利用腔體震盪光譜法研究溴乙醯溴的光分解反應

Abstract

腔體震盪光譜法(CRDS)是種高靈敏度的吸收光譜技術。原理主要是量測穿透過一對高反射鏡面所組成的腔體雷射光衰減速率，並利用衰減速率的倒數對雷射光頻率做圖可得到吸收圖譜。我們使用準分子雷射產生248 nm的光源光解溴乙醯溴，並使用CRDS來偵測溴分子的離去通道。壓力相關的實驗可以確定溴分子並非來自於分子之間的再結合。藉由能量相關的實驗可得知，溴分子的產生是來自於溴乙醯溴吸收一個光子的解離過程。利用光譜模擬來分析溴分子在基態的振動分支比為V0 : V1 : V2 = 1 : (0.5±0.1) : (0.2±0.1)，是屬於較熱的振動態，振動溫度為584±45K。利用雷射誘導螢光技術(LIF)測得溴乙醯溴經由248 nm激發後，激發態生命期為150 ns，相較於其他含溴分子而言是有足夠的時間進行self-quenching。利用紫外-可見光光譜儀測得溴乙醯溴在248 nm下氣態吸收截面積為4.63×〖10〗^(-19) 〖cm〗^2，搭配CRDS的量測可推出光解溴乙醯溴解離出溴分子的量子產率為0.24±0.08。藉由光解二溴丙醯溴以及二溴異丁醯溴讓我們了解立體效應在溴分子生成通道是可以忽略的。

Cavity ring-down spectroscopy (CRDS) is a high sensitivity spectral absorption technique, which is based on measurement of the decay rate of a pulsed light trapped in an optical cavity containing a pair of highly reflective (R>99.9%) mirrors. The plot of decay rate as a function of laser wavelength gives the absorption spectrum. We use an eximer laser (248 nm) to photolyze bromoacetyl bromide, and then detect Br2 absorption transition (B3Π+0u←X1Σ+g) from elimination channel products by using CRDS. Energy and pressure dependence measurements are used to prove the Br2 elimination involving one photon dissociation process. Combining the experimental data and spectral simulation, we can obtain the branching ratio of Br2 vibrational levels which is V0:V1:V2=1:(0.5±0.2):(0.2±0.1). It means the Br2 fragments we observed is vibrationally hot. We measure the aborsorption cross-section in gas phase by using UV-Vis spectrometer, and the quantum yield is determined to be 0.24 ± 0.08 for the Br2 elimination channel of bromoacetyl bromide. We also determine the lifetime in excited state by using Laser-induced fluorescence (LIF) technique, and the radiative lifetime is approximately 150 ns. To realize self-quenching , a Stern-Volmer plot is obtained.