Rydberg transitions and photoionisation cross section of NH3

Abstract

Abstract Partial photoionisation cross sections of NH3 are calculated for processes involving transitions from the ground state to 2A1 and 2E excited electronic states of the neutral target and residual ion respectively for a wide range of wavelengths using the R-matrix method. Some near threshold structures are resolved. A comparative study is reported for the lone pair 3a 1 electrons with the isoelectronic system, H2O [Modak and Antony 2020 J. Phys. B: At. Mol. Opt. Phys. 53 045202]. We observe that the lone pair of NH3 is loosely bound compared to H2O. A local rise in cross section data for 2A1 (3a 1) electronic state due to autoionisation is also observed from higher excited states. This was earlier predicted by Brion et al [Brion et al 1977 J. Elect. Spectrosc. Rel. Phenom. 12 323]. This is the first report which confirms predissociation due to the dissociative photoionisation of 2E state. A major aim of this study is to resolve Rydberg transitions above the ionisation threshold and quantify them for photoionisation rate coefficients. A recent study by Saraswathya et al [Saraswathya et al 2010 Spectrosc. Lett. 43 290] is the only reported data for few of these Rydberg transitions. These transitions are usually absent in experiments due to the difficulty in resolving narrow energy range. In this investigation, we observed Rydberg transitions above the first ionisation threshold of NH3. An autoionisation peak is also identified in the background of Rydberg transitions.