2491-Å Absorption System of NO2 and a Double-Minimum Potential

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Abstract
The 2491-Å absorption system of N16O2 and the corresponding system of N18O2 have been photographed at temperatures up to 600°C. For each molecular species measurements yield the ground-state frequencies ν1″, ν2″, 2ν1″,and ν1″ + ν2″ as well as the excited-state frequencies ν1′, ν2′ and 2ν3′. A large isotope shift observed for the (0–0) band is found to be consistent with the assignment 2ν3′ = 713 cm−1 even though this frequency is quite small compared to the frequency ν1′ = 1156 cm−1. Two facts which demand explanation are the isotope shift of the frequency ,2ν3′, and the relative intensity of the cold band involving 2ν3′. These facts cannot be explained in terms of harmonic potential in the antisymmetrical coordinate Q3′, but can be explained in terms of a double-minimum potential in this coordinate. The barrier of this potential is about 722 cm−1 and the inversion doubling is near 103 cm−1. At vibrational equilibrium the calculated N–O bond lengths are 1.426 and 1.202 Å.

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