Wavelength-dependent Photodissociation Dynamics of Benzaldehyde

The ultrafast dynamics of benzaldehyde upon 260, 271, 284, and 287 nm excitations have been studied by femtosecond pump-probe time-of-flight mass spectrometry. A biexponential decay component model was applied to fit the transient profiles of benzaldehyde ions and fragment ions. At the S₂ origin, the first decay of the component was attributed to the internal conversion to the high vibrational levels of S₁ state. Lifetimes of the first component decreased with increasing vibrational energy, due to the influence of high density of the vibrational levels. The second decay was assigned to the vibrational relaxation of the S₁ whose lifetime was about 600 fs. Upon 287 nm excitation, the first decay became ultra-short (～56 fs) which was taken for the intersystem cross from S₁ to T₂, while the second decay component was attributed to the vibrational relaxation. The pump-probe transient of fragment was also studied with the different probe intensity at 284 nm pump.