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研究生:蔡明蒼
研究生(外文):Ming-Tsang Tsai
論文名稱:步進式傅立業轉換光譜法之化學動力學應用,從可見光到紅外光範圍
論文名稱(外文):Chemical Kinetics Application of step-scan Fourier Transform Spectroscopy, From Visible to Infrared range
指導教授:林金全林金全引用關係
學位類別:博士
校院名稱:國立臺灣大學
系所名稱:化學研究所
學門:自然科學學門
學類:化學學類
論文種類:學術論文
論文出版年:2008
畢業學年度:97
語文別:英文
論文頁數:178
中文關鍵詞:步進式傅立業轉換光譜法時間解析光譜轉動能量轉移紅外放光丙酰
外文關鍵詞:step-scan Fourier transform spectroscopytime-resolved spectrarotational energy transferinfrared emissionCH radicalpropionyl chloride
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本研究利用步進式傅立業轉換光譜儀探討化學動力學反應,偵測訊號波長範圍介於可見光區到中紅外光區之間。根據偵測訊號波長範圍,本研究將分為可見光區實驗以及紅外光區實驗兩個部分來介紹。
關於可見光區實驗,我們觀察特定激發態的CH自由基(A 2Δ, v’=0, 2≦N’≦5, Fi and B 2Σ–, v’=0, 0≦N’≦5, Fi)與氬氣碰撞後產生的轉動能量轉移速率常數。利用雷射光解-激發的技術,我們將待測物激發至包含電子自旋解析的特定轉動能階,俟其與載體氣體碰撞能量轉移後,記錄特定轉動能階螢光隨時間變化的光譜圖,並利用動力學模型適解出自旋解析的轉動能量轉移速率常數。實驗所使用的載體氣體為氬氣。CH自由基是利用248 nm雷射光解三溴甲烷(CHBr3)分子所產生,並使用特定雷射波長激發電子基態的CH分子到位於第一與第二電子激發態待觀察的初始振動轉動能階。
實驗觀察可得到在同一初始轉動態,能量轉移的躍遷最高可達到三個轉動量子數量級。對於第一與第二電子激發態,激發轉動量子態最高分別可以達到4和6。我們計算出碰撞誘導轉動能量轉移速率常數在第一電子激發態的數值介於0.4-6.7×10-11 cm3 molecule-1 s-1之間,而在第二電子激發態的數值則介於0.3-18.3 ×10-11 cm3 molecule-1 s-1之間。比較理論計算與實驗結果,發現碰撞誘導轉動能量轉移速率常數不論是在數量級或是對於初始轉動態與多量子態轉移的依附性皆相當地一致。在最低初始轉動態實驗,轉動能量轉移著重在電子自旋改變,其次為同一電子自旋的轉動量子態改變。當增加初始轉動態的量子數時,電子自旋的守恆會在轉動態能量轉移速率中,明顯地趨於重要。在此條件下,電子自旋部分在碰撞誘導轉動能量轉移過程中,將被視為旁觀者而忽略,並可歸納為Hund’s case (b)的特性。此現象在兩個電子激發態都被明顯地觀察到。
比較兩電子激發態的碰撞誘導轉動能量轉移速率常數,發現第二電子激發態的數值普遍大於第一電子激發態的數值。綜合兩電子激發態的結果,我們歸納出以下幾點碰撞誘導轉動能量轉移的特性:(1)在最低轉動激發態,相較於轉動能階的變化,轉動能量的轉移較優先於電子自旋轉換的部分。(2)當轉動能量轉移發生於激發轉動態具有相同電子自旋部分時,轉動量子數增加的躍遷會主導低轉動激發態的碰撞誘導轉動能量轉移。此現象會隨著初始轉動激發態的增加而有相反的趨勢。當初始轉動激發態較高的時候,碰撞誘導轉動能量轉移會較優先發生於轉動量子數減少的躍遷。
在紅外光區的實驗,我們進行丙酰氯分子(CH3CH2COCl)的光分解研究,並利用光分解產物的特徵放光,推測光分解的反應途徑。本實驗使用248 nm雷射激發CH3CH2COCl分子至第一電子激發態,利用步進式傅立業轉換光譜儀結合多重反射金鏡組合進行解離產物的偵測。我們同時觀測到光分解產物HCl與CO的紅外放光隨時間變化的趨勢,進而推求出光分解產物個別的振動與轉動能階的相對分佈與能量。實驗所使用的載體氣體為氬氣與氧氣。光分解產物的放光強度會隨著載體氣體增加有顯著增強的趨勢,但與載體氣體的不同並沒有顯著的相關性。利用譜線強度積分與雷射能量的關係,確定CH3CH2COCl分子為單一雷射光子吸收的條件下進行光分解反應。
根據光分解產物振動與轉動躍遷光譜的指認,可得知HCl產物的振動(v)與轉動(J)能階最高分別可達到3與12,而CO產物的振動能階最高可分布到v=4。在實驗時間小於15微秒內,各個振動能階的相對轉動能階佈居數呈現兩種不同的波茲曼分布。當時間大於20微秒後,相對轉動能階的佈居數均呈現單一波茲曼分布。依據光分解產物的實驗結果配合理論計算的,我們推測光分解產物HCl與CO是經由相同反應途徑所生成。其光分解反應解離機制為CH3CH2COCl氣態分子於電子基態(S0)先吸收一248 nm雷射光子能量躍遷至第一電子激發態(S1),經由載體氣體淬熄進行內轉換(Internal Conversion, IC)過程,激發態CH3CH2COCl分子以未放光躍遷(Radiationless transition)形式轉換至高內能的電子基態(S0)。此時,含有高內能的CH3CH2COCl分子其能量高於反應能障,可以經由三體同時分裂途徑生成HCl,CO與C2H2等光分解產物。
ACKNOWLEDGEMENT IV
CHINESE ABSTRACT V
ABSTRACT VIII
FIGURE CAPTIONS X
TABLE CAPTIONS XVII
1. INTRODUCTION 1
1.1 Step-scan Fourier-transform spectrometer 1
1.2 Rotational Energy Transfer (RET) study 19
1.3 IR emission study 27
2. EXPERIMENTAL METHOD 49
2.1 RET experiment 49
2.1.1 Reaction chamber 49
2.1.2 Laser system 49
2.1.3 Sample preparation 51
2.1.4 Signal detection and optimization 51
2.1.5 Data acquisition 53
2.2 IR emission experiment 55
2.2.1 Reaction chamber 55
2.2.2 Multipass optical system 56
2.2.3 Laser system 57
2.2.4 Sample preparation 57
2.2.5 Signal detection 58
2.2.6 Data acquisition 61
2.2.7 System calibration 62
3. RESULTS 68
3.1 LIF experiment 68
3.1.1 Kinetic collision model 68
3.1.2 Simulation model approach 71
3.1.3. Theoretical calculation 74
3.1.4 CH B state 76
3.1.4.1. Excitation and emission spectrum 76
3.1.4.2. Fine-structure RET rate constants 78
3.1.4.3. Average RET rate constant 80
3.1.5 CH A state 82
3.1.5.1. Excitation and emission spectrum 82
3.1.5.2. Fine-structure RET rate constants 84
3.1.5.3. Average RET rate constant 86
3.2 IR emission experiment 87
3.2.1 Spectral intensity calibration 87
3.2.2 Spectrum analysis 89
3.2.2.1 Spectrum assignment 89
3.2.2.2 Relative population calculation 92
3.2.3 Rotational and vibrational temperature 95
3.2.3.1 Rotational temperature 95
3.2.3.2 Vibrational temperature 98
3.2.4 Dependence of laser power and quencher pressure 99
3.2.4.1 Laser power dependence 100
3.2.4.1 Quencher pressure dependence 101
4. DISCUSSION 137
4.1 LIF experiment 137
4.1.1 The principle of detail balance 137
4.1.2 Characterization of RET rate constants 141
4.2 IR emission experiment 145
4.2.1 Energy disposal of rotational and vibrational levels 145
4.2.1.1 Rotational energy disposal 145
4.2.1.2 Vibrational levels 147
4.2.2 Photodissociation mechanism 151
5. CONCLUSION 171
6. REFERENCE 174
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