Ultrafast S_1 to S_0 internal conversion dynamics for dimethylnitramine through a conical intersection

摘要

Electronically nonadiabatic processes such as ultrafast internal conversion (IC) from an upper electronic state (S_1) to the ground electronic state (S_0) though a conical intersection (CI), can play an essential role in the initial steps of the decomposition of energetic materials. Such nonradiative processes following electronic excitation can quench emission and store the excitation energy in the vibrational degrees of freedom of the ground electronic state. This excess vibrational energy in the ground electronic state can dissociate most of the chemical bonds of the molecule and can generate stable, small molecule products. The present study determines ultrafast IC dynamics of a model nitramine energetic material, dimethylnitramine (DMNA). Femtosecond (fs) pump-probe spectroscopy, for which a pump pulse at 271 nm and a probe pulse at 405.6 nm are used, is employed to elucidate the IC dynamics of this molecule from its S_1 excited state. A very short lifetime of the S_1 excited state (～50 ± 16 fs) is determined for DMNA. Complete active space self-consistent field (CASSCF) calculations show that an (S_1/S_0)_(CI) CI is responsible for this ultrafast decay from S_1 to S_0. This decay occurs through a reaction coordinate involving an out-of-plane bending mode of the DMNA NO_2 moiety. The 271 nm excitation of DMNA is not sufficient to dissociate the molecule on the S_1 potential energy surface (PES) through an adiabatic NO_2 elimination pathway.