利用步進式時域解析傅立葉轉換紅外光譜法研究乙醯氯之光分解反應

Abstract

本論文研究乙醯氯分子(CH3COCl)在淬熄氣體Ar或O2的存在情況下，吸收單一248 nm雷射光子所進行之光分解反應，並利用步進式時域解析傅立葉轉換紅外光譜法在吸收的條件與偵測光解產物HCl、CO與CO2。吾人分析光分解產物HCl與CO的時域解析紅外放射光譜，進而推求出不同時間下光分解產物的振動與轉動能階的相對分佈與能量，觀測得知HCl產物的振動與轉動能階最高分別可達到3與約10，而CO產物的振動與轉動能階最高分別可達到4與約30。光分解產物HCl與CO的初始分佈與能量以溫度外推的方式推算取得。 根據前人研究，在無碰撞的氣態環境下，乙醯氯分子的光分解通道以碳氯鍵斷鍵產生CH3CO跟Cl為主要通道；而在凝態中，唯一光分解通道為產生T型的HCl•CH2CO複合物。本研究的實驗條件為碰撞可發生的氣態環境下觀測光分解的反應，吾人首次發現Ar或O2的加入有助於碰撞誘發內轉換(collision-induced internal conversion)，促使吸收一248 nm雷射光子能量躍遷至第一電子激發態(S1)的乙醯氯分子以非放光性躍遷(radiationless transition)形式轉換至高內能的電子基態(S0)，再進行解離而產生光分解產物HCl跟CO。本研究可直接藉由其特徵紅外放射譜線直接確認HCl跟CO的生成，而光分解產物CH2的生成則以其跟O2反應所產生的CO2特徵紅外放射譜線間接確認。HCl，CO與CH2等光分解產物推測是經由一新的三體同時分裂之光解途徑所生成。

In one-photon dissociation of acetyl chloride CH3COCl at 248 nm, time-resolved Fourier-transform Infrared emission spectroscopy is used to detect the primary fragments of HCl, CO, and CH2 in the presence of Ar or O2. Emission from both CO and HCl was observed. Spectrum analyses are carried out for HCl and CO fragments to yield the information of population distribution and energies of rotational and vibrational levels. Assignments of the HCl spectra are shown vibrational level is populated up to 3 and rotational level J up to ~10. For CO product, the rotational level J is up to ~30 and vibrational level is up to 4. Nascent populations and energies are also obtained by extrapolated the temperatures to t=0 for both HCl and CO fragments. In previous works, the photodissociation of acetyl chloride is dominant by the C-Cl bond cleavage in collision-free gas phase and exclusive HCl elimination channel from the HCl．CH2CO complex with a T-shaped structure is confirmed in condensed phase. It is for the first time to find that the Ar or O2 addition in the gas phase photodissociation of CH3COCl may facilitate collision-induced internal conversion process with which the HCl and CO elimination are associated. In this study, the HCl and CO fragments are confirmed directly by the particular infrared emission observation; on the other hand, the CO2 emission is postulated by the third photofragment CH2 reacted with O2. The observed fragments are anticipated to result from a dissociation channel via a converted three-body photodissociation mechanism, which is unfounded ever before.