Fine Structure and Λ Splitting – Resolved Rotational Energy Transfer of SH (X2Π, v〃=0 and A2Σ+, v′=0) by Collisions with Ar

SH自由基在大氣、環境、生物體甚至是天文學上都有其重要性。 儘管相關的氣態化學反應動力學(Chemical reaction kinetic)已經有不少的文獻報導, 但是在微觀的動態學(dynamic)方面研究卻鮮少人提及。 特別是SH自由基經非彈性碰撞(inelastic collision)導致的內能能量轉移的領域尚未見到相關的報導。研究SH自由基的內能能量轉移行為理當是對了解相關化學反應動態學有相當的助益. 本論文針對SH自由基(A2Σ+, v′=0 & X2Π, v〃=0)與氬氣(Ar)碰撞誘導轉動能量轉移(rotational energy transfer)過程做實驗與理論兩方面的探討。實驗上觀測到的能量轉移現象可以用量子碰撞計算的結果來佐證。我們應用雷射誘導螢光法(Laser induced fluorescence, LIF)的技術,取得螢光激發(excitation)與放光(emission)光譜可觀察出基態(X2Π, v〃=0)與激發態(A2Σ+, v′=0)精細結構(fine structure)能階的相對數量分佈，進而分析出轉動能量轉移行為。 將SH(A2Σ+, v′=0)與的實驗結果輔以動力學模型可以擬合出SH激發態精細結構能態(fine structure state)的態至態(state to state)能量轉移速率常數。理論方面，使用量子散射理論搭配前人以ab initio方法求得之SH-Ar位能面，我們能計算出轉動能量轉移之碰撞截面與速率常數並與實驗值核對。儘管無法從實驗上量測出SH(X2Π, v〃=0)的態至態速率常數， 我們依然能從時間解析的雷射誘導螢光光譜圖中觀測轉動能量轉移的行為，並且與由速率常數理論值模擬出的轉動分佈比較，檢驗理論數據與實驗值的相符程度。從速率常數的傾向與趨勢所歸納出的一系列碰撞傾向規則(propensity rules)能從微觀的碰撞動態學角度對其行為提出合理解釋。特別是SH(A2Σ+, v′=0)精細結構碰撞傾向與SH(X2Π, v〃=0)Λ分裂態的碰撞傾向可分別由角動量向量模型以及位能面之干涉效應(interference effect of interaction potential)的觀點來說明。 全篇論文可視為首次對SH自由基的轉動能量轉移行為做完整的、有系統的探討。Rotational energy transfer (RET) among the SH (A2Σ+, v′=0) and (X2Π, v〃=0) ro-vibrational levels by collisions with Ar has been studied in both theoretical and experimental manners. The SH (A2Σ+) state is probed using a laser-induced dispersed fluorescence technique, following photodissociation of H2S at 248 nm. The Ar pressure is adjusted appropriately to allow for significant observation of RET process in single collision condition. The spin-resolved rate constants are then evaluated with the aid of a kinetic model: Firstly, a set of rate constants is determined from experimental data by assuming single collision approximation. Then such initial guess values are substituted into rate equation and an iterative process is performed in order to obtain converged rate constants which incorporated the multi collision effect. The theoretical counterparts of both SH (A2Σ+ v′=0) and (X2Π v〃=0) can be calculated by using quantum scattering method with newly fitted potential energy functions based on ab initio potential energy surfaces. Although the Λ doublet resolved rate constants of SH (X2Π, v〃=0) are merely determined via theoretical calculation, experimental time resolved rotational populations are measured via laser induced fluorescence (LIF) technique and compared with simulation in order to examine the reliability of theoretical data. For the SH (A2Σ+ v=0) case, the experimental and theoretical kinetic results are essentially consistent in the trend of N (spinless angular momentum quantum number) and ΔN dependence. The principle of microscopic reversibility is also examined for both experimental and theoretical kinetic data, showing that translational energies of the RET collisions are close to thermal equilibrium at room temperature. Fine structure conserving propensity is found in the fine structure resolved rate constants. Such propensity can be rationalized according to angular momentum vector model. For the SH (X2Π, v〃=0) results, RET rate constants of Λ doublet levels are determined theoretically. In order to examine the reliability of theoretical data, experimental time resolved SH (X2Π, v〃=0) rotational populations are measured via LIF technique and compared with theoretical simulation. Several propensity rules are found in the Λ resolved rate constants, either favor the final level with certain reflection symmetry or tend to conserve the parity. Explanations are given for these propensity rules, basing on both the high collision energy limit and the interference effect in interaction potentials