Vacuum ultraviolet spectrum of the iodine molecule

Abstract

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 ²Π _3/2g 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 [σ _g²π _u⁴Π_g³²Π_3/2g]np σ_u Π_1u, [σ _g²π _u⁴π _g³²Π_3/2g]np π _u Σ ⁺ (0_u⁺), [σ _g²π _u⁴π _g³²Π_3/2g]nf σ _u Π_1u, [σ _g²π _u⁴π _g³²Π_3/2g]nf π _u, Σ ⁺(0_u⁺), and [σ _g²π _u⁴π _g³²Π_3/2g]nf δ _u Π_1u 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 [σ _g²π _u⁴π _g³²Π_1/2g] 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 [σ _g²π _u 3π _g⁴²Π_3/2u], [σ _g²π _u³π _g⁴²Π_1/2u], and [σ _g²π _u⁴π _g²σ _u²Σ _u⁺]. Three of the observed band systems do not appear to involve Rydberg states and their upper levels are very likely the ¹Π(1_u), ³Π(1_u), and ³Π(0_u⁺) states arising from the configuration σ _gπ _u³π _g⁴σ _u². 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.