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We have measured the spectra of two-photon absorption coefficients for CdSe and GaSe based on a z-scan technique. By using extremely high peak intensities from subpicosecond laser pulses, we observed novel behavior of effective two-photon absorption for CdSe. We compared our results for both crystals with theory as well as with the previous values measured at three and two wavelengths, respectively. For both CdSe and GaSe crystals our measured values are lower than all those reported previously. OCIS codes: 180.0190, 190.4180.

In contrast to spectroscopy at longer wavelengths, typical attributes of ultraviolet resonance Raman (UVRR) spectroscopy of biologic tissue are higher absorption coefficient, [micro], and higher photobleaching rate, [kappa]. This study was aimed at measuring [micro] and [kappa] during UVRR spectroscopy of human colon tissue at 251 nm. [micro] was used to estimate the penetration depth of the excitation light; [kappa] was used to predict the rate of signal decrease that was due to photobleaching as a function of laser fluence and tissue thickness. The fitting of the equations through description of a three-state transition model to experimental data that consisted of purine UVRR signal gave [micro] = 0.0169 [+ or -] 0.0023 [micro][m.sup.-1] and [kappa] = 0.572 [+ or -] 0.168 (mJ/[micro][m.sup.2).sup.-1]. [kappa] remained independent of power P for P < 1 mW, but higher power values resulted in a higher photobleaching rate. As predicted by the model, signal decrease that was due to photobleaching was slower as sample thickness was increased. OCIS codes: 300.6450, 170.5660, 170.6390, 260.5130, 260.5740, 260.7190.

A vacuum-ultraviolet photoabsorption imaging method was used, based on time and space resolved absorption of quasimonochromatic VUV beams from laser plasma.

A new study investigates the time-resolved dielectric and photoabsorbance properties of a nematic eutectic mixture doped with a photoactive azo mesogen at room temperature.

The molecular dynamics computer simulation technique has been used to investigate the first photoabsorption band of water around 8 eV under ambient and supercritical conditions. This work produced a detailed analysis of the environmental effects on spectral line shape.

Research was conducted to examine quantitatively the spectroscopic characteristics of the peridinin-chlorophyll-protein from Amphidinium carterae (A-PCP) using the three-dimensional structure of A-PCP obtained from X-ray crystallography. A suitable simple model of excited-state energy coupling among the single set of eight peridinin and two chlorophyll molecules belonging to the monomeric unit of the protein trimer was also used. It appears that the absorption and CD spectra of both Heterocapsa pygmaea PCP and A-PCP are understood in terms of interactions among the eight peridinin and two chlorophyll a molecules grouped in two symmetrically related clusters of four peridinins and one chlorophyll.

Research was conducted to examine the X-ray photoemission and photoabsorption of tris(8, hydroxyquinoline) aluminum, an organic electroluminescent material. The electronic properties and damage mechanisms of N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD) were also studied. It was shown that valence band and core level photoelectron spectroscopy and absorption spectroscopy are well-suited to examine the element-specific structure of organic molecules used in organic light-emitting devices.

A technique for discriminating between scattering and absorbing inclusions located in the center of a scattering slab is presented. The technique is based on an empirical model that provides a simple mathematical expression to describe the change in the time-resolved transmission resulting from the presence of an inclusion. Experimental results from various configurations show that the technique allows for proper recognition of the type of an inclusion whether it is scattering or absorbing. This technique is a significant step toward tissue differentiation.

The effects of optical absorption in the quasi-neutral base region of npn heterojunction bipolar phototransistors on the extent of electron quasi-Fermi level splitting at the emitter-base heterojunction interface were studied. The increase in minority carrier concentration produced by optical generation in the base led to a decrease in the quasi-Fermi level splitting at the heterojunction interface which reduces the net electron injection into the base and emitter injection efficiency.

We present three-dimensional (3-D) microstructures fabricated from photopolymerizable resin with a resolution of 0.62 [[micro]meter]. The method used to fabricate such small structures has been developed using a femto-second-pulsed near-infrared (IR) laser. Structures such as microcoils and microtubes are presented which demonstrate the effectiveness of our method. In this method, a two-photon absorption process is successfully utilized to confine the solidification of photopolymerizable resin to the focused spot of the laser, which is possible due to the quadratic dependence of the two-photon absorption rate on light intensity. The experimental system, experimental conditions, and related issues for this novel method of photopolymerization are also discussed. [285] Index Terms - Photopolymer, three-dimensional microfabrication, two-photon absorption.

Visible-light-emitting diodes of three different colors have been used to detect an absorbing compound (potassium permanganate) in trace quantities in aqueous solution. Photothermal absorption in a closed cell caused deflection of a water meniscus held at a small pinhole. The displacement was monitored with optical-fiber interferometry. The technique was limited by LED emission intensities and environmental acoustic noise, giving minimum detectable absorption coefficients of 2 x [10.sup.-4] [cm.sup.-1] at 478 and 658 nm and 3 x [10.sup.-4] [cm.sup.-1] at 524 nm. The magnitude and form of meniscus deflection signals were shown to be in good agreement with theory.

Research was conducted to examine the infrared photodissociation spectra of acrylonitrile cluster anions and cations using a coaxial infrared laser-ion beam spectrometer developed for the experiment. Ab initio calculation reveals that the cluster ion consists of a dimer ion core solvated with neural acrylonitrile molecules. The acrylonitrile cluster did not show any optical absorption in the wavenumber range studied.

Absorption cross-sectional measurements and model predictions of 25 disks with moderate electrical conductivity were made. The disk radii span part of the resonance region, and their thicknesses are an order of magnitude less than the skin depth. A photoacoustic apparatus, with a 35-GHz microwave source, was used to measure the absorption cross sections. The disks tend to be more absorbing when they are edge-on rather than broadside to the incident microwaves. Also they absorb most efficiently near a size parameter of 1. Model predictions generally agree with the measured magnitudes and trends in the data.

We report the passive mode locking of a diode-pumped Cr:forsterite laser using a semiconductor saturable absorber. The laser generates 10-pJ pulses as short as 1.5 ps in duration with ~650 mW of absorbed pump power. We measured the transient reflectance of the saturable absorber mirror under different incident energy fluences by using a standard pump-probe method. The performance of this laser is also compared with a high-power fiber-laser-pumped femtosecond forsterite laser mode locked with the same absorber.

With a first application of semiconductor lasers to absorption measurements of seeded atomic Rb in high-enthalpy flow fields, a diagnostic technique for time-resolved determination of flow velocity and gas temperature with a line-shape analysis was developed. In our measurements a GaAlAs diode laser was used to scan repetitively at 15 kHz over 1.3 [cm.sup.-1] across the [D.sub.2] resonance transition ([5S.sub.1/2] [approaches] [5P.sub.3/2], 780.2 nm) of seeded atomic Rb to obtain multiple absorption line shapes. The time-dependent signal contains highly resolved spectral line-shape information, which we interpret by fitting the spectrally resolved line shapes to Voigt profiles. Kinetic temperatures in the range 900-1400 K and gas velocities in the range 3900-6200 [ms.sup.-1] were obtained from the Doppler-broadened component of the line shape and from the Doppler shift, respectively, of the absorption frequency.

The first demonstration of phosphorescence in molybdenum(IV) complexes is reported with an investigation of the electronic absorption and emission spectra of salts of two oxomolybdenum(IV) cations and the new dimethoxomolybdenum(IV) complex (trans-Mo(OCH3)2(CN-t-BU)4)(super 2+). All three compounds are found to phosphoresce in the solid state at room temperature and at 77 K. For (MoOCl(CN-t-Bu)4)+, phosphorescence occurs in solution at room temperature.

A new spatial light modulator that uses the electroabsorption and the electro-optic effects in a GaAs single crystal is proposed. The device has the same structure as a Pockels readout optical modulator and can be operated at a frame rate higher than 500 Hz. When the electroabsorption and the electro-optic effects are combined, the dynamic range (contrast ratio) becomes larger than that which results when either effect is used singly. It was experimentally confirmed that the modulator has a high contrast ratio (greater than 2000:1), high sensitivity, and consequently large [Gamma] characteristics.

In solar tower plants, where a rotationally symmetric field of heliostats surrounds the tower, an axisymmetric secondary concentrator such as a compound parabolic concentrator (CPC) or a tailored concentrator or a cone is the obvious choice. For locations at higher latitudes, however, the reflecting area of the heliostats may be used more efficiently if the field of heliostats is located opposite to the sun as seen from the tower. Then the field is asymmetric with regard to the tower. In the case of an asymmetric field, an axisymmetric concentrator necessarily has a concentration significantly lower than the upper limit. Furthermore, the area on the ground from which a tilted axisymmetric concentrator accepts radiation is an ellipse, including also heliostats very distant to the tower producing a large image of the sun. For these reasons we investigate asymmetric secondaries. From the shape of the edge ray reflectors constructed for rays in the central south north plane we conclude that a skew cone reflector might be appropriate for the field, and optimize its free parameters by means of ray tracing. Asymmetric concentrators may increase the concentration by up to 25% at the same efficiency compared to optimized axisymmetric CPC or cone reflectors.

The ultraviolet-photon absorption constants for thin CsI, KI, RbI, NaI and CsBr evaporated films were obtained in the 140-200 nm range. The constant calculated for CsI was in agreement with previous reports while the absorption data for the other materials were determined for the first time. Post-evaporation thermal treatment of the films, which is believed to improve the photoemission behavior of CsI, NaI and CsBr films, had no impact on their photoabsorption characteristics.

Stable isomers, transition states and their photoabsorption spectra for the Na4+ cluster were obtained by ab initio MRCI computations. Three stable geometries of symmetry C2v, D2h and D2d, and two transition states of symmetry Cs and C2 were determined. The D2d configuration is a new geometry not formerly obtained in ab initio computations. The photoabsorption spectra of the three isomers and the transition states were utilized to interpret experimental absorption data.

Temporally- and spatially-resolved absorption and emission spectra were measured at various detection angles to examine NiO laser ablation at 266 nm. Ground states, excited states, lower lying states and other electronic states were differentiated from emission and absorption spectroscopy. Results show that excited Ni neutrals are produced mainly by the recombination of electrons and Ni+ ions.

Surface photoabsorption was employed in the in situ measurement of surface structure transient in heterostructures prepared by altering P precursor partial pressure. The surface photoabsorption transient was correlated with heterostructure abruptness. The inclusion of a pause between the two layers following the flow rate reduction of the P precursors phosphine was found to minimize the magnitude of the memory effect.

Optical absorption and photoluminescence studies were conducted on low pressure metal-organic vapor phase epitaxy-grown strained-layer InAsxP1-x/InP(001)(x

The reactions in illuminated TiO2 nanocrystallite layers was studied by exposing them to pulsed laser photolysis in the presence and absence of added reactants. Results showed that the decay of absorption in the nanosecond time range is mainly due to gradual electron trapping, but the absorbance changes are not affected by scavengers. The trapping of electron was found to successfully compete with recombination even in the absence of scavengers.

An experiment was undertaken to study the reliance of the negative nonlinear absorption (NNA) effect on modulation frequency and degree, sample temperature and erbium concentration present in the erbium yttrium aluminum garnet crystal. The NNA effect was dependent on both the wavelength of the incident laser and sample length and that the process of improved absorption was similar to a four-level laser system. It was asserted that the NNA effect was described by calculating the rate equations based on an analytical model for a five-level system of the erbium ion.

Research was conducted to examine the relationship between pi-conjugation size and electronic absorption spectrum. 1,4-benzoquinone, 1,4-naphthoquinone, 1,4-anthraquinone and 9,10-anthraquinone type indoaniline dyes with the same aniline moieties were synthesized to elucidate the nature and appearance of the hysochromic shift. Results indicate that only the lowest unoccupied molecular orbital energies (LUMO) rise in energy when the quinone system expands, leading to a widening of the highest occupied molecular orbital-LUMO energy gap.

The monolayer growth of gallium arsenide at 650 degrees Celsius by metalorganic chemical vapor deposition was monitored using surface photoabsorption. Scanning photoluminescence measurements of the surfaces were performed to investigate the accuracy of the growth control. Results showed that the observed spectra and model computations in both line width and peak energies were highly compatible.

A study was conducted on GexSi1-x/Si heterojunction internal photoemission detectors with medium- and long-wave infrared regions. Photoresponse measurements were compatible with a theory which considers excited carrier scattering. Results show that at 40 K, heterojunction internal photoemission detectors are capable of background-limited operation in long-wave infrared regions.

Absorption in two types of long-wave infrared detector structures prepared through ultrahigh vacuum chemical vapor deposition was measured. The heterojunction internal photoemission and multiple quantum well structures exhibited dominant free-carrier absorption for normally incident radiation. Resulting absorption values were found in agreement with the classical expression for free-carrier absorption.

The Monte Carlo simulation model was used to investigate the distributions of the number of primary electrons produced per incident mono-energetic x-rays in the 1- to 41-keV energy range comprising the xenon L- and K-absorption edges. The results of the simulation results were compared with recent data and experimental values measured with a gas proportional scintillation counter. Findings indicate a discontinuous behavior with incident x-ray energy which results in an ambiguity in the determination of the x-ray energy in certain narrow ranges containing the edges.

We have measured the absorption coefficient of pure and salt water at 15 wavelengths in the visible and near-infrared regions of the spectrum using WETLabs nine-wavelength absorption and attenuation meters and a three-wavelength absorption meter. The water temperature was varied between 15 and 30 [degrees] C, and the salinity was varied between 0 and 38 PSU to study the effects of these parameters on the absorption coefficient of liquid water. In the near-infrared portion of the spectrum the absorption coefficient of water was confirmed to be highly dependent on temperature. In the visible region the temperature dependence was found to be less than 0.001 [m.sup.-1]/[degrees]C except for a small region around 610 nm. The same results were found for the temperature dependence of a saltwater solution. After accounting for index-of-refraction effects, the salinity dependence at visible wavelengths is negligible. Salinity does appear to be important in determining the absorption coefficient of water in the near infrared region. At 715 nm, for example, the salinity dependence was -0.00027 [m.sup.-1]/PSU. Field measurements support the temperature and salinity dependencies found in the laboratory both in the near infrared and at shorter wavelengths. To make estimates of the temperature dependence in wavelength regions for which we did not make measurements we used a series of Gaussian curves that were fit to the absorption spectrum in the visible region of the spectrum. The spectral dependence on temperature was then estimated based on multiplying the Gaussians by a fitting factor. Key words: Absorption, water, temperature, salinity.

UV-induced excess losses have been measured at various pulse energy densities and exposure times in germanosilicate optical fiber preform cores. The corresponding refractive-index changes have been determined through a Kramers-Kronig analysis. Because of the nonlinear behavior of the excess losses as a function of both exposure time and fluence per pulse, one should be careful when comparing the refractive-index modulation deduced from such measurements with that obtained from Bragg grating reflectivity. Indeed nonlinear effects such as saturation imply that it is necessary to take into account the local character of the change in absorption to calculate the evolution of the refractive-index modulation accurately as a function of the exposure time. Implications of these results are discussed. Key words: Optical fiber, UV absorption, Kramers-Kronig analysis.

A two-dimensional rate equation model, taking into consideration the transverse absorption loss of pump laser power, is proposed to evaluate the characteristics of a dye laser amplifier with a large input laser beam diameter pumped by high average power copper vapor lasers. The calculations are in good agreement with the measurements taken with a Rhodamine 6G dye, and the model can be used for evaluation of the dye concentration at any wavelength. Key words: Dye laser, gain, two dimensional, copper vapor laser.

We report a new material combination, C/Si, for normal-incidence multilayer mirrors in the wavelength region 25-30 nm. The multilayers, fabricated by ion-beam-sputtering deposition, were characterized by near-normal-incidence reflectance measurements by using a discharge source and a grazing-incidence monochromator. The highest measured near-normal-incidence reflectance was R = 23% (25.6 nm), R = 20% (28.3 nm), R = 25% (30.4 nm) at incident angles of 10 [degrees], 12 [degrees], and 4 [degrees], respectively. The multilayers were also characterized by transmission electron microscopy, which revealed sharp layer interfaces and low interfacial roughness. Key words: X-ray optics, multilayer mirrors.

Molecular nitrogen adsorbs on the photochemically generated coordinatively unsaturated Rh(CO) center. The Rh(N2)(CO) species is initially generated and acts as an intermediate surface species for forming Rh(N2)2 species by continued photolysis. Both Rh(N2)(CO) and Rh(N2)2 surface species can be photoexcited and N2 and CO photodesorption occurs from these excited surface species. These were the conclusions from an examination of the ultraviolet photolysis of Rh(super I)(CO)2/Al2O3 in the presence of nitrogen gas at 175 K. It is the first spectroscopic study of the bonding of a molecule to the coordinatively unsaturated Rh site in Rh(CO)/Al2O3 preparations.

The complexes formed from one, two and three electron reduction of delta-(Ru bpy3)2+ and delta-(Os bpy3)2+ were examined by obtaining their electronic absorption and circular dichroism spectra. The results showed that the added electrons are localized on individual bipyridine ligands. The luminescence spectra of the triply reduced species were also obtained.

The cathode spot formation in air within the first 170 ns was investigated by laser absorption photography and ps-pulse interferometry. The discharge was initiated between electrodes made from Ag or Pd with cathode-anode distance below 300 [[micro]meter], the arc duration was some milliseconds, and the arc current 5-10 A. Picosecond holographic interferometry and momentary absorption photography yielded spatial - temporal density distributions in the ignition phase of the cathode spot. An absolute electron density value on the order of 4 x [10.sup.26] [m.sup.-3] has been found. In contrast to vacuum, the cathode spot plasmas broaden little with increasing distance from the cathode, thus narrow plasma channels are observed in the vicinity of the cathode surface having diameters

Equations that relate photothermal lens focal lengths and photothermal deflection angles to saturation absorption coefficients are derived. These equations are derived for two-level absorbers with both homogeneously and inhomogeneously broadened transitions. Initial and time-dependent photothermal lens signals are calculated. Equations describing the zero-time signals are exact to within the simplifying assumptions of the derivation, while the time-dependent signals are approximate. The approximation is performed by the use of a finite series of Gaussian functions to model the temperature change profile distorted by nonlinear absorption. The excitation irradiance-dependent signal behavior for rectangular and exponential excitation pulse time profiles for homogeneously and inhomogeneously broadened transitions are compared. Absorbed energies are used to calculate effective absorbances obtained by the use of conventional and photothermal lensing spectrometry. The conclusions drawn from these comparisons are that pulsed laser photothermal spectroscopy is sensitive to the excitation laser's pulse temporal profile and the transition broadening mechanism.

Optical absorption in the visible region is matched to photoelectric emission of cesium - antimonide photocathodes through the use of thin-film equations to explain the greater absorption (and hence greater sensitivity) of opaque-type photocathodes with the same optical constants and thickness of semitransparent ones. The effect is also the same for other popular photocathodes that are used for optical detection.

A study presents a numerical characterization of photon transport in a multi-scattering, inhomogeneous, absorbing, finite body irradiated by the photon passage pulse. The analysis, which was carried out under the assumption that the photon passage is an isotropic diffusion process, found the terminal slope of the transmittance curves's logarithm to be independent of the source and detector's location.

We have determined the relationship between residual proton (hydrogen) concentration and optical absorption at 1.06 [mu]m in highly deuterated potassium dihydrogen phosphate (KDP). The proton concentration was determined by nuclear magnetic resonance (NMR) spectroscopy and the absorbance was measured by calorimetry. We obtained a hydrogen concentration dependence of 396 [+ or -] 55 parts in [10.sup.6] (ppm) [cm.sup.-1]/% hydrogen for the o- (ordinary-) polarized absorbance at 1.064 [mu]m, and 45 [+ or -] 7 ppm [cm.sup.-1]/% hydrogen for the e- (extraordinary) polarization. In addition, the KDP crystals we tested have residual (hydrogen-independent) o- and e-polarized absorbances of 958 [+ or -] 228 ppm [cm.sup.-1] and 343 [+ or -] 28 ppm [cm.sup.-1], respectively, whose origin is unknown. We also found that the crystal deuterium-hydrogen (D-H) ratio can be related to the D-H ratio of the growth solution by a simple ideal mixture model with a segregation coefficient of 0.684 [+ or -] 0.044. Key words: Deuterated KDP,[KH.sub.2.PO.sub.4], optical absorption.

The reverse mirage or photothermal beam deflection (PBD) effect is simulated via a mathematical model and the results of the simulations are compared with experimental PBD data. The model is based on a one-dimensional thermal wave approach and simulates PBD with the laser probe on the nonilluminated side of the absorbing medium. A theoretical background on the model is presented. Meanwhile, experimental PBD data are obtained via a step-scan Fourier transform infrared PBD measurements. Details of the experiment, particularly the construction of a beam deflection accessory and a sample cell, are described and illustrated. Results of the PBD experiments are also given.

By introducing the material-independent quantities (electric displacement D and flux density B) into the finite-difference time-domain (FDTD) model, a generalized-material-independent perfectly matched layer (GMIPML) absorber used to absorb electromagnetic waves propagating in three-dimensional (3-D) general anisotropic dielectric and magnetic media is proposed. Within the proposed GMIPML absorber, D and B are directly absorbed, whereas E and H are simultaneously absorbed through the relations between E and D, as well as H and B. It is shown that with the help of this GMIPML absorber, Berenger's perfectly matched layer (PML) absorbing boundary condition (ABC) can be simply and effectively extended to 3-D arbitrary anisotropic materials consisting of both arbitrary permittivity and permeability tensors. Index Terms - FDTD, general anisotropic media, GMIPML absorber.