Wavelength modulated absorption

Silicon carbide 3C-SiC Fisv and Xic Eg, ind 2.417(1) 2 Wavelength-modulated absorption... 

Fig. 6. Wavelength-modulated absorption spectra of the TO phonon assisted indirect exciton of silicon at 77K for X = 0 and X = 3.51 X 10° dyn cjj along f 001 for the electric field vector of the light E polarized parallel and perpendicular to the stress axis.

Fig. 7. Experimental values (solid lines) of the wavelength modulated absorption spectra of the peak of Fig. 6 after appropriate background subtraction and the theoretical fit (dashed line) obtained from Eqs. (22).

Fig. 10. Wavelength-modulated absorption spectra of the LA-phonon assisted indirect transition in Ge at 77K for X = 0 and 3.73 x 10 dyn-cm along [ lll for E 1 X and E X It. For the X = 0 spectra the TO-phonon assisted peak is also observed as marked.

Fig. 14. Preliminary infrared absorption spectra of the metal-molecule stretch mode for C Oand C Ointhec(4 x 2) structure on Pt(l 11) at 100 K recorded by a reflection wavelength modulation spectrometer. The recorded first derivative spectrum has been numerically integrated. Both spectra are reproduced in the figure.

In these WMS signals, the wavelength crosses the water absorption feature twice for each modulation cycle. Therefore, when water vapor is in the optical path, the amplitude modulation of the received laser power contains a periodic component having precisely twice the wavelength modulation frequency and fixed in phase relative to the wavelength modulation. 

Groll H., Schnurer-Patschan C., Kuritsyn Yu. and Niemax K. (1994) Wavelength modulation diode laser atomic absorption spectrometry in analytical flames, Spectrochim Acta, Part B 49 1463-1472. 

There are two regimes of FM spectroscopy, the first is when the modulation frequency is greater than the line width of the absorption feature, and the second is when it is less. It is in the latter regime that the Brookhaven group performs their experiments, which is sometimes called wavelength modulation. The Brookhaven group scans an absorption profile by changing the carrier frequency. The output, /(coc), has both absorption and dispersion elements that depend on amplitude or phase modification, respectively, as is shown in Eq. 10,... 

Synchroton radiation has been employed as a spectral source for a study of the absorption of HCN and DCN in the wavelength range 80—120nm. A vacuum-u.v. spectrophotometer for absorptions in the region 105—200 nm has been described. Solid-, liquid-, and gas-phase samples could be analysed at temperatures from —200 to 100 °C and at pressures between 0 and 150 atmospheres. The absorption spectrum of tra j-di-imide in the vacuum-u.v. has been measured. First-derivative u.v. spectroscopy has been employed in the analysis of Watts nickel plating solutions for trace amounts of saccharin. Impurity levels of 0.1 p.p.m. have been recorded. A wavelength modulated derivative spectrophotometer with a multi-pass absorption cell has been developed for the automatic analysis of atmospheric pollutants. Traces of SOj, NO, and NO2 were detected with limits of 15, 13, and Sp.p.b., respectively. A double-beam single-detector absorption spectrometer has been constructed. Independence... 

All of the flicker noises can be effectively eliminated by the use of double-beam optics in conjunction with a background correction system such as Zeeman splitting or a well-aligned (or wavelength-modulated) continuum source. Thus the ultimate limiting noise in atomic absorption is source shot noise, which can be reduced (relative to total source intensity or I, ) by increasing the source intensity, up to the point of optical saturation. 

The possibility of modulating the emitted wavelength of DLs at GHz frequencies by the modulation of the diode current allows one to reduce the low-frequency (flicker) noise in the baseline, which again improves the detection limit. Emission wavelength modulation also permits to correct for unspecific background absorption, thereby improving the selectivity of the technique. 

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

Fig. 1.61 Section of CS2-overtone spectrum. Comparison of optoacoustic detection (a) and absorption spectroscopy with wavelength modulation (b) (G.H. Wenz, Thesis, K.L.)...

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