第 5 章 實驗結果與討論
5.4 討論及與其他文獻之比較
5.4.4 與王鳳燕實驗室(2019)之比較
今年(2019)，中國復旦大學的王鳳燕教授利用離子影像技術，配合 2+1 共振增 強多光子離子化(Resonance enhanced multiphoton ionization, REMPI)，觀察在 230 nm 左右，甲酸之光解產物 CO(v = 0) 在不同轉動態下，平移動能之分佈40，如圖 5-17。他們發現相較於傳統過渡態路徑，經由 OH 漫遊機制產生之 CO(v = 0)，其 移動動能及轉動動能皆較小(J ≤ 20)。這是首次透過實驗，觀察甲酸之漫遊通道。也 預估透過此漫遊路徑產生之 H2O，振動能將會較大。
圖 5-17 CO 在不同轉動態之動能分布40
雖王鳳燕教授在論文中未提及兩者之比例，吾人嘗試將上圖中，擬合曲線之半 高寬和峰值相乘，得到一粗估之曲線下積分值。根據上述結果，可得在 CO(v=0，
J=9)兩者之分支比約略為 0.3: 0.7，而在 J=20 其分支比約 0.05:0.95。
雖然吾人之實驗偵測不到 v=0，且光解波長也較短。但吾人仍將 CO 在(v=1, J=9,20)下，兩轉動態之比例，進行以下對比，如圖 5-17：
圖 5-18 CO (v=1, J=9,20)中，較低轉動溫度之分支比 平行虛線為自王鳳燕實驗室結果得到之分支比(v=0,J=9,20)
於 0-4 μs，吾人經由光譜模擬得到之分支比與王鳳燕實驗室差異甚大。吾人猜 測應為基線校正之誤差導致；於 4-6 μs，此時之光譜不須進行基線校正，分支比與 王鳳燕實驗室得到之數據相近。
綜合黑崎之分子動力學模擬以及王鳳燕團隊與吾人之實驗分析結果可得知：
在漫遊機制下，轉動能較低，而振動能較高。根據雙振動態分佈，可得知在產生 CO 之光解反應中，漫遊機制之比例約為 0.41±0.02。
而根據諸熊教授理論計算之結果11，產生 CO 之過渡態路徑及漫遊路徑分別對 應到以下兩個過渡態：
S0-TS2 (Conventional) S0-TS5 (OH-Roaming)
吾人可藉由比較兩者 C-O 之距離，來估計產物 CO 之振動能大小。CO 分子之 鍵長為 1.128 Å，在 S0-TS2 中，C-O 之距離為 1.164 Å；而在 S0-TS5，C-O 距離為 1.196 Å。因此吾人推測，經由 TS5(OH-Roaming)而產生之 CO，振動能較高。其結 果也與吾人之實驗符合。
由於漫遊機制並非經由內在反應座標(intrinsic reaction coordinate，IRC)進行，
因此無法經由過渡態之構型直接推測轉動能量，仍需配合半古典軌跡(quasi-classical trajectory，QCT)之運算來輔助指認，但根據王鳳燕實驗室之結論 40， Roaming 之轉動能較低。綜合以上比較，吾人可以推得出：甲酸在 193 nm 下肢光 解反應中，漫遊機制所產生之 CO，其轉動能較低、振動能較高。
結論
本論文改進前人之時間解析傅立葉轉換紅外光譜儀系統，並以此改良後之光 譜系統，研究氣態甲酸分子吸收 193 nm 之單光子光解反應。甲酸吸收 193 nm 後，
由電子基態躍遷至激發態(S0→S1，nco→π*co)。
吾人發現當以雙轉動分布擬合 CO v=1 之轉動光譜時，可以發現，在 0-6 μs 有 較小之轉動分布，其轉動能量較低，在 0-1 μs 比例約為 0.41±0.08。而若分析雙振 動分布，可發現在 v=1 時，高振動能之分支比為 0.42±0.03。與 v=1 低轉動之比例 接近，因此推測低轉動之漫遊通道伴隨著高振動能。
對比王鳳燕教授之實驗結果，在 J=9、J=20 時，結果大致符合，且配合諸熊教 授理論計算中，過渡態構型來輔助解釋。因此吾人認為，本實驗所得到 CO 低轉動 能/高振動能之分布，可能即為諸熊教授提出之漫遊機制。
但由於甲酸之吸光截面積較小，且在短波長光解牽涉之反應通道較多，因此 CO 之量子產率減少；此外在 0~4μs，高振動態 CO2之光譜(Δv3 = −1)甚至會與 CO 光譜重疊。若粗略地將其視為一平滑之基線而去除，將會嚴重影響光譜擬合。若未 來想對甲酸之漫遊機制做更深入之研究，可能可以藉由調整碰撞氣體的比例，將 CO2與 CO 比例降至最小；或是選擇其他選擇性較好之技術，使 CO 之訊號不受 CO2干擾。
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