高振動激發態薁分子與單原子氣體─氪氣碰撞能量轉移研究

Abstract

本研究主要透過交叉分子束及時間切片速度相配離子影像〈time-slice velocity map ion images〉偵測技術，探討高振動激發態薁分子〈Azulene〉與惰性氣體原子─氪〈Kr〉間能量轉移的動態學。其中，高能薁分子是以266 nm〈4.66 eV〉雷射將其激發至S4的電子態，再經快速內轉換〈Internal conversion〉機制所產生的基態電子態上高振動能階之熱分子〈 ‡〉。由於薁分子本身之獨特光物理特性，我們得以利用其產生一高純度的熱分子束，並簡單藉由157 nm及118 nm兩真空紫外光加以定量。從偵測之熱薁分子散射影像，我們便能直接獲得分子碰撞後動能及角度的分佈訊息，並量得特定初始碰撞能量下〈170 cm-1、465 cm-1及780 cm-1〉能量轉移機率分佈函數─ 的全貌。當碰撞能量足夠低時，影像上明顯存在形成Az-Kr的凡得瓦分子，這些凡得瓦分子造成小部分平移動能轉移至薁分子的轉動或振動能階上〈T  V/R的能量轉移〉。此外，實驗上也發現T  V/R的能量轉移效益相當高，某些情況下，分子間的相對動能幾乎完全被轉換成分子的轉動或振動能量。另ㄧ方面，薁分子的振動能量僅小部分比例會轉成平移動能的形式，且此V  T的能量轉移機率分佈曲線需以複合指數函數〈multiexponential function〉描述，薁分子向後散射方向影像並存在高能量轉移的分佈成分，來自於超級碰撞〈supercollision〉的結果。實驗上，碰撞能量465 cm-1及780 cm-1時，薁分子散射方向 所觀測之轉移量 的碰撞成分約為1%及0.3%。

The energy transfer dynamics between highly vibrationally excited azulene was studied using a crossed-beam apparatus along with time-sliced velocity map ion imaging technique. "Hot" azulene (4.66 eV internal energy) mole- cules and Kr atoms in a series of collision energies (170, 465 and 780 cm-1) was formed via the rapid internal conversion (IC) of azulene initially excited to the S4 state by 266 nm photons. Attributing to the unique photophysical properties,we can create a near pure highly vibrationally excited molecular beam and characterize the relative concentration of hot molecules. The shapes of the collisional energy-transfer probability distribution functions were measured directly from the scattering results of highly vibrationally excited or "hot" azulene. At low enough collision energies an azulene-Kr complex was observed, resulting from small amounts of translational to vibrational/rotational (T-V/R) energy transfer. T-V/R energy transfer was found to be quite efficient. In some instances, nearly all of the translational energy is transferred to vibrational/rotational energy. On the other hand, only a small fraction of vibrational energy is converted to translational energy (V-T). The V-T energy transfer distribution functions were best fit by multiexponential functions. We find that substantial amounts of energy are transferred in the backward scattering direction due to supercollisions at high collision energies. The probability for supercollisions, defined arbitrary as the scattered azulene in the region 160o<<180o and Edown>2000 cm-1, is about 1% and 0.3% of all other collisions at collision energies 465 and 780 cm-1, respectively.