Passive infrared windows

Chemical Vapor Deposited Diamond for Passive Infrared Windows in Aggressive Environments... 

An important consequence of the presence of the metal surface is the so-called infrared selection rule. If the metal is a good conductor the electric field parallel to the surface is screened out and hence it is only the p-component (normal to the surface) of the external field that is able to excite vibrational modes. In other words, it is only possible to excite a vibrational mode that has a nonvanishing component of its dynamical dipole moment normal to the surface. This has the important implication that one can obtain information by infrared spectroscopy about the orientation of a molecule and definitely decide if a mode has its dynamical dipole moment parallel with the surface (and hence is undetectable in the infrared spectra) or not. This strong polarization dependence must also be considered if one wishes to use Eq. (1) as an independent way of determining ft. It is necessary to put a polarizer in the incident beam and use optically passive components (which means polycrystalline windows and mirror optics) to avoid serious errors. With these precautions we have obtained pretty good agreement for the value of n determined from Eq. (1) and by independent means as will be discussed in section 3.2. 

Fluorine [Matheson] is pretreated to remove HF and SiF4 impurities and is handled in a passivated copper vacuum line designed for fluorine use. Approximately I g of NaF and four stainless steel balls are placed in a 200-mL high-pressure stainless steel Hoke cylinder [Koch Associates] and a high-pressure stainless steel needle valve [Koch Associates] is attached to the cylinder. The cylinder is evacuated on the copper line and cooled to — 196°. A total of 50 mmol of fluorine is carefully condensed into the cylinder. The cylinder valve is closed. The cylinder is allowed to warm slowly to ambient temperature in an empty, precooled Dewar flask (- 196°). It is removed from the vacuum line, shaken to loosen and spread the NaF, placed on its side for 2 days, and shaken periodically. The cylinder is reattached to the vacuum line, and the space between the valves (interspace) is evacuated. A stainless steel infrared cell (8.0 cm) with AgCl windows is attached to the vacuum line and evacuated. A high-pressure infrared spectrum (2 atm) indicates that traces of CF4 are present, but HF and SiF4 are absent. ... 

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