Showing papers on "Absorption spectroscopy published in 1982"

Vibronic interactions in Nd:YAG resulting in nonreciprocity of absorption and stimulated emission cross sections

Abstract: In this paper we discuss the quantum efficiency of the metastable4 F_{3/2} state in Nd:YAG. We show that the vibronic interaction disrupts the equality between absorption and emission cross section ("reciprocity") upon which spectroscopic quantum efficiency measurements are based. Even if the electron-lattice coupling is weak, a phonon whose energy matches the energy difference between two electronic states can have a significant effect. Our spectroscopic measurements clearly show nonreciprocity and establish the importance of vibronics in Nd:YAG. The data are consistent with a high quantum efficiency.

Infrared Vibrational Predissociation Spectroscopy of Water Clusters by the Crossed Laser-molecular Beam Technique

Abstract: Water clusters formed in a molecular beam are predissociated by tunable, pulsed, infrared radiation in the frequency range 2900–3750 cm−1. Absorption spectra of the clusters are obtained by detecting the recoiling fragments off‐axis from the molecular beam as a function of laser frequency using a rotatable mass spectrometer. By carefully adjusting the expansion conditions of the molecular beam and monitoring the largest cluster observable, excessive contamination by clusters larger than the specific one of interest is avoided. It is found that the spectra of clusters containing three or more water molecules absorb over the same frequency range as the liquid. Dynamical information on the predissociation process is obtained from the measured angular and velocity distributions of the fragments. An upper limit to the excited vibrational state lifetime of ∼1 μs is observed for the results reported here. The most probable dissociation process concentrates the available excess energy into the internal motions of...

Association of Inclusion Compounds of β-Cyclodextrin in Aqueous Solution

Abstract: Inclusion compounds in the systems naphthalene–β-cyclodextrin (CDx) and 2-methoxynaphthalene (MN)–o-dicyanobenzene (DB)–β-cyclodextrin in aqueous solution were studied by means of absorption and fluorescence spectra. In the former system a naphthalene excimer fluorescence was observed. By the analyses of the concentration dependence of the absorption spectra and the excimer fluorescence intensities, the species responsible for the excimer fluorescence is identified as a 2:2 inclusion compound which is formed by association of two 1:1 naphthalene–CDx inclusion compounds, not a 2:1 inclusion compound in which two naphthalene molecules enter into the cavity of one CDx molecule. In the latter system, absorption and fluorescence spectra due to a charge transfer complex of MN with DB were observed. By analyses similar to the naphthalene case, the existence of a 1:1:2 association compound formed between a MN–CDx inclusion compound and a DB–CDx inclusion compound is confirmed. Fluorescence quenching by I− and IO3...

Numerical Experiments on the Absorption Lineshape of the Exciton under Lattice Vibrations. I. The Overall Lineshape.

Abstract: A standardized formulation is introduced for the lineshape problem–the effect of lattice vibrations on the exciton absorption, and used in a Monte-Carlo calculation of the density of exciton states and the absorption spectra in the regime of weak scattering. For the direct edge, we obtain the asymmetric lineshape with a strongly decaying low-energy side while the high-energy side can be approximated by a life-time broadened Lorentzian. The linewidth is found to depend on temperature as T 2/3 , T 1 and T 3/2 for one-, two-, and three-dimensional lattices in agreement with renormalized perturbation theory in one and two dimensions. The discrepancy of the perturbational result ( T 2 ) in three dimensions is ascribed to the density of states which is most sensitive to disorder in this case. The shift of the absorption peak is found to be linear in T for low T , the origin of its sublinear behaviour for higher T is clarified.

Extended-x-ray-absorption-fine-structure study of small Fe molecules isolated in solid neon

Abstract: We have used rare gas matrix isolation techniques in combination with extended x-ray absorption fine structure (EXAFS) to study the variation in interatomic distances for small Fe molecules in solid neon. A considerable contraction in the interatomic distances was observed for the metal molecules. An Fe-Fe distance of 2.02 \ifmmode\pm\else\textpm\fi{} 0.02 \AA{} for the lowest concentration of metal was observed. This is in good agreement with early EXAFS measurements in ${\mathrm{Fe}}_{2}$-Ar. We also carried out a careful study of the x-ray-absorption near-edge structure (XANES), and observed the appearance of considerable structure for a 1.5-at.% Fe sample. The XANES spectra were analyzed in terms of $1s$-to-($3d,4s$) and $1s$-to-$4p$ transitions.

Excited state geometry changes from preresonance Raman intensities: Isoprene and hexatriene

Abstract: A method is presented for using a single preresonance Raman spectrum and an absorption spectrum to obtain changes in equilibrium geometry upon electronic excitation. The relative displacements along each of the vibrational normal coordinates are obtained from the Raman intensities, while the overall scaling of the displacements is determined by the absorption band shape. The absorption spectra, as well as Raman excitation profiles, are calculated using either a sum over vibronic states or a formally equivalent time‐dependent method [S.‐Y. Lee and E. J. Heller, J. Chem. Phys. 71, 4777 (1979)]. The time‐dependent method is computationally much faster than the vibronic sum for large multidimensional systems. Our analysis, which assumes isolated molecules and separable, harmonic surfaces, yields a good fit to the vapor phase absorption spectrum of trans‐hexatriene with a Lorentzian linewidth of 175 cm−1. However, the diffuse absorption spectrum of isoprene cannot be adequately reproduced using Lorentzian line...

Relationship between the area of L2,3 x‐ray absorption edge resonances and the d orbital occupancy in compounds of platinum and iridium

Abstract: The relationship between the intensity of the L 2 and L 3 x‐ray absorption threshold resonances and d orbital occupancy has been investigated for a series of compounds of platinum and iridium and the pure metals, using Xα‐SW molecular orbital calculations to obtain the unoccupied d orbital states. The change in d orbital occupancy caused by formation of the core hole was found to be fairly constant in different compounds and the pure metal. The difference between the d orbital occupancy in platinum metal and the d orbital occupancy in a given platinum compound agrees well with the calculated effective charge on the platinum atom in the compound. The areas of the threshold resonance lines were obtained by a deconvolution of the absorption edge into a Lorentzian component and an underlying ’’step’’ representing the onset of absorption to continuum states. For a series of platinum compounds, a linear relationship was obtained between the unoccupied d orbital states calculated in the core hole potential, and the sum of the areas of the L 2 and L 3 threshold resonance lines (corrected for the different degeneracies of the initial state). However, this relationship could not be applied to IrO2 because of the absence of transitions to empty d states at the top of the valence band.

Transition strengths in the visible–infrared absorption spectrum of I2

Abstract: The contributions of the 1Πu←X, B←X, and A←X transitions to the absorption coefficient of I2 in the 4200–8000 A region are reassessed in light of recent results concerning the 1Πu and A potentials and their diffuse absorption spectra. Earlier estimates of the continuum underlying the B←X discrete spectrum are augmented by additional high‐resolution ’’between‐the‐lines’’ absorption measurements, corrected for residual B–X absorption using the very reliable spectroscopic constants now available for this system. The 1Πu–X and A–X systems are now estimated to be about 10% weaker than in the previous analysis [J. Tellinghuisen J. Chem. Phys. 58, 2821 (1973)]. For the region 2.6–2.8 A sampled in absorption, the estimates of the B–X transition strength ‖μe‖2 remain close to the previous values.

The effect of the metal atom on the absorption spectra of phthalocyanine films

Abstract: The visible and near ultraviolet absorption spectra have been measured for thin films of H2Pc [C32H18N8], MgPc, FePc, CoPc, CuPc, and ZnPc. Magnesium and iron phthalocyanine have very different absorption spectra. For most of the phthalocyanines, structure of magnitude 0.21 eV is seen on the visible and Soret bands. A d band is indicated in the region 4.0–4.7 eV. This is the spectral region most sensitive to substitutions of the central metal atom.

Photodissociation processes in the OH molecule

Abstract: The potential energy curves of several excited states of OH of 2Sigma(+), 2Sigma(-), 2Pi, and 2Delta symmetries, and the transition moments connecting the excited states with the ground state and with each other, are calculated using two previously published theoretical models. The importance of the excited states in photodissociation is explored. Direct photodissociation by absorption into repulsive electronic states, as well as the contribution of absorption into bound electronic states, are discussed. Because they may participate in the photodissociation processes, several quartet states and one of the sextet spin multiplicity are investigated. The mechanism by which bound states can be dissociated are considered and estimates of the dissociation efficiencies of the mechanisms are made. Absorption into the bound 3 2Pi state may be an important dissociation channel. It is shown that OH is dissociated by the absorption of Lyman alpha radiation, a channel of particular significance in shocked interstellar gas and in cometary atmospheres.

Ultraviolet Transmission Characteristics of a Fluorophosphate Laser Glass

Abstract: Ultraviolet transmission characteristics of LG-810 fluorophosphate laser glass were investigated as a means of diagnosing transition metal contamination. Absorption bands normally exhibited by this glass were shown to derive primarily from metallic impurities. Uv-transparent glasses were produced having absorption cut-offs close to those reported for BeF/sub 2/-based glasses.

Electron Spectroscopy for Atoms, Molecules, and Condensed Matter

Abstract: The basic theoretical considerations involved in developing instruments for electron spectroscopy for atoms and solids are discussed. To be competitive with x-ray emission or absorption spectroscopy methods, electron spectroscopy would have to achieve such high resolution that well-defined electron lines would be obtained with line widths equal to or close to the atomic levels themselves. Chronology of development of equipment to achieve these demands on resolution is discussed. Successful instrument development led to application of electron spectroscopy to the chemical analysis of all elements in the periodic system and the coining of the acronym ESCA (electron spectroscopy for chemical analysis). Numerous applications of ESCA are described. A brief overview of recent development of new electron spectroscopic instrumentation is included, and several reviews and books published on the subject during the 1970s are among the 260 references cited. (BLM)

Excitation spectra of surface-enhanced Raman scattering on silver-island films

Abstract: Both the spectral dependence and the magnitude of the excitation spectra of surface-enhanced Raman scattering are found to be quantitatively related to the absorption spectra for silver-island films. This relationship is maintained even when the local electromagnetic enhancement at the islands is varied by coating the film with a layer of absorbing dye molecules. The results illustrate a simple experimental way to determine independently the role of the electronic plasma resonances in the enhancement process on these films and shed new insights into the details of the electromagnetic interactions responsible for surface-enhanced Raman scattering.

Frequency Stabilization of AlGaAs Semiconductor Laser Based on the 85Rb-D2 Line

Abstract: The frequency of an AlGaAs semiconductor laser was stabilized by using the linear absorption spectrum of the 85Rb-D2 line. By controlling the injection current, the frequency stability of 3.0×10-10σ1.4×10-12 was obtained for 10 msτ500 s. First observation of the saturated absorption spectrum of the 85Rb-D2 line is demonstrated, which can be used as a frequency reference to improve the frequency stability.

Direct evidence for the site of substitutional carbon impurity in GaAs

Abstract: Direct evidence that substitutional carbon in GaAs is predominantly on the As sublattice is obtained from Fourier transform infrared spectroscopy absorption measurements of the carbon‐induced localized vibrational mode (LVM). The previously reported LVM absorption band of carbon is measured under high resolution conditions and is found to be the near superposition of at least four bands. It is shown by comparison with similar measurements of silicon‐doped GaAs and by physical arguments that the only satisfactory explanation is that the bands arise from carbon on As sites with different nearest‐neighbor configurations of the two Ga isotopes. There is no experimental indication of carbon on the Ga sublattice. These are the first observations of such shifts in LVM spectra for simple substitutional impurity defects in semiconductors.

Difference frequency laser spectroscopy of the ν3 band of the CH3 radical

Abstract: The ν3 fundamental band of the CH3 radical has been detected in absorption with a tunable difference frequency laser. Zeeman modulation is found to be inefficient, because the spin‐rotation interaction is small. The molecular constants in the ν3 vibrational state have been determined with the molecular constants in the ground state fixed at the values determined from the analysis of the ν2 band. The main parameters thus obtained are the band origin ν0=3160.8212(12), B3=9.471 10(14), C3=4.701 67(15), (Cζ)3=0.345 88 (22), and q3=0.006 42(25), all in cm−1 with one standard deviation in parentheses. The transition frequencies and the molecular constants of the ν3 band may be useful in a search for interstellar CH3.

High resolution absorption and emission spectroscopy of a silane plasma in the 1800–2300 cm−1 range

Abstract: High resolution infrared absorption and emission spectra have been obtained from an electrical discharge in silane. In addition to extracting vibrational and rotational temperatures for silane itself, emission from the ground electronic state of the free radical SiH is observed and vibrational and rotational temperatures of 2000 and 485 K determined.

Reaction kinetics of a high pressure helium fast discharge afterglow

Abstract: This work reports the study of elementary processes which govern the relaxation of the afterglow of a 3 kA 7 ns duration discharge in helium at atmospheric pressure. Populations of the dominant energy storing species were monitored as functions of time through measurements of the emission asnd absorption of radiation from the plasma. The atomic helium spectral lines, radiated during the discharge pulse, were found to rapidly transform into molecular helium bands in the visible and far ultraviolet (600–1000 A) regions during the afterglow. The observed features of this He2 radiation were consistent with a collisional‐radiative scheme of electronic recombination of molecular He2+ ions, the concentration of which was inferred from Hβ line broadening. A significant fraction of the He+2 recombination events were found to result in helium atoms in the metastable 2 3S state, and the time dependence of this dissociation process correlated with that of He2 populations characterized by quantum number n = 4. The con...

Integrating sandwich: a new method of measurement of the light absorption coefficient for atmospheric particles.

Abstract: A new technique has been developed for determination of optical depths δa as low as 0.0005 for thin layers of absorbing materials or particles. The measurement involves optical amplification of the absorption and is not affected by the scattering properties of the absorber. This is accomplished by introducing the absorber into the virtually isotropic radiation field between two high-reflectance diffusing wafers and measuring the resultant attenuation of the transmitted light. The technique has been directed toward determination of the absorption coefficient of atmospheric aerosols in remote and relatively unpolluted locations. Provided appropriate collection filters and sampling conditions are used, the method can establish an absorption coefficient for the aerosol as low as 5 × 10−9 m−1 within a 10-h sampling period. A proportionally higher absorption coefficient requires proportionally less sample time. This paper discusses instrument design, the theoretical optical model, laboratory calibration, and a field test of the technique.