子計畫一：飛秒超強光雷射之研究；子計畫二：光化學的超快過程之研究

Abstract

我們在本計畫中與實驗團隊合作，研究高功率雷射所致2-甲基及3-甲基環戊酮的游 離性解離反應中，甲基取代位置的效應。雷射功率較低（小於1014 瓦／平方公分）時， 分子解離符合統計結果，可用RRKM 理論解釋。當功率增加，另一種解離途徑──場 輔助解離隨之浮現。故本計畫另一目的在於以理論研究場輔助解離，將以時間相關絕熱 理論描述之。 初步研究成果顯示，某些時段中位能面呈現可解離狀態，而其他時段則為不可解離 的穩定狀態，這與場輔助解離現象相符。由於時間相關絕熱理論本身尚未有深入研究， 我們除了將它應用於場輔助解離之外，亦將推廣之以解釋表面加強拉曼散射、尖端加強 拉曼散射等。 光致質子（或氫原子）轉移反應中，所觀察到的同位素效應不如預期的明顯，因此 我們在本計畫中將以不同的角度處理光致質子（或氫原子）轉移問題：將此過程視為兩 個激發態位能面（初始激發態位能面與轉移後的最終激發態位能面）之間的躍遷，亦即 視為非絕熱躍遷。初步結果顯示，強耦合狀況下可得到Marcus 類型方程式，具有顯著 的溫度效應；弱耦合狀況下則得到量子速率表示式，溫度效應減弱。預期同位素效應並 不明顯，乃是因為在非絕熱躍遷中，Franck-Condon 因子佔有非常重要的角色。 目前商業化的量子化學計算程式已可提供可靠的非簡諧位能面，故能用以研究凝態 分子的振動弛豫、孤立分子（無碰撞環境）的分子內振動弛豫等等。本計畫將利用我們 於1970 年代提出的耦合振子模型，結合絕熱近似、ab initio 計算位能面來研究此類重要 問題。與此相關的予體分子及受體分子間振動能轉移問題也將進行研究。

In collaborating with the experimental group, in this project we shall study the high-power laser ionization-dissociation of 2-methyl- and 3-methyl-cyclopentanone, that is, studying the effect of the methyl substitution. Although in the low intensity range, I<1014 W/cm2, the molecular dissociation takes place in a statistical manner so that the RRKM theory can be used. As I increases, another channel of molecular dissociation, field assisted dissociation (FAD), opens up. Another purpose of this project is to theoretically study FAD; it will be treated by using the time-dependent adiabatic theory. In our preliminary work, we have shown that at a certain time-interval, the potential surface become dissociative while at other intervals the surface becomes stable. This is compatible with the FAD behaviors. Since this time-dependent adiabatic theory is not much studied, we shall not only apply it to study FAD but also generalize it so that it can be applied to surface-enhanced Raman scattering, tip-enhanced Raman scattering etc. Due to the observation that the isotope effect in photo-induced proton (or hydrogen) transfer is not as significant as is to be expected, in this project we shall propose a different approach for treating the photo-induced proton (or hydrogen) transfer in which we regard this process as a transition between the two excited surfaces (that is, the initial excited state surface and the final transferred state surface). In other words, we shall regard the photo-induced proton (or hydrogen) transfer as a non-adiabatic transition. Our preliminary results indicate that in the strong coupling case we obtain the Marcus-type of equation which exhibits a significant temperature effect and that in the weak coupling case we obtain the quantum rate expression which exhibits a weak temperature effect. The isotope effect will be expected to be not significant because in a non-adiabatic transition, the Frank-Condom factors play a very important role in the transition. Due to the fact that the commercial quantum-chemical calculations programs can provide reliable anharmonic potential surfaces, it has become possible to study the vibrational relaxation for molecules embedded in a condensed phase and the intramolecular vibrational relaxation for an isolated molecule (that is, a molecule in collision-free condition). In this project, we shall employ the coupled oscillator model (which we proposed in 1970’s) combined with the adiabatic-type approximation to study this important problem. Another process, the vibrational energy transfer between the donor molecule and the acceptor molecule will also be studied. For these investigations the ab initio potential surfaces will be employed.