The 248 nm photolysis of NO2/N2O4: Time-resolved Fourier transform infrared emission from NO and NO2, and quenching of NO (v=5–8)

Abstract

The photolysis of NO2 and N2O4 has been studied at 248 nm by observations of time-resolved Fourier transform infrared emission from the photofragments. The photolysis of NO2 produces emission in the Δv=−1 and −2 fundamental and overtone bands of NO(X 2Π), and spectral analysis yields a broad Gaussian-type distribution in the vibrational levels v=2–8, in good agreement with one of two previously reported initial nascent quantum state distributions. Quenching of the higher levels (v=5–8) of NO in collisions with NO2 produces rate constants which increase with increasing v with values between 0.91 and 3.5×10−11 cm3 molecule−1 s−1. The process is shown to have a larger component of resonance energy transfer from NO(v) to NO2(0,0,1) than previously reported values for the rate constants at lower v which are further from resonance. A fast component of IR emission from the nascent excited states of NO2 is observed, together with slower decaying emissions near 1450 and 2750 cm−1, assigned as Δν3=−1 and Δν1=Δν3=−1 transitions from high vibrational levels of the ground state formed by quenching of electronically excited NO2 produced from the photolysis of N2O4. A comparison is made of these IR bands with similar features seen in the IR emission from NO2 following electronic excitation in the visible region below its dissociation limit. Further emission near 1880 cm−1 accompanies the photolysis of N2O4, and is tentatively assigned to the direct formation of NO as a photolysis product, with a non-negligible quantum yield in low vibrational levels.