Vibration-free enclosure

If the molecule moves without hindrance in a rigid-walled enclosure (the free enclosure ), as assumed in free volume theories, then rattling back and forth is a free vibration, which could be considered as coherent in such a cell. The transfer time between opposite sides of the cell t0 is roughly the inverse frequency of the vibration. The maximum in the free-path distribution was found theoretically in many cells of different shape [74]. In model distribution (1.121) it appears at a > 2 and shifts to t0 at a - oo (Fig. 1.18). At y — 1 coherent vibration in a cell turns into translational velocity oscillation as well as a molecular libration (Fig. 1.19). 

Most process Raman instruments have few, if any, moving parts and thus are quite stable and robust. This reduces the number of parts likely to need replacement. The laser and detector shutter are the most common service items. Special utilities and consumables are not required. Proper enclosure selection allows instruments to survive harsh environments, such as installation outdoors or around vibrations. An instrument built to accurate specifications and well installed is unlikely to require much care or daily attention. However, it is important to remember that there is no such thing as a maintenance-free instrument. Limited additional maintenance will ensure robust performance for years. 

The whole apparatus was mounted on a heavy metal block free of extraneous vibration and surrounded by a removable Lexan enclosure which permitted constant temperature and humidity control. The apparatus was located in a laboratory controlled at 23 1°C and relative humidity of 72 2%. Temperature variations in the enclosure never exceeded 0.1°C and the relative humidity never varied more than 0.5% during a single experiment. 

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