Tracking the photodissociation dynamics of liquid nitromethane at 266nm by femtosecond time-resolved broadband transient grating spectroscopy

Highlights: • The photodissociation of liquid NM is monitored by TG technique. • WLC is used as the probe to simultaneously monitor the reactants and the products. • The dissociation ratio of liquid NM is about 27%. • NO₂ is directly observed in the TG spectra. Femtosecond time-resolved transient grating (TG) technique was employed to get insight into the photodissociation mechanism of liquid nitromethane (NM). Broadband white-light continuum was introduced as the probe to observe the evolution of electronic excited states of NM molecules and the formation of photodissociation products simultaneously. The reaction channel of liquid NM under 266 nm excitation was obtained that NM molecules in excited state S₂ relax through two channels: about 73% relax to low lying S₁ state through S₂/S₁ internal conversion with a time constant of 0.24 ps and then go back to the ground state through S₁/S₀ internal conversion; the other 27% will dissociate with a time constant of 2.56 ps. NO₂ was found to be one of the products from the experimental TG spectra, which confirmed that C–N bond rupture was the primary dissociation channel of liquid NM.