Abstract: 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 internal conversion process over a period of 210 fs. The excited molecule populated on the S₁ state then decayed to the vibrationally hot ground state (S₀) through another internal 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.

Key words: Time-resolved mass spectrum, Ultrafast dynamics, Pump-probe, Time-of-flight mass spectroscopy, Internal conversion