利用步進式時域解析霍氏轉換紅外光譜法研究甲酸甲脂於193 nm及248 nm光解產生一氧化碳及二氧化碳之內能分佈

Abstract

本實驗利用步進式時域解析霍氏紅外放光光譜法研究甲酸甲酯(HC(O)OCH3)於193 nm及248 nm之光解反應，藉由觀測產物CO的振-轉動放光譜線，分析其內能分佈，並進一步探討其反應機構。 在248 nm之光解實驗中，可觀測到產物CO分布到v≤11、J≤27之放光譜線。於低壓(0.71 Torr甲酸甲酯, 0.16 Torr氬氣)的實驗中，其平均轉動能量為3.0±0.1 kJ mol-1；平均振動能量為76±1 kJ mol-1。而在高壓(1 Torr甲酸甲酯, 3 Torr氬氣)的實驗中，其平均轉動能量為2.4±0.1 kJ mol-1；平均振動能量為66±1 kJ mol-1。在這兩組實驗中，吾人於各振動態下僅觀察到一種轉動分佈應是經由一般傳統過渡態(TS1)所產生，並非像林金全的研究成果[J. Phys. Chem. A. 120, 5155 (2016)]所提到，於振動態v = 1及v = 2觀測到兩種轉動分佈。其中低轉動能態係經由漫遊機制所產生。 在193 nm之光解實驗中，可觀測到產物CO分布到v≤4、J≤25之放光譜線，平均轉動能量為3.6±0.3 kJ mol-1，而平均振動能量為14±2 kJ mol-1。於193 nm的實驗中，吾人也未觀測到經由漫遊機制所產生CO的放光或是一般傳統過渡態(TS1)所產生的CO，而是觀測到經由產物HCO二次解離所產生CO的放光，。吾人也觀測到CO2的放光訊號，其呈現兩種振動能分佈趨勢，最大振動能量分別為165 kJ mol-1及263 kJ mol-1，其分支比為0.35 : 1，而所得之平均振動能量分別為114 kJ mol-1及250 kJ mol-1。依據理論計算吾人觀測到兩種振動能分佈之反應途徑並非像李世煌的研究[J. Chem. Phys., 129, 194304 (2008)]中所提到係經由CH3OCO二次解離所產生的CO2，而可能是經由其他兩種反應途徑產生，其分別為(a)先經由過渡態(TS3)產生HOCOCH3再經由過渡態(TS6)產生高振動能態的CO2 + CH4及(b)經由過渡態(TS7)直接產生低振動能態的CO2 + CH4。

Photodissociation of methyl formate(HC(O)OCH3) with light at 193 nm and 248 nm has been investigated with a step-scan time-resolved Fourier-transform emission spectrometer. Rotationally-resolved bands in region 1865－2300 cm-1 werw observed and assigned as emission of CO (v≤11,J≤27) and CO(v≤4,J≤25) for the experiment at 248 nm and 193 nm, respectively. Two different conditions were used for experiments at 248 nm. At low pressure (methyl formate 0.71 torr, Ar 0.16 torr), the average rotational energy is 3.0 ± 0.1 kJ mol-1 and the average vibrational energy is 76 ± 1 kJ mol-1. At high pressure (methyl formate 1 torr, Ar 3 torr), the average rotational energy is 2.4 ± 0.1 kJ mol-1 and the average vibrational energy is 66 ± 1 kJ mol-1. In contrast to Lin’s results[J. Phys. Chem. A. 120, 5155 (2016)], reporting that the rotational distribution of CO is bimodal at v = 1 and v = 2, with the low-J component from the roaming path, our results show that the rotational distribution of CO is Boltzmann in each vibrational state; CO comes from decomposition of HC(O)OCH3 via the conventional TS. For the experiment at 193 nm, the average rotational energy is 3.6 ± 0.3 kJ mol-1 and the average vibrational energy is 14 ± 2 kJ mol-1. The rotational distribution of CO is Boltzmann in each vibrational state; it was produced via the secondary dissociation of HCO product rather than via the conventional transition-state for decomposition of HC(O)OCH3. A broad emission band of CO2 was observed. The vibrational distribution of CO2 is bimodal and the maximum vibrational energy is 165 kJ mol-1 for channel A and 263 kJ mol-1 for channel B. According to theroetical calculations, CO2 in channel A is produced from the direct decomposition of HC(O)OCH3 via TS7 and CO2 in channel B is from the secondary decomposition of the isomer HOCOCH3 via TS6.