The ultrafast nonradiative processes and photodissociation dynamics investigation of S1 state in propanal.

Abstract

The ultrafast nonradiative dynamics in the S1 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 320nm. The population of the S1 state undergoes ultrafast internal conversion (IC) to the highly vibrationally hot S0 state in a timescale of <100fs and nonradiative deactivation by intersystem crossing (ISC) to triplet T1 state occurring with a time constant of about several hundreds of femtoseconds. The ISC process is then followed by the dissociation on the T1 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 540fs, which agrees well with the measured ISC relaxation time constant of 430fs. The CO molecule is proposed to form at about 170fs after the excitation, supporting the dissociation mechanism via the molecular channel following the IC decay of the S1 state. The energy of the first excited electronic state of the C3H6O+ is obtained to be 12.25eV.