利用同時計測電子及離子速度影像研究構象異構物離子的特異性光分解動態學

Abstract

在蛋白質摺疊、分子辨識過程中，特定構象異構物相關動力學已 經被認為是生物化學中很重要的現象。令人感的有趣的是，不同構 象異構物之間雖然只有很小的能量差別，但是最後卻有完全不一樣 的反應性。直到最近，科學家用雷射多光子激發游離來產生特定的 離子構象異構物以便能夠研究構象異構物控制化學反應。為了能夠 暸解離子的內能分布來辨別離子構象異構物，科學家們必須先測量 雷射多光子游離的電子光譜。在分析電子光譜之後，才能夠瞭解不 同結構（包含構象異構物）的離子狀態下，其能量分布。偶而，在 特定的雷射波長激發下，能量最終會分佈在特定的離子構象異構物 上。在這個情形下，科學家便可以再加入其他光子給離子，做進一 步的離子構象異構物光分解動力學研究。然而，最終離子構象異構 物的分佈狀態是根據 Frank-Condon 原理而定，跟分子最初基態、中 間過度態及最後離子態的躍遷機率和分子結構有關。一般而言，每 個分子有其不同的特性而且通常最後能量總是不對等地分佈在各個 不同的結構上。換句話說，在上述傳統的實驗方法下，只有非常少 的機會可以觀察到特定構象異構物的分解反應。為了深入暸解構象 異構物控制化學反應，我們計畫建造具有飛行質譜與離子速度影像 的同時計測電子及離子影像儀。可調波長的雷射將被用來共振激發 分子到最終離子態。同時紀錄能量解析的電子與相互反彈飛散開的 離子，我們將能夠仔細釐清不同構象異構物的內能分佈狀態及其分 解動態學。此外，將以全始量子化學計算來解釋分子的電子光譜、 電子激發態、及各種分解途徑的構象異構物分子位能曲面。特別在 此註明，本計畫所使用的電子及離子影像儀也將被用在軟X 光游離 與光分解動態學研究中。

Conformational dependent dynamics has been considered as significant phenomenon in biological chemistry such as protein folding and molecular recognition. It is fascinating that small amount of energy difference between molecular conformers will result totally different reaction preferences. Recently, using laser multi-photon ionization to prepare specific conformer ions has been conducted to explore conformationally controlled chemistry. In order to know the internal energy of ions, which are related to conformational structures, one should measure the laser multi-photon electron spectroscopy in advance. After analyzing the electron spectra, it will give the energetic distributions of ionic states owing to different ionic structure, including conformers. Occasionally, energy could participate on a specific conformer ion rather than the others at certain laser wavelength. In this case, one can dissociate the molecular ion with additional photon to examine the conformerspecific fragmentation. However, the participations of final comformeric states are managed by Franck-Condon principle, which are depend on transition probability and molecular geometry of initial ground state, intermediated state and final ionic states. Generally, each molecule has its own nature to populate on the final ionic state and is usually widely distributed on various structures with different amount of internal energy. In other words, it’s very uncommon to perform conformer-specific fragmentation in conventional experimental technique as described above. To gain more insight into conformationally controlled chemistry, we plan to build an instrument that contains time-of-flight spectrometers in combination with velocity map imaging for electron-ion coincidence measurement. A wavelength tunable laser will be used to resonantly excite molecules into final ionic states. By recording energy-resolved electron with recoiled fragment ions in coincidence, we are able to clarify the energy content of specific conformer and its dissociation dynamics in detail. Ab initio quantum chemistry calculations will be performed to obtain the photoelectron spectra, electronic states, and the potential energy surfaces of various conformer dissociation channels. It should be noted that this electron-ion coincidence spectrometer will also be used in studies of soft X ray photoionization and photodissociation dynamics.