Time-resolved Photodissociation of Acetaldehyde Molecules by Ultraviolet Excitation
作者单位：Institute of Atomic and Molecular Sciences“Academia Sinica” Department of PhysicsTaiwan University
会议名称：《第十五届全国化学动力学会议》
会议日期：2017年
学科分类：081704[工学-应用化学] 07[理学] 08[工学] 0817[工学-化学工程与技术] 070303[理学-有机化学] 0703[理学-化学]
关 键 词：acetaldehyde roaming time-resolved VMI REMPI radiationless
摘 要：The broad near-UV absorption continuum of acetaldehyde(CH3CHO) spanning 240-330 nm has been studied extensively due to the significant roles of the resulting radical products in atmospheric chemistry. In this absorption range, the ground state of acetaldehyde is mainly excited to the first electronic excited singlet state(S) followed by fluorescence or radiationless processes such as internal conversion and intersystem crossing that then lead to different fragmentations. Recently, a roaming pathway, which is anomalous compared with the conventional transition state mechanism, has been proven to play a significant role in the CO+CH channel from photodissociation of acetaldehyde using static spectroscopy methods. In order to take a different view of the roaming pathway in the time domain, the picosecond(ps) time-resolved time-of-flight(TOF) spectroscopy and velocity map ion imaging(VMI) are implemented to study time evolutions of fragments from the excitation of acetaldehyde to the S state. A 10% gas mixture of acetaldehyde is expanded into the reaction chamber and subsequently pumped and probed by two time-delayed laser beams(pulse width .7 ps). The probe wavelengths are chosen either at 33 nm through 2+1 resonance enhanced multiphoton ionization(REMPI) of CH or at 30 nm through 2+1 REMPI of CO and 1+1 REMPI of HCO. In addition to resonance detection products, products from dissociative ionization are observed due to the relatively high peak intensities of ps pulses. From the TOF measurements, the CH signals probed at the REMPI wavelength of ground state show an exponential growth with a rise time .2 ns, 00 ps, or 00 ps at 286 nm, 276 nm, or 266 nm pump wavelength, respectively, which are presumably dominated by radiationless processes. From pump-probe vibrational state selected ion imaging data, the energy partitions of products are partially disentangled with picosecond time resolution.