Accessing a low-lying bound electronic state of the alkali oxides, LiO and NaO, using laser induced fluorescence

Abstract

Abstract The first laser based probe for the sodium and lithium monoxides is established. The Li(Na)+N2O reactions studied in a multiple collision entrainment mode produce the LiO and NaO ground X2Π and low-lying monoxide excited states. In contrast to the alkali halides, laser induced excitation spectroscopy confirms that the LiO and NaO B 2Π states, counter to recent predictions, are located at energies well below the ground state dissociation asymptote and, as predicted, possess significant binding energies. An assignment of the laser induced excitation spectra (LIF) for the B 2Π-X 2Π transitions of LiO in the region 3940–4300 A is based on a direct correlation with the observed chemiluminescence (CL) from the lowest level of the LiO B 2Π state ( ∼4000–7000 A ) and high quality ab initio calculations for the ground state. The self-consistent assignment of the observed LIF and CL spectra makes use of the complimentary extended progressions in the X 2Π (CL) and B 2Π (LIF) vibrational level structure which results from the significant shift of the B 2Π excited state potential relative to that of the ground state. The experimental data are consistent with an excited state vibrational frequency separation of order 130 cm−1, and T e ( B 2 Π) ≈ 26078 ± 800 cm −1 . The latter value, in correlation with the ground state dissociation energy of LiO, suggests a B 2Π excited state dissociation energy well in excess of 2000 cm−1. The radiative lifetimes of the lowest levels of the LiO B 2Π state, isoergic with the highest levels of the LiO ground state, are determined to be in excess of 600 ns. The corresponding NaO excitation spectra in the range 6680–7250 A also correlate well with ab initio calculations for the ground electronic state of NaO. Within this study, we provide optical signatures which one might consider to monitor LiO or NaO in process streams. In correlation with the observed chemiluminescence from B 2Π states of the higher alkali oxides KO, RbO, and CsO, this study suggests regions where one might probe their excitation spectra.