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  • 學位論文

臭氧層破洞的光化學: 過氧化氯的紫外光分解截面積及產物的量子產率

Photochemistry of the Ozone Hole: Ultraviolet Photodissociation Cross Sections and Product Quantum Yields of ClOOCl

指導教授 : 李遠哲
共同指導教授 : 林志民(Jim J-Min Lin)

摘要


過氧化氯分子的光分解速率被認為是臭氧洞形成的最關鍵因素。光分解反應速率正比於光分解截面積的大小,而光分解截面積等於吸收截面積與光分解量子產率的乘積。過去20年來科學家利用光經過含過氧化氯的樣品槽後的衰減量來決定吸收截面積,但由於過氧化氯極不穩定且實驗室所合成出的樣品中含有大量的不純物,造成量得的吸收光譜會有極大的誤差。雖然如此,累積過去的知識讓大氣化學家相信已定量地解臭氧層破洞的機制,但在2007,年美國航空暨太空總署的噴射推進實驗室發表新的過氧化氯吸收截面積[J. Phys. Chem. A 111, 4322 (2007)]遠小於過去科學界所認同的值,若將此值套入現有的大氣化學模型中,有一半臭氧的消失將無法解釋,使得化學家開始懷疑並重新審視現有的臭氧洞光化學機制。本實驗利用全新的方法,把不純的樣品經由質譜儀篩選後偵測過氧化氯受雷射光解後的衰減訊號來計算吸收截面積,此方法可確保過氧化氯的訊號不受不純物干擾而獲得最準確的值。本論文量測了雷射光波長266及330奈米下的吸收截面積,其值遠大於2007年噴射推進實驗室所發表的結果,將此值代入大氣化學模型的計算便可以妥善解釋臭氧洞的形成並證實現有的臭氧洞光化學模型仍然可行。以往由於吸收截面積測量值的誤差非常大,以致於很少人在乎光分解量子產率的精確值,1999年美國加州理工學院發表過氧化氯吸收紫外光分解後會產生兩個氯原子和一個氧分子以及兩個氧化氯自由基等兩個光分解通道[J. Phys. Chem. A 103, 1691 (1999)],其量子產率分別為0.9、0.1,其中產生氯原子的通道為摧毀臭氧的真兇。本實驗利用分子束的方法,量測過氧化氯分子在雷射波長248奈米下的光分解產物量子產率。並藉由實驗結果所推得的產物速度分佈與各向異性參數,了解過氧化氯分子的光分解動態。本論文的結果,期望能對臭氧層破洞的化學機制有更深入的了解並對地球自然環境的重視能有所貢獻。

並列摘要


The photolysis rate of chlorine peroxide (ClOOCl) is considered the key process in the ozone hole formation. Photolysis rate is in proportion to photodissociation cross section, which is equal to the product of absorption cross section and photodissociation quantum yield. In the past 2 decades, scientists determined the absorption cross section of ClOOCl by radiation intensity attenuation after passing through the sample cell. Since ClOOCl is extremely unstable and the laboratory synthesized sample has massive impurities, the measurements of ClOOCl absorption spectrum have large errors. Despite possible error in estimating the ClOOCl photolysis rate, accumulation of past knowledge let chemists believe that we have already understood the mechanism of the ozone hole formation. However, NASA-JPL published in 2007 their new ClOOCl absorption cross section measurements [J. Phys. Chem. A 111, 4322 (2007)], which is much smaller than that the previously accepted value. If we apply this value to the existing atmospheric chemistry model, half of the ozone depletion cannot be explained. It caused the chemists to start afresh to suspect and to carefully examine the existing photochemistry mechanism of the ozone hole. This experiment uses the brand-new method that we determined the photodissociation cross section of ClOOCl at 266 and 330 nm with molecular beam and mass-resolved detection by measuring the decrease of the ClOOCl intensity upon laser irradiation. This method may guarantee the ClOOCl signal not to be disturbed by impurity and get the most accurate value. Our value is far larger than the JPL published result. After substitution our value to the atmospheric chemistry model, it may properly explain the ozone hole formation and confirmed the existing model of the ozone hole is still valid. Since error of previously observed absorption cross section is huge, few people care about the precision level of the photodissociation product quantum yields. In 1999, CIT published a paper that after absorption ultraviolet light, photodissociation of ClOOCl produces: 2Cl + O2 and ClO + ClO two product channels [J. Phys. Chem. A 103, 1691 (1999)], the determined quantum yields are 0.9 and 0.1, respectively. Only the channel of Cl atom production will destroy the ozone. We utilize the molecular beam experiment to measure the product quantum yields at 248 nm excitation. Product velocity distributions and anisotropic parameters let us interpret the photodissociation dynamics of ClOOCl molecule. The results of this thesis may contribute to a more thorough understanding of the ozone hole formation and to the protection of the natural environment of the Earth.

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