Dependence Of Signal Intensity Research Articles

Temperature Dependence of Photothermal Divergence Signal of GaAs

We measured the temperature dependence of the photothermal signal intensity for undoped and doped GaAs samples using the photothermal divergence method proposed by Saito et al. [Appl. Opt. 31 (1992) 2047]. The temperature dependence of signal intensity and cutoff frequency of the signal agreed with a theoretical prediction and enabled determination of the temperature dependence of the thermal conductivity of the samples.

Surface coil MR imaging of the human brain with an analytic reception profile correction.

An analytic correction method was developed and applied to correct the signal intensity nonuniformity caused by the inhomogeneous reception profile of surface coils used--because of their higher sensitivity--for magnetic resonance imaging of the brain at 1.5 T. The reception profile was analytically modeled by using the Biot-Savart law. The correction considerably improved visual interpretation of the images by modeling and removing the dependence of signal intensity on distance from the coil. The method can be used for single or multiple coils of any planar geometry and in any orientation. Application of this technique allows imaging of the brain with surface coils, yielding increased resolution and signal-to-noise ratio, without the penalty of image nonuniformity.

High-field cross polarization NMR from laser-polarized xenon to surface nuclei

A method for NMR investigations of surface nuclei using cross polarization from optically polarized xenon (OPCP) is described. We find this methodology results in enhancement factors of approximately 103 upon application to surface protons. The dynamics of129Xe transfer to protons is examined in some detail, including the time, temperature, and multiple contact dependences of signal intensities. Furthermore, we discuss the sensitivity of the transfer process to spatial diffusion. Finally, we report on application of the OPCP experiment to a low total surface area sample.

Pump-probe laser photolytic fragmentation fluorescence spectrometry of methyl vinyl ketone, methacrolein and crotonaldehyde

The two-laser “pump-probe” photolytic fragmentation fluorescence spectrometry of methyl vinyl ketone, methacrolein and crotonaldehyde is described. Probe laser induced C 2 ( C 1 II g → A 1 II u9 Deslandres-d'Azambuja band system) fluorescence is detected. The relative C 2 fluorescence intensity and spectral patterns exhibited by each C 4H 6O isomer are dependent upon the probe laser wavelength employed. The dependence of the fragment fluorescence signal intensity on the probe laser fluence implies that the “probe” laser induces photofragmentation of intermediate specie(s) produced by the “photolysis” laser. The vinyl radical (C 2H 3) is believed to be an important intermediate produced from the C 4H 6O isomers by the photolysis laser, which can subsequently be converted to C 2 via the absorption of probe laser photons.

Characterization and optimization of a radiofrequency glow discharge ion source for a high resolution mass spectrometer

A radiofrequency (rf) powered glow discharge ion source has been designed and coupled to a double-focusing mass spectrometer. An electrical interface has been developed to provide an effective rf power transfer, thereby ensuring rf shielding and grounding, and to couple the ion source to the accelerating potential. The device permits the direct analysis of almost any solid sample type (conducting, semiconducting and non-conducting). In order to establish optimum conditions for the trace analysis of solids, mass spectrometric studies of the dependence of ion signal intensities in an rf glow discharge on operating parameters (applied rf power and discharge gas pressure) were carried out. The energy distribution of the ion species was measured by the accelerating voltage scan. It was shown that the singly charged atomic ions of the sample have about a 10 eV higher average energy than the discharge and residual gas ions. An effective energy separation of the analyte ions from the discharge and residual gas ions was achieved by setting the position and width of the energy window of the double-focusing mass spectrometer. The optimization of the operating parameters of a glow discharge ion source and correct adjustment of the ion transfer optics of a mass analyser permit an improvement in the analytical characteristics of the elaborated device.

Optically detected magnetic resonance study on the origin of the pheophytin triplet state in [formula omitted] complexes

In order to determine the origin of the photoexcited pheophytin triplet state ( 3Phe) in plant Photosystem II preparations ( D1 D2- cyt b-559 complexes) an absorption and fluorescence detected magnetic resonance (ADMR, FDMR) study was conducted in zero magnetic field at low temperatures. The ADMR signal intensity dependence on excitation light flux was linear showing that 3Phe is formed by a 1-photon process. Upon successive exposure of the sample to strong white light it was shown that 3Phe is not directly correlated with the primary donor triplet state 3P680. This is consistent with the absence of a double resonance signal connecting 3Phe with 3P680 down to an instrumental sensitivity of ΔI I ( Δ transmission transmission ) below 10 −6. An intermediate state of photoinhibition between intact and fully degraded reaction centres is responsible for the formation of 3Phe. It may be a conformationally changed state of the reaction centre protein, most likely having a larger distance between P680 and the pheophytin whose triplet state is observed. Factor analysis of the absorption spectra of a series of gradually degraded samples yielded two spectral components which were interpreted as the spectra of fully intact and completely degraded reaction centres.

Interatomic distances in layered silicates and their intercalation compounds as studied by cross polarization NMR

Contact time dependences of signal intensities in 29Si and 13C CP/MAS NMR have been measured for kaolinite and kaolinite-dimethylsulphoxide (DMSO) and kaolinite-DMSO- d 6 intercalation compounds. Cross relaxation times between 1H and 29Si and between 1H and 13C have been estimated experimentally, which reflect the internuclear distances. Relaxation times have been calculated theoretically using reported crystal structure models, and we have discussed the validity of those models.

On the interpretation and rotational assignment of degenerate four-wave mixing spectra: Four-photon line strengths for crossover resonances in NO A 2Σ+–X 2Π

We present here a set of equations specifically adapted to simulation of fully resonant, high-resolution, phase-conjugate degenerate four-wave mixing (DFWM) in molecular gases. Signal-intensity dependence on molecular wave functions, lifetimes, and laser beam polarizations is explicitly included in these equations. The emphasis of the presentation is on both physically intuitive interpretation and a practical, ‘‘cookbook’’ approach to spectral simulation. We present experimental verification of our calculations drawn from the spectrum of dilute NO in N2 at low pressures. Both degenerate two-level and three-level (crossover) resonances were observed. The experimental spectral intensities are accurately reproduced by the expressions presented here. We point out some of the subtleties of DFWM spectra that could be used as aids to interpretation, especially the use of laser polarization as a probe for spectral line assignments.

Surface reactivity and stability of Ga-deposited GaAs oxide mask for selective area growth of GaAs

We evaluated the surface reactivity (energy exchange during molecular scattering) of the photo-oxidized GaAs mask for selective area growth as a function of the amount of Ga deposited onto it through substrate temperature dependence of the mass spectrometer signal intensity of trimethylgallium (TMG) scattered from the mask surface. We found a correlation between the surface reactivity for TMG and the stability of the mask which was measured by temperature programmed desorption; the surface reactivity of an oxide having high stability was found to be low. The evaluated surface reactivities give a guide for choosing an effective mask material for selective area growth.

Simultaneous Measurements of Energetic O- Ions and O Atoms in Sputtering of Zinc Oxide Target

Energetic negative ions and neutral atoms were observed simultaneously using a time-of-flight apparatus for conventional planar magnetron sputtering of a ZnO:Al target in Ar and O2 gases. Energetic ions such as O- and O2- and energetic neutral O atoms were detected in time-of-flight spectra. The gas pressure dependence of signal intensities of the spectra were measured, and the behavior of the energetic particles was examined. The ratio of the secondary electron emission coefficients of O- ions and O atoms at the collector was estimated from the results, and the relative intensities of the fluxes of O- and O were estimated. The scattering mechanism of energetic O- ions with Ar gas and O2 gas is discussed.

CIDEP Study of Radicals Produced Photochemically in the Organic Solution of 9-Acridone and Phenols

Abstract The photoreaction of the solution systems of 9-acridone (or N-methyl-9-acridone) and phenols was investigated by optical spectroscopy, ESR, and time-resolved CIDEP techniques. CIDEP signals of acridone semiquinone and phenoxyl radicals were detected together and the hyperfine coupling constants of acridone semiquinone were determined for the first time by simulation method. It was also clarified that radical pair mechanism (RPM) with triplet precursor and triplet mechanism (TM) were the main spin polarization mechanisms in these systems. The results indicate that the excited triplet state of acridone abstracts a hydrogen of phenols. Temperature, phenol concentration, light intensity and time dependence of the phenoxyl CIDEP signal intensity and/or pattern supported the conclusion.

Dependence of the X-Ray Photoacoustic Signal Intensity of Pure Copper and Brass on Sample Thickness

The hard X-ray photoacoustic signal intensities (I) of pure copper and brass have been measured in the Cu K-edge region as a function of sample thickness (d) (from 5 µm to 100 µm). It is shown that the photoacoustic signal intensities of pure copper and brass decrease above the sample thickness of 10 µm for each modulation frequency. The absolute values of the power law, d(ln I)/d(ln d), of pure copper and brass depend on modulation frequency.

Photothermal Effect in Compound Semiconductors CdInGaS4 and CdIn2S4

Photothermal spectroscopy using thermistor is applied to study the heat generation by nonradiative processes of compound semiconductors CdInGaS4 and CdIn2S4 together with the optical absorption and reflectance measurements. It shows that the increases of heat generation are different from those of the total rate of optical energy absorption below the fundamental absorption edge regions for CdInGaS4 and CdIn2S4. The incident light power dependence of photothermal signal intensity using Ar ion laser is non-linear for CdInGaS4 and CdIn2S4.

Dependence of the x-ray photoacoustic signal intensity of copper on sample thickness and modulation frequency

The hard x-ray photoacoustic signal intensities of copper have been measured at an x-ray absorption near-edge structure (XANES) region as functions of sample thickness (from 5 to 300 μm) and modulation frequency (from 5 to 50 Hz). It is shown that the photoacoustic signal intensities show maxima at the sample thickness close to 10 μm for each modulation frequency. The x-ray photoacoustic signal varies from f−1.0 ( f denotes modulation frequency) up to 20 μm sample thickness, whereas it varies from f−0.5 beyond a 20 μm sample thickness, which is not consistent with the Rosencwaig–Gersho theory for the photoacoustic effect in solids.

Dependence of the X-Ray Photoacoustic Signal Intensity of Brass on Sample Thickness and Modulation Frequency

Hard X-ray photoacoustic signal intensities of brass have been measured in the Cu K-edge region as functions of sample thickness d (from 10 µm to 300 µm) and modulation frequency f (from 5 Hz to 50 Hz). The photoacoustic signal intensities are proportional to d-1 for each modulation frequency. The X-ray photoacoustic signal varies as f-1.0 up to a sample thickness of 20 µm, whereas it varies as f-0.5 beyond the sample thickness of 20 µm.

Solid‐state photochemical generation of polymeric polyradicals: Poly(4‐vinylphenoxy) and copolymers with styrene

AbstractSynthesis of poly(4′‐vinyl‐2,4,6‐tri‐tert‐butyl‐diphenyloxalate) and its 65 : 35 and 19 : 81 copolymers with styrene are described. Rigid phase broad‐band photolysis of the homopolymer with a quartz filtered xenon arc at 77 K results in production of up to 25% of the theoretical number of spins/mol expected for quantitative production of pendant phenoxyl radicals, and shows no major loss of radical signal in the ESR at temperatures below 100 K. Curie law analysis of the temperature dependence of the ESR radical signal intensity for the neat photolyzed homopolymer 1 shows curvature consistent with antiferromagnetic pairing of radical spins at low temperatures. Since through‐bond conjugation of radical spins is not possible in this system, the antiferromagnetic interaction is interpreted in terms of intrachain and/or interchain through‐space exchange interactions.

Echo-induced EPR spectra of nitroxides: Study of molecular librations

Abstract

Echo-induced EPR spectra of nitroxides in organic glasses: model of orientational molecular motions near equilibrium position

The shapes of echo-induced EPR spectra (i.e of the field dependence of ESE signal intensity) for 15N nitroxides in organic glasses are shown to be in good agreement with those computed for the model where magnetic relaxation is caused by stochastic orientational motion of a molecule near its equilibrium position.

Quantitative dependence of MR signal intensity on tissue concentration of Gd(HP-DO3A) in the nephrectomized rat

Cardiac-gated SE 20 224 ± 20 MR images were obtained from nephrectomized rats before and after intravenously administering 153Gd-Gd(HP-DO3A). The concentration of Gd, [Gd], was linear in dose in myocardium, skeletal muscle, and blood. Under steady-state conditions, where d[Gd] dt = 0 , image intensities (IIN) in regions of interest were compared with the measured [Gd]. IIN was linear in myocardium at ≤0.61 μmol/g-myocardium (≤0.5 mmol/kg dose) and in skeletal muscle at ≤0.63 μmol/g-muscle (≤0.75 mmol/kg). Above 0.6 μmol Gd/g-tissue, IIN did not increase further. The in vivo data were consistent with measured ex vivo and in vivo relaxivities. A 29% greater slope for IIN versus [Gd] in myocardium [14,439 ± 4350 IIN/(μmol/g)] than in muscle [10,258 ± 5,296 IIN/(μmol/g)] was attributed to a significant difference in blood content: 25% versus 2% weight blood in myocardium and skeletal muscle, respectively. Two components were apparent from plots of ex vivo 1 T 1 versus [Gd] in myocardium and muscle, and only one for blood.

Design and performance of a novel electrospray interface

In this work a new electrospray system has been developed which employs heat as a means of desolvation and requires no counterflow of heated gas. This article describes the operation and performance of this device, with particular emphasis on the differences between it and those described earlier. Results are presented that illustrate the dependence of signal intensity on ion source and spray chamber temperatures and on the composition and flow rate of mobile phase into the electrospray. Results on proteins electrosprayed from aqueous solutions are presented including a bacterial protease which has a tight tertiary structure. The ability to obtain fragmentation data by collisionally induced dissociation in the interface is also discussed.