本篇論文中我們以飛秒雷射激發-探測光游離光裂解實驗技術(Femtosecond Pump-probe Photoionization-Photofragmentation technique)結合質譜偵測技術,研究[phenol-N-methylformamide]+ 及[phenol-N,N-dimethylformamide ]+([PhOH-NMF]+及[PhOH-DMF]+)陽離子錯合物的質子轉移過程及錯合物異構反應的相關動力學。我們在分子束中分別製備PhOH-NMF及PhOH-DMF中性錯合物,以resonance-enhanced multiphoton ionization (REMPI)技術(1+1, λ_pump=261.3~272 nm)經由S1態游離化PhOH-NMF及PhOH-DMF錯合物,再將探測脈衝延遲入射(λ_probe=392~408 nm)得到[PhOH-NMF]+及[PhOH-DMF]+離子碎裂強度隨時間變化的瞬時訊號。這兩者的瞬時訊號在不同波長下隨時間變化的趨勢與前人研究的[PhOH-NH3]+類似,證實[PhOH-NMF]+及[PhOH-DMF]+系統有質子轉移現象發生,但是質子轉移的速率較[PhOH-NH3]+的系統慢了許多,顯示在本系統中質子轉移途徑上有能量障礙。理論計算結果亦顯示PhOH-cisNMF陽離子態質子轉移途徑上存在著小能障與實驗結果吻合,而PhOH與amide形成氫鍵錯合物時存在兩種異構物,我們進行RRKM計算結果顯示異構化反應的時間尺度約在幾十皮秒。[PhOH-NMF]+以多指數模型適解得到三個生命期,分別為τ_1≈0.23 ps 指認為PhOH-NMF由雷射游離至[PhOH-NMF]+FC所引發的初始波包運動,τ_2=3 ps~18 ps為質子轉移過程,τ_3=40ps~81 ps維錯合物進行異構化。[PhOH-DMF]+以連續模型適解得到三個生命期,分別為τ_1≈0.3 ps為初始波包運動,τ_1≈2 ps為振動能緩解,τ_3=15ps~17 ps維錯合物進行異構化。
We report studies of ultrafast proton transfer (PT) reaction of phenol-N-methylformamide(PhOH-NMF) and phenol-N,N-dimethylformamide (PhOH-DMF) cation complexes by using femtosecond pump-probe photoionization-photofragmentation spectroscopy (fs-PIPF). Neutral PhOH-NMF and PhOH-DMF complexes prepared in a free jet are photoionized by femtosecond [1+1] resonance-enhanced multiphoton ionization via S1 state, and the subsequent dynamics occurring in the cations is probed by delayed pulses that result in ion fragmentation. The observed probe-wavelength dependence of the transients are consistent with a spectral evolution from the initial non-PT to final PT states. The combination of experimental results and DFT calculations suggest that a low barrier is present along [PhOH-NMF]+ cation proton transfer reaction coordinate. There are three distinct time scales in [PhOH-NMF]+ experiments : τ_1≈0.23 ps is assigned to an initial wave-packet motion, τ_2=3 ps~18 ps is assigned to the proton transfer, and τ_3=40ps~81 ps is attributed to to complex isomerization. The experiments revealed that PT in [PhOH-DMF]+ cation also proceeds in three distinct steps: τ_1≈0.3 ps an initial wave-packet motion, followed by a slower relaxation τ_2≈2 ps and a final step τ_3=15ps~17 ps of complex isomerization.