Molecular Photodetachment Spectrometry. I. The Electron Affinity of Nitric Oxide and the Molecular Constants of NO−

Abstract

We apply laser photodetachment and photoelectron spectrometry for the first time to the study of molecular negative ions. We describe in detail the study of the nitric oxide in N${\mathrm{O}}^{-}$; a following paper reports results for ${\mathrm{O}}_2^{-}$. We use the N${\mathrm{O}}^{-}$ results to develop and illustrate in detail the principles and applications of the technique. A mass-selected N${\mathrm{O}}^{-}$ beam (680 eV) is crossed with a linearly polarized monochromatic (4880-Å) argon-ion laser beam, and electrons photodetached into a $\frac{4\pi }{2000}$-sr solid angle perpendicular to the crossed beams are energy analyzed using a hemispherical electrostatic monochromator. The data yield a set of vertical detachment energies between vibrational states of N${\mathrm{O}}^{-}$ and NO, and relative intensities for these transitions. Angular distributions about the polarization direction are studied by rotating the laser polarization while maintaining the mutually perpendicular ion-beam-laser-beam-electron-collection geometry. For each transition we measure the anisotropy parameter $\beta$, corresponding to the form [$1+\beta P_2\left(cos\theta \right)$] for the angular distribution. Several arguments, including data on N${\mathrm{O}}^{18-}$ photodetachment, are used to identify the initial and final vibrational states. Molecular rotational effects, and effects associated with the spin-orbit splitting of the final $\mathrm{NO}\left(X{}_{}{}^2{}_{}{}^{}\Pi \right)$ state are identified and included in the analysis. A Franck-Condon-factor analysis of the observed relative cross sections, parametrized by trial values of the N${\mathrm{O}}^{-}$ molecular constants, is used to determine that for N${\mathrm{O}}^{-}$ ${\omega }_e^{\prime \prime }=1470\mathrm{}\pm 200$ ${\mathrm{cm}}^{-1\mathrm{}}$, ${\mathcal{r}}_e^{\prime \prime }=1.258\mathrm{}\pm 0.010$ Å, and $B_e^{\prime \prime }=1.427\mathrm{}\pm 0.02$ ${\mathrm{cm}}^{-1\mathrm{}}$. Using these constants, the measured vertical detachment energy is reduced by rotational and spin-orbit effects to the electron affinity $E_A\left(\mathrm{NO}\right)={24}_{-5\mathrm{}}^{+10\mathrm{}}$ meV.