NEGATIVE ION PHOTOELECTRON SPECTROSCOPY OF $AI_{3}O^{-}$ AND $AI_{4}O^{-}$

Abstract

The 488 nm photoelectron spectra of $Al_{3}O^{-}$ and $Al_{4}O^{-}$ are reported and interpreted in light of ab initio calculations by Alex $Boldyrev^{1}$ on these anions and the corresponding neutral clusters. The interpretations of the photoelectron spectra were facilitated by using the calculated structures, vibrational frequencies, etc. to generate simulated photoelectron spectra for comparison with the observed ones. The $Al_{3}O^{-}$ spectrum displays transitions from the planar $C_{2r}, ^{1}A_{1}$, ground state of the $Al_{3}O^{-}$ anion to the $^{2}B_{2}$ ground state and to one excited electronic state of neutral $Al_{3}O$. The ground state transition displays a harmonic progression in an in-plane Al-O-Al bending mode with a frequency of $160 \pm 10 cm^{-1}$ in the neutral molecule and $205 \pm 15 cm^{-1}$ in the anion. The electron affinity of $Al_{3}O$ is $1.151 \pm 0.005 $eV. The $Al_{4}O^{-}$ spectrum displays two vibrationally resolved transitions from the $C_{2v} {^{2}}A_{1}$ ground state of $Al_{4}O^{-}$ which has an $Al-O-Al_{3}$ structure with the AI-O unit terminally bonded to an equilateral $Al_{3}$ unit, to the low-lying $^{1}A_{1}$ and $^{3}B_{1}$ excited states of neutral $Al_{4}O$. The transition to the calculated $D_{4h}$ ground state of $Al_{4}O$ is not observed due to its poor Franck-Condon overlap with the anion. The excellent agreement of the simulated and observed photoelectron spectra of $Al_{3}O$ and $Al_{4}O^{-}$ provides strong support for the $theoretical^{1}$ results.