利用腔體震盪吸收光譜法偵測二氟二溴甲烷光分解後，溴分子的振動分佈

Vibrational distribution of Br2 molecule following photodissociation of CF2Br2 by using cavity ring down absorption spectroscopy.

許靜怡(Ching-Yi Hsu)

指導教授 ： 林金全

國立臺灣大學/理學院/化學研究所/碩士(2005年)

https://doi.org/10.6342/NTU.2005.02671

摘要

腔體振盪吸收光譜法(Cavity Ring-down Spectroscopy)技術是近年來發展迅速的一種測量吸收光譜的新方法，其振盪腔體(Ring-down Cavity)是由兩片在特定波段反射率很高的(通常反射率高達99.9%以上)的反射鏡所組成，雷射光進入腔體後會在腔體內來回的反射直到雷射光強度衰退至近乎零，雷射光強度對時間呈現一指數衰減(exponential decay) ，當雷射光頻率與待測分子之能階躍遷(transition)達共振(resonance)時，會使雷射光的衰變加速，以雷射頻率對雷射波長作圖，即能得到待測分子的吸收光譜。
我們著重於研究二氟二溴甲烷(CF2Br2)被248nm光分解後，光分解產物溴分子的量子產率及初生態的振動分布。二氟二溴甲烷(CF2Br2)有兩個主要的分解通道
CF2Br2 → CF2Br + Br ΔH=274 kJ/mol

CF2Br2 →CF2 + Br2 ΔH=231 kJ/mol
在過去的研究中，大部分的研究團隊都認為最主要的光分解通道為解離一個溴原子；但我們利用腔體振盪吸收光譜法，成功地偵測到了另一個光分解的通道; 溴分子的解離通道。我們得到溴分子的量子產率為0.038±0.009。另一方面我們發現在二氟二溴甲烷光分解後所得到的溴分子的初生態振動分布相較於三溴甲烷屬於冷振動的分布。我們認為這種光分解所得的溴分子，是來自於二氟二溴甲烷吸收248 nm後，從激發的電子能態(excited electronic states)經由內轉換(internal conversion)和高振動態的電子基態(highly vibrational levels of ground state)，進而分解成產物。我們利用已知理論計算從能量的觀點去証實產生溴分子的這個解離通道確實可以發生。

關鍵字

腔體震盪吸收光譜法 ； 二氟二溴甲烷 ； 光分解

並列摘要

Cavity ring-down Spectroscopy (CRDS) is a relatively new direct absorption technique and its applications are developed very quickly in recently years. The method is based on measurement of the decay rate of a pulse light trapped in an optical cavity which is formed by a pair of highly reflective(R>99.9%) mirrors. A plot of decay rate as a function of laser frequency gives the absorption spectrum.
As for photodissociation studies of CF2Br2, the major dissociation channels are found to be
CF2Br2 → CF2Br + Br ΔH=274 kJ/mol

CF2Br2 → CF2 + Br2 ΔH=231 kJ/mol
We used a cavity ring-down spectroscopy (CRDS) to study of nascent Br2 following photodissociation of CF2Br2. The quantum yield of Br2 is found to be 0.038±0.009 following photodissociation of CF2Br2 at 248nm. According to the absorption spectrum, the nascent vibrational distribution was obtained. A comparison with the CHBr3 case reveals that nascent vibrational distribution leads to vibrationally cool. The excited parent molecules (CF2Br2) may transfer into highly vibrational levels of their electronic ground state via internal conversion. The results agree with a given theoretical calculation.

並列關鍵字

cavity ring down absorption spectroscopy ； CF2Br2 ； photodissociation

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