Absorption line profile

This is the Menzel-Minnaert-Unsold interpolation formula (often used assuming Roo = 1). It gives a better approximation to stellar absorption-line profiles (which are definitely not flat-bottomed) than does the exponential formula the shape of the corresponding curve of growth is much the same, but its use leads to a b-parameter that is about 25 per cent higher for the same observational data. Denoting the central value of p by po, the Doppler part of the curve is given by... 

Measurement of integrated absorption requires a knowledge of the absorption line profile. At 2000-3000 K, the overall line width is about 10-2 nm which is extremely narrow when compared to absorption bands observed for samples in solution. This is to be expected, since changes in molecular electronic energy are accompanied by rotational and vibrational changes, and in solution collisions with solvent molecules cause the individual bands to coalesce to form band-envelopes (p. 365). The overall width of an atomic absorption line is determined by ... 

For an accurate data analysis, a detailed understanding of systematic effects is necessary. Although they are significantly reduced with the improved spectroscopy techniques described above, they still broaden the absorption line profile and shift the center frequency. In particular, the second order Doppler shift and the ac-Stark shift introduce a displacement of the line center. To correct for the second order Doppler shift, a theoretical line shape model has been developed which takes into account the geometry of the apparatus as well as parameters concerning the hydrogen atom flow. The model is described in more detail in Ref. [13]. 

Not all transitions which are observed in the emission spectrum have the unexcited state (the ground state) as their lower energy level. In other words they require partial excitation before atomic absorption can occur. However, in flames, most atoms exist only in the ground state, and only transitions with the ground state as their lower energy state exhibit sensitive absorption.13,14 Because the number of such transitions is small, the probability of overlap of the atomic absorption line profile of one element with the emission line profile of another element is extremely small. The spectral selectivity of AAS is therefore excellent in this respect. 

Zeeman background correction also depends upon line splitting, but in this instance most commonly the absorption line profile (the n component) is split into two or more components (the a components) by the application of an intense... 

The absorption line profile in the atomizer (e.g. in the flame) will still peak at the initial emission line peak. Absorbance will be reduced as the emission line becomes broader, and even more dramatically when the emission line shows reversal. Thus atomic absorption signal decreases with increasing lamp current (Figure 2). As might be expected, the drop off in signal is greater for more volatile elements such as cadmium and zinc. 

More frequently the absorption line profiles overlap partially and, additionally, to obtain isotopically pure sources is difficult. Thus, part of the absorption observed will be due to the other isotope. Using a natural lithium hollow cathode lamp and a lamp made from enriched (93%) lithium-6, Chapman and Dale [236] were able to determine lithium isotopic... 

Le Fevre published an extensive article on the work done regarding molecular refractivity. To Shore is due a report on the scattering theory of absorption-line profiles and refiractivity. Theories of hypoduromism are reviewed in the artides of Rhodes and Chase and of Weissbluth. The papers on optical rotatory dispersion and circular dichroism have been reviewed by numerous authors for various substances. 

Owing to the line broadening mechanisms, the physical widths of spectral lines in most radiation sources used in optical atomic spectrometry are between 1 and 20 pm. This applies both for atomic emission and atomic absorption line profiles. In reality the spectral bandwidth of dispersive spectrometers is much larger than the physical widths of the atomic spectral lines. 

Frequency tripling of KrF laser output in Xe results in radiation near 83 nm, tunable over a region of 0.5 nm, and this has been used to measure absorption line profiles in the system in H2, and the corresponding B" - X... 

Collisions between molecules are the greatest cause of line broadening at the pressures normally employed for MMW spectrometry. In the Lorentz theory (ref 2, p. 338) the lifetime of the rotational state involved in the transition is ended abruptly by collision with another molecule which stops the rotation. When the molecule starts to rotate again, its phase with respect to the other molecules is random. For an assembly of molecules this will give rise to an absorption line profile with a FWHM of Xjlm, where r is the mean time between collisions. This is the linear sum of two terms, one for the upper and one for the lower state, having the shape of the Lorentz function (Figure 1.4) when Av [Pg.12]

The result of this power broadening or saturation is to reduce the absorption in the line centre compared with the absorption in the wings of the line. This in turn leads to loss of analytical signal intensity and an apparent broadening of the absorption line profile. The resulting effect on the line shape function can be described by an equation due to Karplus and Schwinger, for low powers and incomplete saturation (ref 3, p. 50) ... 

Estimation of Low-Energy Excitation Modes. An absorption line profile of each dye molecule at low temperature consists of two components a sharp zero-phonon line and a broad phonon side band. Tbe energy difference between the zero-phonon line and the phonon side band coincides with the low-energy excitation mode or phonon frequency of the matrix when multi-phonon processes can be ignored. 

The half-width Av can be evaluated if the absorption line profile is assumed to be triangular. Since a line profile is a Gaussian distribution, this becomes a good approximation. Under these conditions, the relation... 

Fig. 1.6 Modulated laser sources, (a) Wavelength-modulation (b) corresponding absorption line profiles (c) phase modulation (d) corresponding sideband spectmm...

The enhanced sensitivity of intracavity absorption may be utilized either to detect minute concentrations of absorbing components or to measure very weak forbidden transitions in atoms or molecules at sufficiently low pressures to study the unperturbed absorption line profiles. With intracavity absorption cells of less than 1 m, absorbing transitions have been measured that would demand a path length of several kilometers with conventional single-pass absorption at a comparable pressure [15, 19]. 

Fig. 2.7 Comparison of the saturation of a homogeneous absorption line profile (a) and an inhomogeneous profile (b) in a standing-wave field, (c) The traveling saturating wave is kept 2ita) = a)o and a weak probe wave is tuned across the line profile...

E.A. Sviridenko, M.P. Erolov, Possible investigations of absorption line profiles by intracavity laser spectroscopy. Sov. J. Quantum Electron. 7, 576 (1977)... 

However, when the field is on, only the background is measured since the a absorption line profiles are shifted away from the emission line while the static polariser, constructed from synthetic crystalline quartz, rejects the signal from the it components. Background correction is achieved by subtraction of the field-on signal from the field-off signal. With this principle of operation, background absorption of up to 2 absorbance units can be corrected most accurately even when the background shows a fine structure. 

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