Portland State University. Department of Chemistry
Robert J. O'Brien
Date of Publication
Master of Science (M.S.) in Chemistry
Hydroxyl group, Fluorimetry
1 online resource (84 p.)
For many years there has been a great interest among the scientific community in the study of the hydroxyl radical, HO. This interest stems from the fundamental role played by this molecule in the photochemistry of the atmosphere, mainly as a cleansing agent of environmental pollutants. Knowing the concentration of the radical would enable scientists to corroborate current atmospheric models and to predict future trends in the atmosphere. Even though there is a great interest in the determination of atmospheric concentrations of this molecule, the task has been very difficult. This is mainly due to the lack of a method sensitive enough to detect concentrations around 106 molecules per cubic centimeter. The most accurate method presently available is the method of laser induced fluorescence using the fluorescence assay with gas expansion technique (LIF-FAGE). This method involves low pressure excitation of HO from its ground state to its lowest electronic excited state and observing the consequent fluorescence around 309 nm. The procedure is done at a pressure of 5 Torr to maximize the fluorescence lifetime of the radical and to minimize the interference of photolytic species. Background determination is achieved by chemical modulation using isobutane in a second channel of the same cell which removes the HO signal.
In this study an assessment of the level of ozone interference in LIFF AGE has been done by calculating the relative population distribution of HO among its rotational levels and from this, determining its temperature. When the laser passes through the excitation detection cell it photolyses the ozone present producing in this way the highly reactive 0 1(D). When this molecule reacts with water or with isobutane it produces HO, and this is the source of interference in the actual measurements.
In the determination of the relative population distributions of the different HO species, it was found that the naturally occurring HO has a thermal distribution with a temperature of about 300 K. The HO molecules produced from the reaction of 0 1(D) with isobutane also showed a thermal distribution with a temperature of about 230 K. On the other hand, the HO produced from the reaction of 0 1(D) with water did not show a thermal distribution. Two distinct temperatures were observed for this case: one around 200 K for values of K = 1 to 4, and the second one around 3000 K for values of K = 5 to 6. These values agree with previous experimental results for LIF methods by other authors except for the fact that the deviation from the first temperature determined by other authors starts at K = 6 or 7.
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Martinez, Carmen Ivette, "Study of photolytic interference on HO measurements by LIF-FAGE" (1989). Dissertations and Theses. Paper 3931.