Lineshape effects in atomic absorption spectroscopy

Deviations from Beer's Law caused by the relative lineshape functions of source and absorber in atomic absorption experiments are considered. The validity of using a modified form of the law incorporating a correction factor γ, which is particularly convenient in time-resolved atomic absorption experiments, to account for these deviations, has been critically examined for a wide range of source and absorber lineshapes using numerical evaluation of the transmission integrals involved. It is concluded that there is, in general, no theoretical justification for the use of such a γ factor, except in the case of large source linewidth/absorber linewidth ratios when the line broadening in the absorber involves a significant homogeneous (Lorentzian) contribution. The use of empirically determined γ factors, much different from unity to analyse experimental data, should be viewed with suspicion unless direct evidence is presented to show that under the experimental conditions γ happens to be constant or slowly varying.     

Simultaneous detection of 14NO and 15NO using Faraday modulation spectroscopy

We describe a technique of simultaneous detection of ¹⁴NO and ¹⁵NO by means of Faraday Modulation Spectroscopy (FAMOS) based on a cw distributed feedback quantum cascade laser (QCL) operating near 5.4 μm. FAMOS is a spectroscopic method for selective, sensitive, and time-resolved detection of free radical molecules such as NO, in the mid-infrared spectral region. The selected spectral lines are the Q (1.5) for ¹⁵NO located at 1842.76 cm^?1 and the P (9.5) for ¹⁴NO located at 1842.93 cm^?1. The detection limit (1σ) of 6 ppb $/\sqrt{\mathrm{Hz}}$ for ¹⁵NO and 62 ppb $/\sqrt{\mathrm{Hz}}$ for ¹⁴NO has been achieved. The simultaneous detection was performed using a fast laser frequency switching between the two isotopologues with a time resolution of 2 s. The isotope ratio (δ ¹⁵N) has been determined with a precision (1σ) of 0.52‰ at 800-s averaging time for 100 ppm NO-gas with a time resolution of 2 s. δ ¹⁵N is determined after NO release from nitrite by chemical reduction with potassium iodine.     

The sensitive detection of NO by Faraday modulation spectroscopy with a quantum cascade laser

A spectrometer based on a quantum cascade laser and capable of operating at particular wavelengths in the mid-infrared with very high sensitivity for the detection of open-shell molecules has been developed. It exploits magnetic field modulation in the Faraday rotation configuration. The signals for nitric oxide (NO) that may be observed with this instrument have been studied and their dependence on the J and Ω quantum numbers investigated with a simulation program. It is shown that the Q(3/2) transition of NO in the ²Π_3/2 component at 1875.8 cm^?1 would provide the greatest sensitivity for detection. The experimental observation of the R(21/2) transition of the Ω = 1/2 component gives a detection limit of 41 ppb of NO in air at a pressure of 25 mbar. Detection of NO through the Q(3/2) transition would provide a detection limit of 4 ppb at this pressure.     

Longitudinally modulated ENDOR spectroscopy. Lineshape and signal dependence on relaxation times

A novel technique, named longitudinally modulated electron nuclear double resonance (LOMENDOR), is reported. The lineshape of the signal is studied as a function of the relaxation times of the spin system under study and as a function of the microwave and radiofrequency field strengths. Analytical expressions of the line intensity and width are obtained at low saturation factors. The experimental setup is described and the results are compared with theory. The application of LOMENDOR for the direct measurement of relaxation times is discussed in detail.     

Electrochemical modulation spectroscopy

Applications of modulation spectroscopy techniques in electrochemical kinetic studies are reviewed. These methods permit direct in situ detection and identification of reaction intermediates and the measurement of kinetic rate constants approaching the diffusional limit. The results of spectral studies of adsorbed monolayers on metal surfaces are discussed with particular reference to surface roughness effects and photon-assisted charge-transfer transitions between the metal-adatom complex (considered to be a virtual bound state) and the conduction band of the substrate. Recent developments in the theory of the electroreflectance (ER) of metals are presented. The primary features are shown to arise from free-electron effects. At high positive biases, field-modulated interband transitions are enhanced owing to the reduced shielding of surface d orbitals. Surface plasmon generation in metals is also readily detected by ER measurements because of the ease with which interfacial optical parameters can be varied.     

New directions in modulation spectroscopy

Since its inception in 1964, modulation spectroscopy has achieved a high level of maturity as applied to the study of interband critical point transitions in “perfect” crystals. Several papers at this conference describe applications of the physical principles and experimental techniques of modulation spectroscopy in ways that were completely unforeseen eight years ago. In this paper we review and discuss additional new directions not explicitly covered elsewhere in this volume, with emphasis on electric field modulation of optical properties in systems where the initial and/or final states are not Bloch (band) states. These include molecular spectroscopy (determination of excited state dipole moments by electric field modulated luminescence; electroabsorption in charge transfer complexes), impurity and defect spectroscopy (Stark effect of impurities; piezospectroscopy of deep defect levels), and applications in materials science (phase separation in III–V alloy thin films, and surface reactions on IV–VI epitaxial films).     

Interference phenomena in modulation spectroscopy

Interference phenomena in light reflected or transmitted through a sample can be clearly observed either in a transparency region or in very thin samples. In the case of modulation experiments carried out on such samples the interference is superimposed on a differential spectrum that makes its interpretation more complicated. This is a disturbing aspect of interference. As a rule, one tries to eliminate interference influence (if it is not of special interest). Because this is not always possible, it is useful to know its manifestation in various modulation experiments. Some of the examples of interference manifestation are considered in the present report. However the interference can be used as a source of additional information. This is a useful aspect of interference. Several cases are considered in which interference in modulation spectroscopy is used for determination of the some constants of solids, particularly the electrointerference application for determination of the electro-optical effect constant. A brief review is given of work in this direction. A new method is given for the determination of the refraction coefficient and its variation from the usual interference and modulation interference data. The method provides the determination of these quantities even in the presence of noticeable dispersion, when other methods guarantee the needed accuracy. The imperfectness of the system, which leads to considerable errors in the determination of parameters by usual formulas, is also considered. The results are presented.     

Infrared laser modulation spectroscopy

The lack of sensitivity in far infrared of conventional modulation techniques can be overcome by the use of a laser beam as a powerful high resolution infrared source. As an illustration of these features we describe a thermoreflectance experiment performed on mercury telluride using a frequency stabilised CO₂ laser. In this experiment a thin slab of HgTe was illuminated by the beam of the laser. A continuous shift of the Γ_6v?Γ_8c energy interval was produced by a slow temperature variation while the sample was submitted to a slight temperature modulation obtained by low frequency Joule heating. Synchronous and direct detection of the reflected beam gave the relative variation of reflectivity as a function of the sample temperature. Several spectra obtained at different emission lines enable us to determine the energy difference (E_g) between Γ_6v and Γ_8c inverted states. As a first approach a qualificative fit has been obtained with a simple model of dielectric constant and its temperature derivative. These results give the first direct determination of E_g near room temperature E_g = ? 117.04 meV at T = 286 ± 2 K.     

Pseudo field modulation in EPR spectroscopy

This paper develops methodology for computer simulation of the effect on an experimental EPR spectrum that would occur if an additional field modulation were applied followed by eventual phase sensitive detection at the modulation frequency or at one its harmonics. The algorithm, which is called pseudomodulation, transforms the digitized spectrum and also filters the noise. If a second harmonic spectrum is desired in order to make subtle changes in curvature more apparent, it is shown that it is always preferable to obtain an experimental second harmonic spectrum. The signals are identical, but because of the filtering properties of the pseudomodulation algorithm, the noise is lower. Pseudomodulation should be applied to simulated spectra prior to fitting a model to data in order more precisely to simulate the experimental signal. It is argued that such fits ought to involve not only first harmonics but also higher harmonics, since the various harmonics are sensitive in different ways to input parameters in the spin Hamiltonian. Application of pseudomodulation to the EPR spectrum of the blue copper-protein azurin is described.     

Surface studies by modulation spectroscopy

Surface barrier and electronic surface states are important parameters for characterizing semiconductor surfaces. Qualitative information on the surface electric field can be deduced from electroreflectance studies. Energetic positions of surface states have been determined by spectroscopic methods using effect modulation techniques. Besides the field effects of surface conductivity and absorption, which are limited to low densities of surface states, new information on surface states was gained by investigating the spectral dependence of surface photoconductivity. Also surface phonons were detected in “spectral oscillations” of photoconductivity. The measurement of surface photovoltage at photon energies where the bulk is transparent promises a new tool for surface state research in the future. To demonstrate these modulation techniques examples are given for Ge, Si, ZnO, and CdS surfaces.     

Natural logarithm wavelength modulation spectroscopy

Natural logarithm wavelength modulation spectroscopy(ln-WMS) is demonstrated in this Letter. Unlike the conventional wavelength modulation spectroscopy(WMS)-2 f technique, it is a linear method even for large absorbance, which is the core advantage of ln-WMS. The treating method used in ln-WMS is to take the natural logarithm of the transmitted intensity. In order to determine the proper demodulation phase, the η-seeking algorithm is introduced, which minimizes the absolute value of the first harmonic within the non-absorbing region. Subsequently, the second harmonic of the absorption signal is extracted by setting the demodulating phase as 2η. To illustrate the validity of ln-WMS, it was applied to water vapor experimentally. The result shows that even if the absorbance(base-e) is between 1.60 and 6.26, the linearity between ln-WMS-2 f and volume fraction is still established. For comparison, measurement with conventional WMS-2 f was also done, whose response no longer kept linearity. The η values retrieved in continuous measurements and the residuals were shown so as to evaluate the performance of the η-seeking algorithm. Time consumed by this algorithm was roughly 0.28 s per measurement. As an alternative WMS strategy, ln-WMS has a wide range of potential applications, especially where the absorbance is large or varies over a wide area.     

Multi-harmonic detection in wavelength modulation spectroscopy systems

Two multi-harmonic detection methods for wavelength modulation spectroscopy (WMS) systems are presented and compared. The two possibilities discussed in this paper are: simultaneous curve fitting of multiple harmonic spectra, and reconstruction of the transmission from harmonic coefficients. The optimum number of harmonics is four and 25 harmonics, respectively. Compared with standard single-harmonic curve fitting, the methods give about a factor of 3 better performance than standard second-harmonic curve fitting. Concluding, multi-harmonic detection is better than single-harmonic detection and should be used if the system bandwidth is high enough to allow for proper detection of the higher harmonics.     

载流子复合对时间分辨法拉第旋转光谱的影响

利用二能级体系速率方程,推导了半导体中探测光探测到的法拉第旋转光谱的理论模型,发现电子-空穴对的复合对法拉第旋转信号随时间的衰减有重要影响,并利用该模型对GaAs量子阱中实验测得的法拉第旋转光谱进行拟合,得到GaAs量子阱材料中的电子自旋弛豫时间为73.5 ps,而直接利用单指数进行拟合得到的电子自旋弛豫时间仅为51.3 ps. 因此,直接利用单指数对法拉第旋转光谱进行拟合得到电子自旋弛豫时间的传统做法是不准确的. 关键词： 自旋弛豫时间 时间分辨法拉第旋转光谱 GaAs量子阱     

A Faraday Laser Magnetic Resonance spectrometer for spectroscopy of molecular radical ions

We describe a mid-infrared Laser Magnetic Resonance (LMR) spectrometer which is based on a cryomagnet and a sealed-off CO-laser. Faraday rotation combined with a multireflection cell is used for polarization sensitive detection of molecular radical ions. Experimental details and first spectroscopic examples are given.     

Laser FM spectroscopy with photochemical modulation

We have combined the techniques of frequency-modulation spectroscopy (FMS) and photochemical-modulation spectroscopy to carry out high-resolution, high-sensitivity absorption measurements on the formyl and amino radicals. Using the (0,9⁰, 0)-(0,0¹, 0) band of theÃ,² A- ,² A transition of HCO at 614 nm, we obtained a sensitivity limit for absorption of 1.5×10^–6. Reconstructed spectra of several HCO lines are presented.     

Online monitoring of biogenic nitric oxide with a QC laser-based Faraday modulation technique

Online sensing of nitric oxide traces based on a cw quantum cascade laser, operating near 5.2 m, is reported. We utilized a Faraday modulation technique which provides unique selectivity since it exploits the magnetic moment of nitric oxide (NO) molecules. The minimum detectable NO concentration was found to be 4 ppb (sampling time: 10 s). Continuous monitoring of the NO release from nitrite solutions and human sweat samples is described. Detectable NO release rates are down to 10 pmol/s. We compare NO generation induced by a chemical, and by exposure to ultra-violet radiation. PACS 07.88.+y; 42.62.Fi; 07.57.Ty     

Frequency modulation (FM) spectroscopy

Frequency modulation (FM) spectroscopy is a new method of optical heterodyne spectroscopy capable of sensitive and rapid measurement of the absorption or dispersion associated with narrow spectral features. The absorption or dispersion is measured by detecting the heterodyne beat signal that occurs when the FM optical spectrum of the probe wave is distorted by the spectral feature of interest. A short historical perspective and survey of the FM spectroscopy work performed to date is presented. Expressions describing the nature of the beat signal are derived. Theoretical lineshapes for a variety of experimental conditions are given. A signal-to-noise analysis is carried out to determine the ultimate sensitivity limits.     

Ionospheric Faraday amplitude modulation of radio-astronomical signals. I. Solar radio emission

Considerable time shifts of the amplitude maxima of solar radio emission were recorded during solar flare X38 of January 17, 2005 at the European VHF stations spaced by a distance of 100 to 1000 km. It is shown on the basis of the model of polarization interference of radio-astronomical signals [1] that this phenomenon can be stipulated by the amplitude modulation of solar radio emission caused by the rotation of the polarization plane in the ionosphere (Faraday effect). __________ Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 50, No. 12, pp. 1029–1042, December 2007.