Photoelectron spectroscopy of small IBr-(CO2)n (n=0-3) cluster anions.

We report the photoelectron spectra of small IBr-(CO2)n cluster anions (n=0-3). The vibrational state-resolved spectrum of IBr- permits reliable identification of the origins of the excited A' 3Pi2 and A 3Pi1 states of neutral IBr through a high-quality Franck-Condon spectral simulation. As a result, we directly determine several important spectroscopic parameters: the adiabatic electron affinity (EA) of IBr, EA=2.512+/-0.003 eV, the ground electronic state (X 2Sigma(1/2)+) bond strength of IBr-, D0(IBr-)=0.966+/-0.003 eV, its equilibrium bond length, R(e)(X)(IBr-)=3.01+/-0.01 A, and its vibrational frequency, omega(e)(IBr-)=134+/-10 cm(-1). These values represent a substantial improvement over existing experimental information and are in good agreement with recent theoretical studies. The photoelectron spectra of the first three cluster anions, IBr-(CO2)n (n=1-3), do not exhibit resolved vibrational structure, but the similarity to the IBr- photoelectron spectrum indicates minimal electron delocalization onto the solvent. The cluster anion spectra shift to progressively higher electron binding energies, providing information on the magnitude of the solvent perturbation and estimates of the EA of IBr-(CO2)n.