VUV laser-induced photofragmentation of NH3: Internal energy distribution of NH(A 3Π)

Abstract

Ammonia is photolyzed with a xenon excimer laser at 172 nm. A weak emission observed between 539–572 nm is assigned to high vibrationally excited states of NH₂(Ã ²A₁), formed through a single photon process. However, the main feature of the emission spectra is due to NH(A ³Π), formed by a two‐photon process and detected through the identification of the A ³Π → X ³Σ⁻ transition between 328 and 345 nm. A simulated spectrum for T_rot=3000 K and T_vib=10 000 K was found to give the best fit with the experimental spectrum for the observed relative populations of v′=0, v′=1 and their rotational levels. The measured natural lifetime τ₀=400±70 ns should be considered as an average value for v′=0 and v′=1 populated to J′≂25. The quenching rate constant by NH₃ was found to be k_q=3.5±0.7×10⁻¹⁰ molecule⁻¹ s⁻¹ cm³. The different mechanisms for the formation of NH (A ³Π) are discussed.