The ultrafast nonradiative processes and photodissociation dynamics investigation of S state in propanal.

The ultrafast nonradiative dynamics in the S electronic excited state and the corresponding photodissociation dynamics in propanal molecules have been studied with time-resolved photoelectron imaging and time-of-flight mass spectrometry at an excitation wavelength of 320 nm. The population of the S state undergoes ultrafast internal conversion (IC) to the highly vibrationally hot S state in a timescale of <100 fs and nonradiative deactivation by intersystem crossing (ISC) to triplet T state occurring with a time constant of about several hundreds of femtoseconds. The ISC process is then followed by the dissociation on the T surface because the excitation energy is higher than the dissociation barrier along the C-C(HO) bond length coordinate. The dissociation product of the CHO radical has an appearance time of about 540 fs, which agrees well with the measured ISC relaxation time constant of 430 fs. The CO molecule is proposed to form at about 170 fs after the excitation, supporting the dissociation mechanism via the molecular channel following the IC decay of the S state. The energy of the first excited electronic state of the CHO is obtained to be 12.25 eV.