Modulated absorption spectroscopy

Remarks Steimle et al. [OOSte] have applied transient frequency modulation absorption spectroscopy to PtC and TiS molecules as produced in a laser ablation supersonic expansion source, and discussed the results in comparison with LIF detection techniques. 

Fig. 1.8 Principle of phase-modulated absorption spectroscopy (a) no absorption of carrier and sidebands, (b) one sideband coincides with an absorption line...

R. Grosskloss, P. Kersten, W. Demtroder, Sensitive amplitude and phase-modulated absorption spectroscopy with a continuously tunable diode laser. Appl. Phys. B 58, 137 (1994)... 

J.M. Jasinski, E.A. Whittaker, G.C. Bjorklund, R.W. Dreyfus, R.D. Estes, R.E. Walkup, Detection of SiH2 in silane and disilane glow dischaige by frequency modulated absorption spectroscopy. Appl. Phys. Lett. 44, 1155 (1984)... 

A typical modem set-up for modulated absorption spectroscopy, with a multipass cell, is shown schematically in Figure 6.11 the equipment components indicated in this figure can easily be recognized in the photograph of a TDLAS system for the analysis of flowing gases in Figure 6.12. 

Laser excitation here offers few distinctive features at the molecular level, except that at high levels of intensity the increased flux can lead to saturation. Then, a significantly high proportion of the sample molecules undergoes transition to an excited state and, as C varies through depletion of the ground state population, departures from Beer s law arise. This is a phenomenon that is exploited for analytical purposes with pulsed radiation, in concentration-modulated absorption spectroscopy. 

Considerable experimental support for the CT exciton mechanism for carrier generation has been provided by work on electric field modulated absorption spectroscopy. As described Sebastian et al (21), the electric field induced change in absorption coefficient k for excitation of a Frenkel exciton may be given to first order by the expression... 

The pyrolysis of CR NH (<1 mbar) was perfomied at 1.3 atm in Ar, spectroscopically monitoring the concentration of NH2 radicals behind the reflected shock wave as a fiinction of time. The interesting aspect of this experiment was the combination of a shock-tube experiment with the particularly sensitive detection of the NH2 radicals by frequency-modulated, laser-absorption spectroscopy [ ]. Compared with conventional narrow-bandwidth laser-absorption detection the signal-to-noise ratio could be increased by a factor of 20, with correspondingly more accurate values for the rate constant k T). 

Recent work in our laboratory has shown that Fourier Transform Infrared Reflection Absorption Spectroscopy (FT-IRRAS) can be used routinely to measure vibrational spectra of a monolayer on a low area metal surface. To achieve sensitivity and resolution, a pseudo-double beam, polarization modulation technique was integrated into the FT-IR experiment. We have shown applicability of FT-IRRAS to spectral measurements of surface adsorbates in the presence of a surrounding infrared absorbing gas or liquid as well as measurements in the UHV. We now show progress toward situ measurement of thermal and hydration induced conformational changes of adsorbate structure. The design of the cell and some preliminary measurements will be discussed. 

The three most commonly applied external reflectance techniques can be considered in terms of the means employed to overcome the sensitivity problem. Both electrically modulated infrared spectroscopy (EMIRS) and in situ FTIR use potential modulation while polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS) takes advantage of the surface selection rule to enhance surface sensitivity. 

Polarisation modulation infrared rejiection-absorption spectroscopy (PM-IRRAS or JRRAS). Potential modulation IR studies rely on switching the potential at a reflective electrode between rest and active states, generating difference spectra. However, the EMIRS technique has several drawbacks the relatively fast potential modulation requires that only fast and reversible electrochemical process are investigated the absorption due to irreversibly chemisorbed species would be gradually eliminated by the rapid perturbation. Secondly, there is some concern that rapid modulation between two potentials may, to some extent, in itself induce reactions to occur. 

The combination of surface enhanced Raman scattering (SERS) and infrared reflection absorption spectroscopy (IRRAS) provides an effective in-situ approach for studying the electrode-electrolyte interface. The extreme sensitivity to surface species of SERS is well known. By using polarization modulation of the infrared beam for IRRAS, the complete band shape is obtained without modulating the electrode potential. 

In situ studies of catalytic reactions have also been a prime focus of our group. The high-pressure spectroscopic technique used in our research is polarization modulation IR reflection absorption spectroscopy (PM-IRAS). Like SFG, PM-IRAS is a highly surface-sensitive technique that yields vibrational information about adsorbed surface species. Unlike SFG, however, PM-IRAS... 

These systems possess the simplest possible variation in that only one parameter, i.e., the fluorescence quantum yield is proton-controlled. The insulation of the fluor and receptor modules in the ground state by the alkyl spacer leads to essentially pH-invariant absorption (position, shape, and intensity) and fluorescence spectra (shape and position only). The same pKa values are obtained from fluorimetry or absorption spectroscopy so that the detection sensitivity of excited state experiments can be used for the measurement of binding constants of the ground state. 

PM-IRRAS Polarization-modulated infrared reflertion absorption specTroscopy... 

Eaithfull, N.T. (1971b) Flame interference in atomic absorption spectroscopy with a.c. modulated systems. Laboratory Practice 20(8), 641-643. 

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