丙烯酸分子的激发态超快预解离动力学

摘要

The ultrafast predissociation dynamics of acrylic acid after excitation to the second electronically excited state (S2) with a 200 nm pump pulse were studied using a femtosecond pump-probe technique combined with time-of-flight mass spectroscopy (TOF-MS).The time-resolved mass spectra signals of the parent ion and fragment ions were collected.By using the kinetic equations to fit and analyze the time-resolved mass spectra ion signals,the existence of the predissociation channel was revealed.The excited molecule populated in the S2 state decayed to the first electronically excited state (S1) through a fast intemal conversion process over a period of 210 fs.The excited molecule populated on the S,state then decayed to the vibrationally hot ground state (S0) through another intemal conversion process over a period of 1.49 ps.Finally,on the vibrationally hot ground state surface,the molecule dissociated to the neutral fragments,H2C=CH and HOCO,H2C=CHCO and OH via C- C bond fission and C- O bond fission,respectively.The corresponding predissociation time constants were determined to be approximately 4 and 3 ps,respectively.The generation of fragment ions can occur in two ways,both from the dissociation of the parent ion and the ionization of the neutral fragments on the vibrationally hot ground state surface.%利用飞秒泵浦-探测技术结合飞行时间质谱(TOF-MS),研究了丙烯酸分子被200 nm泵浦光激发到第二电子激发态(S2)后的超快预解离动力学.采集了母体离子和碎片离子的时间分辨质谱信号,并利用动力学方程对时间分辨离子质谱信号进行拟合和分析,揭示了预解离通道的存在.布居在S2激发态的分子通过快速的内转换弛豫到第一电子激发态(S1),时间常数为210fs,随后再经内转换从S1态弛豫到基态(S0)的高振动态,时间常数为1.49 ps.分子最终在基态高振动态势能面上发生C-C键和C-O键的断裂,分别解离生成H2C=CH和HOCO、H2C=CHCO和OH中性碎片,对应的预解离时间常数分别约为4和3 ps.碎片离子的产生有两个途径,分别来自于母体离子的解离和基态高振动态势能面上中性碎片的电离.