| Summary: | 碩士 === 國立清華大學 === 生醫工程與環境科學系 === 96 === Adiabatic expansion has the effect of causing adsorption of water on surfaces, but present analytical instruments do not allow implementation of an analysis beginning with pumping from atmospheric pressure, which is an important range in relation to the adsorption of water vapor on a sample surface. The purpose of this work is to establish a new experimental method to investigate the phenomenon of adsorption of water on a surface at pressures near atmosphere.
The main emphases of this work are the design and construction of a system for infrared analysis to investigate the phenomenon of adsorption of water inside a vacuum chamber. The experiment involves (a) a Fourier spectroscopic analysis to obtain spectral signals about the incident infrared, (b) a purge mechanism for a purified gas to remove water vapor and carbon dioxide from the system, and (c) an application of an electric field with polarized modulation to suppress interference from signals of gases absorbing in the light path. After testing and fine-tuning the light path, the experimental system can obtain the interference signals of infrared radiation. The sealing of the gas purge system has been improved to effectively eliminate the permeation of gases such as water vapor and carbon dioxide from the outside. The movable mirror in the Michelson interferometer maintains its stability independent of the rate of flow of the purge gas. During test experiments on filling the chamber with nitrogen or helium mixed with water vapor, the measured temperatures decreased to -47.6 oC（N2) and -68.1 oC（He) during evacuation reflecting the effect of adiabatic expansion. The absorption signal of water vapor after pumping for 4.7 s is about 0.005∼0.01 arbitrary units higher for the N2 / H2O gaseous mixture than for the He / H2O gaseous mixture. Tests of the polarization modulation technique indicate that signals from absorbing gases in the light path are removed when the ratio of the summation signal (amplitude of channel B) to the differential signal (amplitude of channel A) is less than 1.26. The result demonstrates the capability of observing a signal due to molecules adsorbed on a surface with an infrared analysis system and a polarization modulation technique, so as to improve our understanding of adsorption of water in a vacuum chamber.
For purposes of testing, we applied the polarization modulation technique only on a standard sample (poly-l-lysine film on a gold-coated substrate). To investigate the effect of adiabatic expansion leading to the phenomenon of water adsorption, future work will focus on measuring adsorption spectra for materials in vacuum chambers and transforming spectral signals to units of surface coverage.
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