The initiation and subsequent control or exploration study of chemical transformation in real time by using ultrashort laser pulses aim at femtochemistry.The real-time investigations of ultrafast dynamics of excited molecules in gas and condensed phases have attracted a great deal of attention over the last two decades.As a kind of important organic compound,aliphatic ketone is an area of much interest for many research fields,especially for atmospheric photochemistry.Via photodissociation reaction,it can release carbonyl radical whose chemical character is active and can react with hydroxyl easily.As a typical aliphatic ketone,butanone has been a research focus over the past decades.The ultrafast dissociation dynamics of butanone after excitation to the second electronically excited state (S2) with a 195.8 nm pump pulse is studied by the femtosecond pump-probe technique combined with the time-of-flight mass spectrometry (TOF-MS).Time-resolved mass spectrometry (TRMS) has proven to be a powerful technique to study the ultrafast dynamics of excited states in molecules.In this technique,the MCP detector is capable of recording time-resolved ion yield measurements of different cations by monitoring the current output directly from the anode by using an oscilloscope.This enables a time-of-flight mass spectrum to be recorded at each delay time,which is controlled by a delay stage,and the measured total signal is then integrated,yielding a time-resolved ion yield transient,which is conducted by LABVIEW software.The pump wavelength in this work is set to be 195.8 nm and the probe laser wavelength is centered at 800 nm.The complex ultrafast dynamics in butanone with 3s Rydberg state excitation and its possible decay paths and following dissociation mechanism are given.Experimental results show that the Norrish I type dissociation kinetics of butanone exhibit rich features,for it has a methyl group and an ethyl group at α position.The decay time constant of the parent transient is approximately 2.23 ps ± 0.02 ps.There is only one time constant of 2.15 ps ± 0.02 ps for the fitting of the propionyl transient.The best fit of acetyltransient is obtained with four time constants:τ1 =(2.40±0.15) ps,τ2 =(1.10±0.25) ps,τ3 =(0.08±0.02) ps,and τ4 =(17.72±0.80) ps,corresponding to S2 →S1internal conversion,the primary dissociation of the S1 state generating CH3CO((A)),(A) → (X) internal conversion and secondary dissociation of CH3CO((X)) respectively.Two competitive α-CC bond dissociation processes are observed and discussed.They are dissociation channels through intramolecular vibrational energy redistribution (IVR) and/or by getting over the dissociation barrier in α-cleavage of butanone.But hereunder the condition of this experiment,the dissociation is the result of IVR.%本文采用195.8 nm飞秒激光将丁酮分子激发到S2(n,3s)里德堡态,在800 nm探测光的作用下获得时间分辨的飞行时间质谱.对实验结果的分析表明,由于丁酮α位置具有一个甲基和一个乙基,使得Norrish Ⅰ型解离反应表现出丰富的动力学特征.母体离子瞬态衰减的时间常数为(2.23±0.02) ps.丙酰基离子瞬态衰减与母体类似,只有一个为(2.15±0.02) ps的时间常数,说明丙酰基离子来自于母体的解离性电离.乙酰基离子的时间曲线拟合得到四个时间常数:τ1=(2.40士0.15) ps,τ2=(1.10士0.25) ps,τ3=(0.08士0.02) ps,τ4=(17.72士0.80) ps,分别对应于S2→S1的内转换,S1态生成CH3CO((A))的初步解离,CH3CO((A))快速内转换为CH3CO((X)),以及CH3CO((X))基态上的二次解离.丁酮分子α-C-C键的解离存在分子内振动能量再分配(IVR)与势垒解离两种竞争的解离通道,但在该实验条件下,我们认为是通过分子内振动能量再分配通道发生解离的结果.
展开▼
