Heating and Cooling of the FASSST Spectrometer

Being able to heat and cool the absorption cell of a spectrometer has many benefits. Cooling simplifies the spectrum of the gas in the cell by freezing out rotational and vibrational motion of the gas molecules, while heating increases the population of the excited ro-vibrational states. We designed and prototyped a heating and cooling system for the absorption cell of the FASSST spectrometer [1]. Introduction The FASSST (FAst Scan Submillimeter Spectroscopic Technique) spectrometer consists of several major parts. It uses a Backward Wave Oscillator (BWO) to generate radiation, and then radiation is then split by a Mylar beam splitter, one direction traveling down the absorption cell, the second traveling to a Fabry-Perot cavity. It also consists of two hot-electron detectors, one placed at the end of the absorption cell, the second placed outside the Fabry-Perot cavity for reference. At the beginning of this project, the 6-meter long absorption cell of the FASSST spectrometer could be heated with a system of 8 heating tapes on the cell, controlled by 4 variable transformers. There was no cooling system. While this system allowed the aluminum tube to be heated to temperatures up to 200C, it was desired to improve to a system that could perform heating and cooling in a more controlled manner. My objective was to design and prototype a system that would both heat and cool the absorption cell and would be easy to assemble and maintain. The designed system will be implemented later this year. Being able to heat and cool the absorption cell gives the ability to change the population of certain ro-vibrational states. When cooling the absorption cell one can “simplify” the spectrum by decreasing the population of the excited states while increasing the population of the ground state, thus decreasing the intensity of the higher energy ro-vibrational states. Heating increases the population of the excited states. Another benefit of being able to change the temperature is finding the energy of the lower state of a transition by using the ratio of their intensities. The overall assumption we make is that the experimental signal is proportional to the absorption coefficient (small absorption limit). The integrated absorption coefficient for a two level system equals ( lu lu u l h ) B n n c υ α = − − (1) where lu α is the absorption coefficient from state l to state u, lu B is the Einstein coefficient of absorption, υ is the frequency, nu is the population of the upper state, and nl is the population of the lower state. We can say that the population of a state is