利用時間解析傅立葉轉換紅外光譜法研究甲酸在193nm的光解反應：漫遊機制vs過渡態機制

Abstract

本研究主要進行時間解析傅立葉轉換紅外放射光譜儀系統之架設及改善，並利用此系統，研究甲酸在193 nm雷射下，光解產生高振動態(v≥1)一氧化碳之轉動—振動光譜(ro-vibrational spectra)。藉由分析一氧化碳在接近初生態時之轉動/振動態分布特徵，吾人發現在v=1之轉動能階分布可以兩個波茲曼分布解釋，且轉動能量較低的貢獻分支比在0-1 μs約0.41±0.08，而若將振動能階也分析成兩個波茲曼分布，可得出振動能較低之貢獻分支比為0.60±0.03。綜合前人所做之離子影像實驗與諸熊教授之理論計算，可得到漫遊路徑下產生之CO，其振動能量較傳統過渡態路徑來的高，而轉動能量較低。根據雙振動分佈之分析，在193 nm光解，產生CO之反應中，漫遊機制占0.41±0.02。因此對於甲酸光解反應之漫遊機制，本實驗提供另一面向之證據。

In this work, we use time-resolved step-scan Fourier transform infrared spectroscopy to study photolysis mechanism of formic acid at 193 nm. We acquired high-resolution (0.5 cm-1) ro-vibrational spectra of CO produced from photolysis of formic acid in the rage of 1900-2200 cm-1. With the aid of spectral simulation, we can analyze the bi-rotational distribution in v=1 state, and the branching ratio of roaming pathway in v=1 at 0-1 μs is 0.41±0.08. If we fit the vibrational distribution as bimodal, the branching ratio of low-vibrational energy componenet is 0.60±0.03. Comparing with the ion imaging results from Wang’s group and theoredical results from Morokuma’s group, we suggested that the vibrational energy of CO from roaming is higher than that from conventional transition pathway, while the rotational energy of that is lower. According to bi-vibrational distribution,the proportion of the roaming pathway in the reaction of producing CO in the photolysis of formic acid at 193 nm is 0.41±0.02. We succeded in providing a new evidence of roaming phenomena in the photolysis of formic acid.