Ab Initio Calculation on Photo-Dissociation of CO Molecules to Test the Self-Shielding Model

The mass-independently fractionated oxygen isotope in CAI of primitive meteorites is commonly attributed to self-shielding effect of CO [e.g.,1]. Besides its unusual implication that CAI oxygen must be the same as the solar oxygen, the self-shielding hypothesis is based on several dubious premises including no isotopic effect of photo-dissociation of CO. Here, we show from a quantum mechanical calculation that photo-dissociation of CO seems to give rise to yield substantial isotopic effect on the photo-dissociation. Ab initio reaction dynamics simulations were performed to compute the photolysis rate for the E∠ • XΣ electronic transition with non-adiabatic transition to kΠ and dissociative channel. This transition is considered one of the important transition for a CO self-shielding model. We used the BornOppenheimer approximation; in the first step the time independent Schrodinger equation was solved only for the electron-motion, and in the second step we performed the time dependent wave-packet dynamics for the nuclei-motion in the potential energy curves. The theoretical photo-dissociation cross section as a function of wavelength of excitation light was estimated by the Fourier transform of the autocorrelation function, [2]. Assuming the Boltzmann distribution for vibrational and rotational state, we summed up to get the total cross section. Using photo-dissociation cross secions calculated for respective isotopes (i = 16, 17, 18), we will