Ultrafast excited-state dynamics in a prototype of the peptide bond: Internal conversion of the isolated N,N-dimethylformamide

A combination of femtosecond time-resolved photoelectron imaging technique and time-resolved mass spectroscopy technique are implemented to investigate the electronic excited-state dynamics in N,N-dimethylformamide (NNDMF). The ultrafast internal conversion (IC) of S-1 (n pi*) and S-2 (pi pi*) excited states of NNDMF are observed in this experiment. The molecule is excited to the lowest-lying (1)pi pi* state (S-2 state) following absorption of two 400-nm photons. It is found that the population of the S-2 (pi pi*) state undergoes ultrafast IC to the highly vibrationally excited S-1 (n pi*) state within 99 fs by very fast C-N stretching, while the nonradiative deactivation of the S-1 (n pi*) state occurs in 2.4 ps, and it is to a large extent due to the C-N bond cleavage from the S-1 potential energy surface, which would be able to efficiently compete with the IC of S-1 -> S-0 through S-1/S-0 conical intersections.