets has provided extensive information on exciton-exciton interactions. In this paper we shall discuss recent experiments in which we use time-resolved spectroscopy to probe the dynamics of exciton states in superlattice materials with long exciton lifetimes.
Luminescence Recombination in Magnetron Sputtered a-Si:H
Authors: Rhodes A. J. , Bhat P. K. , Searle T. M. , Austin I. G.
Published: February 2023

See ThC3 fox summary.
The Excitation Dependence of the Phosphorescence of Phenazine in Ethanol AT 77K
Authors: Kallir A.J. , Suter G.W. , WiId U.P.
Published: February 2023

The phosphorescence spectrum of phenazine in ethanol at 77K is strongly excitation dependent. In the spectra shown in fig. 1b three components may be recognized: 1. A very weak component A may be observed at about 15500cm-1, i.e. at the blue edge of the phosphorescence spectrum. Its excitation spectrum is significantly different from that observed at 15370cm-1, i.e. that of the main component (see fig. 1a). 2. The main component B with its 0-0 band at 15370cm-1. 3. A weaker component C whose 0-0 phosphorescence band shifts to the red (down to about 15100cm-1) upon excitation into the red edge of the absorption spectrum.
Site Selection Spectroscopy of o- and m-Xylylenes and Their Methylated Derivatives in Shpolskii Matrices
Authors: Lejeune V. , Despres A. , Migirdicyan E.
Published: February 2023

The highly reactive o- and m-xylylenes and their methylated derivatives are biradicaloid species which exist as transient intermediates in chemical reactions. They result from the photodissociation of CH bonds on two different methyl groups of methylbenzenes. Many years ago, we have reported the first direct characterization, by electronic spectroscopy, of these fragments photolytically generated "in situ" from the corresponding methylbenzenes in organic glasses(1) and then in polycrystals(2). Here we present the laser induced fluorescence and excitation spectra of o- and m-xylylenes and their methylated derivatives trapped in well defined lattice sites of Shpolskii matrices at 5-10 K.
Luminescent Properties of Lamellar Solids Derived from Hydrogen Uranyl Phosphate and Cr(III) Werner Complexes
Authors: Olken Michael M. , Verschoor Carla M. , Ellis Arthur B.
Published: February 2023

The emissive layered solid HUO2PO4·4H2O (HUP) readily undergoes intercalative ion exchange reactions with a variety of cationic species, leading, in many cases, to substituted lamellar materials which retain the luminescence characteristic of UO22+.1 We have recently carried out such substitution reactions using the following Cr(III) Werner complexes: Cr(NH3)63+; Cr(NH3)5(H2O)3+; Cr(ethylenediamine)33+ (Cr(en)33+); Cr(2,2′-bipyridine)33+(Cr(bpy)33+); and Cr(urea)63+.
Luminescence Studies of Interacting Metal Complexes in Two Dimensions
Authors: Patterson Howard H. , Roper Gerald , Ludi Andreas , Blom Nils
Published: February 2023

Salts of AAu(CN)2 (A = K+, Rb+, Cs+, Tetrabutylammonium) in the solid state have an interesting two-dimensional layered structure for the gold atoms. For example, for KAu(CN)2 individual layers of gold atoms at room temperature are separated by 9.2 Å while the nearest neighbor gold distance in a layer is 3.36 Å. When the cation is changed, or the temperature is changed, the Au-Au nearest neighbor separation varies.
Emission from Photoproducts of Benzene
Authors: Dharamsi A. N. , Shahmirian V.
Published: February 2023

In our investigation of charge-transfer complexes, we have observed emission from the photoproducts of benzene in the gaseous phase. KrF 248 nm radiation from an excimer laser was incident upon the experimental gases contained in a glass vessel with suprasil windows. Emission was detected in the orthogonal direction by a photomultiplier tube mounted at the exit slit of a monochromator. The incident energy at the input window at 248 nm was ~400 mJ (beam area ~2 cm2, FWHM Δt~20 ns, FWHM Δv~5 nm) . Benzene (Fisher, 99.95% pure, Thiophene ≲ 0.5 ppm) and O2 (99.996% pure) were used without further purification. The cell was evacuated to pressures ~10-7 mbar and flushed out at least twice before experiments were performed.
The Dual Fluorescence of 9,9′-Bianthryl - A Study on Excited State Dipole Moments
Authors: Baumann Wolfram , Spohr Eckhard , Bischof Hilmar
Published: February 2023

The influence of an external electric field on the intensity and spectral position of the fluorescence of solute bianthryl has been studied in various solvents of different polarity. Two different interpretations of the experimental results are given.
Mobility and Conformation of Side Chains in Copolymers of the Methacryl Amide Type: Study of Depolarization of Fluorescence and Nonradiative Energy Transfer
Authors: Mikes Frantisek , Vyprachticky Drahomir , Kralicek Jaroslav , Laosky Jiri
Published: February 2023

See WE20 for summary.
Luminescence Spectra and Kinetics of various Radical Pair Products following Photochemical Hydrogen Transfer in Doped Fluorene Single Crystals
Authors: Borczyskowski C. v. , Steidl P. , Stehlik D.
Published: February 2023

We have shown previously that photoexcited phenazine doped as acceptor in a fluorene single crystal is able to abstract a hydrogen atom from the methylen group of a specific neighbor host molecule. Reaction product is a radical pair (RP) triplet state the molecular structure of which is known in detail /l/. Precursor is the photoexcited triplet state of the phenazine acceptor. The H-transfer is fully reversible. At room temperature the RP product has a lifetime of only ~0.2 μs.
Proposal for New Experiments for the Investigation of Transport Coherence of Excitons in Molecular Crystals
Authors: Kenkre V. M. , Tsironis G. P.
Published: February 2023

A central open fundamental question in exciton dynamics in molecular crystals concerns transport coherence of excitons, particularly at low temperatures. A large amount of effort has been spent over many years on this issue but clear answers have not emerged. Old suggestions that the temperature dependence of energy transfer rates is indicative of transport coherence have been shown to be untenable, or at least inconclusive, since a variety of alternate factors having little to do with coherence can be shown to lead quite reasonably to explanations of those observations. Careful reanalysis1,2 of sensitized luminescence experiments is making clear that the role that capture plays in them (relative to exciton motion) can be considerable and that that category of observations is not well suited to investigations of coherence. We have recently found3,4 that Ronchi grating experiments constitute an ideal candidate for definitive coherence studies. The observable is the time-dependent delayed fluorescence arising from mutual annihilation of triplet excitons. The parameter under experimental control is the extent of the spatial inhomogeneity of the triplets manipulated through the ruling period of the Ronchi grating covering the crystal during illumination.5 We present a detailed analysis of the suggested experiment and describe the expected effect of two separate factors on the outcome of the experiments: (i) finite degree of coherence and (ii) finite rate of vibrational relaxation.
Vibronic Excitons in Crystalline Anthracene
Authors: Lagos Roberto E. , Friesner Richard A.
Published: February 2023

In a recent paper(1) we presented a cluster generalization of the Dynamical Coherent Potential Approximation (DCPA). When applied to exciton-phonon interactions in molecular crystals the theory represents a distinct improvement over perturbative and single site DCPA calculations. We also extended our formalism to the case of several molecules per unit cell. Our model assumed throughout one harmonic vibrational mode per molecule.
Role of Viscosity Effect on Photoisomerization
Authors: Villaeys A.A. , Boeglin A. , Lin S.H.
Published: February 2023

Many rate processes like diffusion, dielectric relaxation, electron transfer reactions, nonradiative decay etc..., in dense media can be treated from a unified quantum statistical mechanical viewpoint [1]. In these unimolecular rate processes, one can regard that two potential surfaces, say a and b, are involved. It is assumed that when the two potential surfaces cross, the resonance interaction is small. A general theoretical approach has been developed that will treat these rate processes by properly choosing the perturbation associated with each rate process.
Nd:YAG Laser-Induced Visible Fluorescence from Singlet Molecular Oxygen Generated in NaOCl-H2O2 Chemical Reaction System
Authors: Inaba Humio , Yamagishi Akio
Published: February 2023

The oxygen molecule is known to absorb and emit photons by single-molecule transitions as well as cooperatively by pair-molecule transitions or transitions involving O4 complexes1,2. From the excited singlet states 1Δg(0) and 1Δg (1), near infrared emissions at 1.06 μm and 1.27 μm take place as showin in Fig. 1, although they are generally weak due to the forbidden nature of the transitions. Also it is to be noted that the excited-state oxygen pair molecules Produce fluorescence spectra in the visible region as indicated in Fig. 1. Since the metastable 1Δg (0) state is the first excited one, the fluorescence lines from the 1Δg (0)· 1Δg (0) state at 630 nm and 700 nm exhibit rather stronger intensities than those for other transition lines. However, the transition from the ground

3

Σ
g
−

(
0
)

state to the excited 1Δg (1) state is almost on resonance with the Nd:YAG laser output at 1.06 μm. Hence strong absorption of this radiation provides large concentrations of singlet oxygen molecules in this state from which various excited states are induced by energy transfer processes2.
Saturation Effects of the 2 eV-Photoluminescence in CdMnTe
Authors: Müller E. , Gebhardt W.
Published: February 2023

The luminescence in the random system Cd0.45Mn0.55Te which is due to a 4T1 → 6A1 transition within the Mn2+-ions covers a phonon-broadened distribution of 4T1-states.1) The data of fig. 1 are obtained from a cw-experiment using a focussed Ar-laser beam at 514 nm. The luminescence was recorded at the short- and longwavelength side of the band at 580 and 680 nm respectively. Both intensities show a nonlinear increase with laser power, but only that from the 580 nm side saturates.
Exciton Holeburning in GaAs/GaAlAs Multiquantum Wells
Authors: Hegarty J. , Sturge M. D.
Published: February 2023

The lowest energy two-dimensional exciton in GaAs/GaAlAs multiquantum wells (MQW) is inhomogeneously broadened at low temperature by fluctuations in the layer width. The broadening permits probing of the excitons as a function of position within the inhomogeneous line[1]. At high densities interaction between the excitons becomes important and leads to saturation and to an increase of the homogeneous linewidth[2]. Using the picosecond pump/probe technique we have measured the saturation behavior as a function of exciton energy within the inhomogeneously broadened line. Figure 1 shows the transmission spectrum T of a weak narrowband probe (dotted) and the change in transmission ΔT of the probe induced by the presence of a pump for two different pump wavelength for 102 Å thick layers of GaAs separated by 200 Å layers of AlGaAs. An increase in transmission (holeburning) at the pump wavelength is observed showing that the interaction between resonant excitons is much stronger than between nonresonant excitons. The strength of the hole decreases rapidly on passing through the center of the line towards the high energy side. This is consistent with an increase in the homogeneous linewidth of the higher energy excitons[1,3]. By delaying the probe we measured the rate of decay of the hole which was found to depend strongly on pump intensity. At low intensity the hole lifetime approaches the luminescent lifetime but becomes shorter at exciton densities greater than ~109 excitons/cm2/layer. We attribute this to exciton-exciton scattering leading to spectral diffusion. The behavior was similar in 51 Å and 102 Å layers. In addition to holeburning, Figure 1 shows an increased absorption in the wings of the line whose shape is almost independent of pump wavelength and which is similar to that observed in bulk GaAs[2].
Photoluminescence Studies of Ga(As,P) Strained-Layer Superlattices†
Authors: Gourley P. L.
Published: February 2023

Semiconductor Strained-Layer Superlattices (SLS’s) are composed of thin (few hundred Å) alternating layers of lattice-mismatched materials that grow in a coherently strained condition which avoids the formation of misfit dislocations.1 The growth of SLS's permits the freedom to combine lattice-mismatched semiconducting materials in a superlattice structure. These SLS's form a new class of semiconductors with useful and tailorable electronic properties.2
Fast Carrier Expansion Effects in the Optical Spectra of CdTe
Authors: Schweizer H. , Zielinski E. , Forchel A. , Mahler G.
Published: February 2023

We have investigated the spectra of the spontaneous and stimulated emission and transmission of CdTe at temperatures below 50K using a N2-laser pumped dye laser for the plasma excitation. Fig. 1 depicts the spatially resolved results of luminescence measurements in a back-scattering geometry. In going from the edge to the centre of the excitation spot of approximately 40 μm diameter we observe a strong line width narrowing. At intermediate positions even two distinct emissions are observed. We attribute this to a fast carrier drift from the surface into the bulk of the sample, resulting in a spatial carrier distribution which - particularly at the center of the excitation - extends deep into the sample. The spatial variation of the luminescence is explained straightforwardly, if the two bands in Fig. 1 are attributed to the spontaneous and stimulated emission of the plasma: The stimulated emission is observed mainly at the center of the excitation since the optical amplification is strongly enhanced by the wide spatial range over which the carriers are distributed. If the plasma were limited to the absorption length of the laser, the optical path would be too short for a significant amplification.
Emission of Free and Bound Excitons in GaSe and InSe Crystals in the Direct and Indirect Transitions Region
Authors: Gnatenko Yu. P. , Skubenko P.A. , Zhirko Yu. I. , Fialkovskaya O.V.
Published: February 2023

The layered GaSe and InSe crystals are characterized by the presence of a great number of intensive emission lines, located near the emission line of a free exciton. The presence of various crystalline modifications notably complicates their interpretation.
Optical Spectra of Excitons in Photon Fields
Authors: Toyozawa Y.
Published: February 2023

Dynamics of excitons in the phonon field and its reflection in the absorption, resonance Raman, and luminescence spectra are studied with particular attention to localization versus the delocalization.
Characterization of the luminescent triplet state of the K2Cr2O7 crystal at 1.2 K via EPR and optical spectroscopy
Authors: van der Waals J.H. , van der Poel W.A.J.A.
Published: February 2023

We are currently investigating the luminescent state of crystals in which the anion is a transition metal complex with the metal formally in a do configuration, tetrahedrally surrounded by oxygen. Examples under study are VO43−, MoO42−, and also the Cr2O72− ion in which two tetrahedra share a common oxygen. First we established the triplet character (S=1) of the emitting state of this type of ion - which had been proposed [1] but never proven [2] - in some qualitative ESR experiments with optical detection on YVO4 and K2Cr2O7 crystals. We then initiated a systematic study of the latter system, the results of which are here reported, by three experimental techniques [3]:

1. Excitation and fluorescence spectroscopy with simultaneous (variable frequency) microwave irradiation to induce ESR transitions between the sublevels of a spin triplet in zero field.

2. ESR with optical detection in an external field.

3. Optical Zeeman spectroscopy.
Low Temperature Luminescence Studies of Two Polymorphs of Bis(Urea) Dioxouranium(VI) Sulfate
Authors: Brittain Harry G. , Perry Dale L.
Published: February 2023

Recently, Toivonen and Niinisto reported that UO2SO4·(urea)2 is capable of existing in two crystalline forms. The structures of these are very similar, with the uranium atom lying at the center of a pentagonal bipyramid formed by oxygen atoms. Of the five equatorial oxygens, three belong to sulfate groups and two to urea molecules: The bridging tridentate sulfate groups (labeled as 3,4,5) join the bipyramids into infinite double chains, and the α and β polymorphs differ in how these chains are packed.
Dosimetry by Thermoluminescence: Datation of Peruvian Pottery
Authors: Quiñones E. F. , Carranza E. López , Rozenberg C. , Bonnier E.
Published: February 2023

Natural radioactivity can produce measurable quantities of radiation damage in rocks and buried pottery and its gradual accumulation in time gives us a “clock” which can be used for datation in Geology and Archaeology. Thermoluminescence (TL) is used to estimate the accumulation of radiation damage in pottery. In this paper we present first results in the datation of some of the sherds coming from the archaeological site Piruro, Tantamayo, Huánuco, situated at 3800 m of altitude in the Peruvian Andes, excavated by Rozenberg and Bonnier. Two of the sherds, the 3063 and the 3071, come from the same layer and have the ages 910±140 and 840±100 respectively which give for the context the age 870 (±30, ±90) A.D. 1110. Other two sherds, the 3131 and the 3162 have the ages 520±60 (A.D. 1460) and 580±80 (A.D. 1400) respectively. All ages are in years refered to 1980.
Fluorescence in the One-Dimensional Ferromagnet CsNiF3
Authors: Soscia M. , Cibert J. , Terrile M.C. , D’aubigne Y. Merle
Published: February 2023

The nature of optical excitations in low dimensional stoechiometric compounds is of current interest. In one-dimensional antiferromagnets (TMMC, CMC)(1) the excitation is reported to be localized. However in rare-earth systems(2) a double-peaked lineshape has been attributed to the dispersion of the exciton.
Deformation-Induced Tunneling in II-VI Semiconductors and their Mechanoluminescence
Authors: Chandra B. P. , Majumdar B.
Published: February 2023

Luminescence produced during mechanical deformation of solids is known as mechancluminescence (ML) or triboluminescence. A large variety of organic and inorganic compounds exhibit the phenomenon of ML [1-3]. The present paper reports the mechanism of ML excitation in II-VI semiconductors.
Laser Spectroscopy of Point Defects in CdF2 on the Transition to a Semiconductor
Authors: Mho Sun-Il , Wright John C.
Published: February 2023

Defect equilibria in fluorite materials have been extensively studied. The behavior does not agree with that expected from simple mass action relationships. Competing equilibria caused by extensive defect clustering are found to be very important such as scavenging of interstitial fluoride

(

F
i

′

)

by the clusters to make negatively charged clusters (1). CdF2 is the only one of the fluorites that can be converted to a semiconductor by heating. It is also a system where scavenging equilibria might be particularly important because the higher electron affinity of Cd2+ would result in stabilization of the scavenged clusters by delocalization of the excess negative charge.
Nucleation of Defects in Alkali Halides
Authors: Ramponi Albert J. , Wright John C.
Published: February 2023

Single crystals of KCl:Sm2+ have been investigated using selective laser excitation into the 5D0 and 5D1 levels of the 4f6 electron configuration for Sm2+ as a model system for the study of point defects in the alkali halides. Fluorescence occurs in the red and near ir via relaxation from the 5D0 excited state down to the various components of the 7FJ. ground state multiplet.
Simple Semicontinuum Vibronic Model of the F Center
Authors: Georgiev M.
Published: February 2023

This is an extension of our earlier work on the F center1. We had calculated the thermal ionization rates of excited F centers (F*) in KCl using experimental data on the F center photoionization efficiency and luminescent lifetime. When plotting them versus the respective reciprocal temperatures, considerable deviations from the Arrhenius temperature behavior were found to occur below 100 K leading to a nonvanishing residual ionizability at low temperature. These were suspected to result from a strong lattice tunneling at the lower temperatures. A reaction-rate theory was applied for a quantitative analysis which expresses the rate constant in terms of four adjustable parameters: vibrational frequency ω, lattice-reorganizational energy Er, reaction heat Q at 0 K, and electron-exchange integral V12.2 The ionization transition was regarded as the one in which the F* electron transferred to a polaron state by virtue of the strong interaction with a promoting mode. Very reasonable values were obtained for the fitting parameters, as the theory was compared numerically with the experiment. V12 was also estimated independently within the BWK approximation resulting in a remarkable agreement between fitted and computed values.
The Calculation of the Formation Energies of Point Defects
Authors: Runciman W.A. , Srinivasan B. , Richardson D.D.
Published: February 2023

There are many different centres formed when uranium is incorporated into LiF or NaF in an oxidizing atmosphere.1 Some of these centres are due to the presence of additional impurity ions, such as magnesium. It is also possible that lattice vacancies may be present. It was decided to calculate the formation energies of different models for centres in order to find the most likely candidate for the observed centres. In general, we require to calculate the interaction energies due to the presence of the defect and the lattice relaxations around the defects. It is assumed that changes in the electronic band structure due to the defect are negligible. This appears reasonable since screening and electronic relaxation will be negligible in an electronic insulator such as LiF. However the remaining calculations are still formidable as there are non-Coulomb short range interaction potentials as well as the Coulomb potentials. In order to make a preliminary survey of a large number of types of centre calculations were made using the Coulomb potential only and without lattice relaxation. Ions are assumed to substitute for lattice ions and are assigned the conventional ionic charges. Using the Madelung constant for the alkali halide lattice the calculations can be reduced to simple hand calculations. In practice we used a programme which could also cope with non-Coulomb potentials.2 There is a problem in finding suitable short-range potentials and this more than complexity of calculation was the reason for their initial neglect. Calculations were made for 57 varieties of uranium centres in lithium fluoride which showed local charge compensation.
Lasing from A Vibrational Transition of a Molecular Defect
Authors: Gosnell T. R. , Tkach R. W. , Sievers A. J.
Published: February 2023

Infrared fluorescence from CN− defects in alkali halides was first observed by Yang and Luty1. Recently, we have observed lasing in the KBr:CN system. This is the first report of a laser operating between pure vibrational levels of a defect in a solid. While CN− will fluoresce at high temperatures, (up to 500 K), lasing is only observed at temperatures below 4 K. This is due to the nature of the pumping.
Slow Transitions in Diamond: the Photoluminescing S1 Centre
Authors: Pereira M. Estela , Isabel M. , Jorge B. , Thomaz M.F.
Published: February 2023

The luminescing S1 centre in diamond has some characteristic features, eg, its lifetime in the milisecond region and two zero-phonon lines 34 meV apart with temperature dependent intensities and different vibronic structure. Although some preliminary studies on it have been carried out (1) a detailed study of this interesting centre has not yet been reported. We found this centre present in the wide class of the so called brown diamonds whose characteristic centres began to be studied recently (2, 3) and studied it under steady state conditions and by time resolved spectroscopy.
Spectral Structure of Secondary Emission in Indirect Exciton-Phonon System
Authors: Iida T. , Sakai M.
Published: February 2023

We study the secondary emission due to recombination of excitons near the indirect band bottom, where the relaxation is governed by the multiple scattering by acoustic phonons and a nonradiative decay process. In this case, the secondary emission can be separated exactly into the Raman scattering (SR) and the hot luminescence (SL) parts.1,2) General expressions of the excitation spectrum (IR) of SR and the lineshape of SL are presented. The nonradiative decay is attributed to the scattering by imperfections, and therefore the decay rate σ represents the sample dependence. Near the band bottom, the scattering by acoustic phonons is quite sensitive to temperature and the kinetic energy of exciton (Eac) moving with the sound velocity. From the temperature dependence of IR, we can determine σ and Eac uniquely. It should be emphasized that in our theory, only σ and Eac values are necessary for the calculation of SL.