Reflection mode spectrometer sensitivity

The experimental aspects of ex situ FUR spectroscopic studies of sensitive electrodes (e.g., Li and Ca surfaces) using a purged spectrometer (not in a glove box) have been described in detail in Refs. 36-38, 92, and 93. Briefly, it is possible to analyze thin surface films on active metal, using a reflectance mode, while the active surface is in contact with a KBr or NaCl polished window. In Refs. 37 and 38, a possible organization of the measurement chamber of an FTIR spectrometer for such measurements is described. It should be noted, however, that the performance of such measurements when the spectrometer is placed in the glove box is much more elegant and easier. 

Figure 8.5. Palm of the hand. Fluorescence spectrum (Fb-mode, full line) and diffuse reflectance absorption spectrum (dashed line, normalized to X = 700 nm, reference filter paper) measured with a sensitized diode array spectrometer.

IRES Versus Other Reflection Vibrational Spectroscopies. In order to achieve a sensitivity sufficient to detect absorption due to molecules at submonolayer coverages, some sort of modulation technique is highly desirable. Two candidates for modulation are the wavelength and the polarization state of the incident light. The former has been successfully applied to single crystal studies by Pritchard and co-workers (5j, while the latter is the basis of the Toronto ellipsometric spectrometer and of the technique employed by Bradshaw and coworkers (6) and by Overend and co-workers (7). The two different techniques achieve comparable sensitivities, which for the C-0 stretching mode of adsorbed carbon monoxide amounts to detection of less than 0.01 monolayer. Sensitivity, of course, is very much a function of resolution, scan rate, and surface cleanliness. 

One of the major advances in the past decade has been the maturation of the electronic revolution. This has had its effect on surface spectroscopy, with regard to instrumentation for transmission IR, but particularly for sensitivity gains that have made reflectance techniques the preferred alternative for fundamental studies. In the transmission mode, the commercial development of the Fourier transform IR spectrometer has led to significant advantages in the determination of the vibrational spectra of adsorbed species. This is covered in the chapter by Bell. 

Infrared-based techniques are used to identify molecules on the surface. IR radiation is used to excite vibrational modes in molecules in the gas phase or adsorbed on a surface. The transmitted or reflected IR spectrum can be analyzed in a spectrometer. Considerable improvement in the sensitivity can be achieved by use of Fourier transform infrared (FTIR) spectroscopy. Attenuated total reflection (the ATR-FTIR method) inside a crystal (germanium) of high refractive index can be used to further enhance the surface sensitivity (using the evanescent field). 

Figure 8 The 10-15 )im spectrum of the outflow from a star known as the Red Rectangle because of the rectangular appearance of its associated reflection nebula on red plates. Bands in this spectral region are largely due to the C-H out-of-plane bending mode whose exact peak position is sensitive to the number of adjacent H atoms involved. The boxes underneath the spectra give the ranges where PAHs with the indicated number of adjacent H atoms emit. The arrow labelled 6 indicates the position for benzene. These spectra were obtained with the Short-Wave-length Spectrometer on board the Infrared Space Observatory at a spectral resolution ranging from 250 to 2000.

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