Photoelectron spectroscopy of As−, As2−, As3−, As4−, and As5−

The negative ion photoelectron spectra of ${\mathrm{As}}^{-},$ ${\mathrm{As}}_2^{-},$ ${\mathrm{As}}_3^{-},$ ${\mathrm{As}}_4^{-},$ and ${\mathrm{As}}_5^{-}$ have been measured. From these, the electron affinities of As, ${\mathrm{As}}_2,$ ${\mathrm{As}}_3,$ ${\mathrm{As}}_4,$ and ${\mathrm{As}}_5$ have been determined to be 0.814, 0.739, 1.45, <0.8, and ∼1.7 eV, respectively. In the case of ${\mathrm{As}}_2^{-},$ the following molecular constants were also determined: $r_e({\mathrm{As}}_2^{-}\text{)=2.239\hspace{0.17em}Å,}$ ${\omega }_e({\mathrm{As}}_{\mathrm{2}}^{\mathrm{-}}{\mathrm{)=293\hspace{0.17em}cm}}^{\text{−1}},$ ${\omega }_e{\chi }_e({\mathrm{As}}_{\mathrm{2}}^{\mathrm{-}}{\mathrm{)=4.9\hspace{0.17em}cm}}^{\text{−1}},$ $D_0({\mathrm{As}}_{\mathrm{2}}^{\mathrm{-}}\mathrm{)=3.89\hspace{0.17em}eV},$ and ${\mathrm{\Delta E[}}^2{\Pi }_g{\text{(3/2)−}}^2{\Pi }_g\text{(1/2)]=0.256\hspace{0.17em}}\mathrm{eV}.$ In the case of ${\mathrm{As}}_3^{-},$ vertical detachment energy (VDE) was measured to be 1.62 eV, and for ${\mathrm{As}}_3,$ ${\mathrm{\Delta E(}}^2{A}_2{-}^2{B}_1)$ was determined to be 0.36 eV. For ${\mathrm{As}}_4^{-},$ VDE was found to be 1.52 eV. The relatively high stability of ${\mathrm{As}}_5^{-}$ suggests that it, like ${\mathrm{P}}_5^{-},$ may be a candidate for forming cluster-assembled, ionic crystals of stoichiometry, ${\mathrm{MAs}}_5,$ where M is an alkali metal atom. Similiarities with other small cluster anions of Group V elements are also discussed.