Vacuum ultraviolet spectrum of the iodine molecule

The absorption spectrum of iodine has been photographed in the higher orders of a 10.7 m concave grating spectrograph in the region 1950–1200 Å. A number of band systems has been obtained, most of which correspond to Rydberg transitions. The limit of the Rydberg series leading to the 2Π3/2g state of the molecular ion has been recorded and the corresponding ionization potential is found to be 75 814 ± 10 cm−1. Five series have been found which terminate at this common limit. They represent transitions from the ground state to [σg2πu4Πg32Π3/2g]np σu Π1u, [σg2πu4πg32Π3/2g]np πu Σ+ (0u+), [σg2πu4πg32Π3/2g]nf σu Π1u, [σg2πu4πg32Π3/2g]nf πu, Σ+(0u+), and [σg2πu4πg32Π3/2g]nf δu Π1u states respectively, where n takes the running values 6, 7, 8, … etc. for the first three series and 4, 5, 6, … etc. for the last two series. The series limit of the bands corresponding to the molecules with ν″ = 1 has been found to be at 75 600 ± 10 cm−1. The first few members of similar series corresponding to transitions to different states involving the common [σg2πu4πg32Π1/2g] core have been identified and the ionization limit of these series is estimated to be at 80 895 ± 50 cm−1. Some of the remaining band systems obtained have been found to be very likely due to transitions to the Rydberg states involving molecular ion cores like [σg2πu 3πg42Π3/2u], [σg2πu3πg42Π1/2u], and [σg2πu4πg2σu2Σu+]. Three of the observed band systems do not appear to involve Rydberg states and their upper levels are very likely the 1Π(1u), 3Π(1u), and 3Π(0u+) states arising from the configuration σgπu3πg4σu2. The positions of these levels are at 64 956, 62 844, and 61 847 cm−1, the corresponding vibrational frequencies being 145, 144, and 109 cm−1 respectively.