Photoelectron Spectra of the Halogens

Abstract

Photoelectron spectra of valence shell electrons in F2, Cl2, Br2, and I2 yield information on the molecular and electronic structure of the lowest several states of the corresponding positive ions, some of which is not available from previous spectroscopic studies. Spin–orbit fine structure is resolved for the ground 2 Π g states of F2 + and Cl2 +; values obtained are ζ = 337 ± 40 and 645 ± 40 cm −1 , respectively. The first excited states of the ions are identified as 2 Π u and approximate values of ζ = 2000–2200 and 6400 cm −1 obtained for Br 2 + (A 2 Π u ) and I 2 + (A 2 Π u ) , respectively. A state in F2 + reported previously [D. C. Frost, C. A. McDowell, and D. A. Vroom, J. Chem. Phys. 46, 4255 (1967)] to lie at ∼ 17.4 eV is not observed here and is attributed to nitrogen impurity. Vibrational frequencies for F2 + and Cl 2 + (X 2 Π g ) are in agreement with spectroscopic work. The corresponding frequencies for Br2 + and I2 + are ω e = 360 ± 40 and ∼ 220 cm−1. A change in bond length to Br 2 + ( 2 Π g ) of Δr e ∼ (−)0.095 Å is estimated from the vibrational envelope of the X 2 Π g state of Br2 +.