MSF: Long Path-length Absorption Cell (LPAC)
The long path-length absorption cell (LPAC) at RAL is a 9-metre long stainless-steel vessel containing multi-pass reflective optics for broadband high-resolution spectroscopy at long optical path-lengths from 32 metres to over 1 km. The cell temperature, logged automatically to computer, can be controlled at any point between 190 and 320 K, and cooling to 77 K is also possible. A state-of-the art high-vacuum system ensures the LPAC and all external optical paths can be fully evacuated.
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The long path cells, which are available at RAL, are both White-type multi reflection cells in which high reflectance mirrors are used to fold paths of up to 512 metres into compact absorption systems. Both these cells have the considerable advantage that they can be cooled to temperatures comparable to those found in the upper troposphere (and if need be, down to 90 K). The cells can be connected to the very high-resolution Bruker spectrometer by using evacuated paths.
A full description of the LPAC is given in the following publication: A coolable long path absorption cell for laboratory spectroscopic studies of gases, J. Ballard, K. Strong, J. J. Remedios, M. Page, and W. B. Johnston, J. Quant. Spectrosc. Radiat. Transfer, 52 (5) 677-691 (1994).