VUV laser-induced photofragmentation of NH3: Internal energy distribution of NH(<i>A</i> 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 Trot=3000 K and Tvib=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 kq=3.5±0.7×10−10 molecule−1 s−1 cm3. The different mechanisms for the formation of NH (A 3Π) are discussed.