Vacuum-Ultraviolet Absorption Spectrum of Carbon Suboxide. II. The Effect of Low Temperature and the Effect of High Pressure of an Inert Gas on the 1780-Å Band

Abstract

The gas-phase absorption spectrum of the 1780-Å band of carbon suboxide has been measured photo-electrically at both room and dry ice temperatures and in the presence of a high pressure of argon using a 1-m vacuum monochromator having a 1.0-Å bandwidth. The addition of 1500 psi of argon greatly broadens the vibrational structure and shifts the whole band to higher energies. On cooling from 298°K to 195°K, the size of the 14 vibrational bands on the 1780-Å transition remain unchanged relative to each other, but the intensity of the transition is decreased. The constancy in relative size of the vibrational bands at different temperatures rules out the possibility that they are a vibrational sequence in the 63-cm−1 ground-state πu bending mode and confirms their assignment as a progression in a 400-cm−1 excited-state bending vibration. The decrease in oscillator strength as the temperature is lowered and the regularity of the progression as it passes over the peak of the band imply that the linear–linear vertical transition is forbidden. The effect of the high pressure of argon on the 1780-Å band indicates that it is borrowing its intensity from a Rydberg band. The presence of a long progression in a bending mode is strong evidence that the equilibrium configuration in the excited state is nonlinear. The Herzberg–Teller theory of vibrationally induced electronic transitions and an INDO molecular orbital calculation have been used to assign the upper states of the 2650-, 1780-, and 1587-Å bands.