The Isotope Ratio C¹²/C¹³ in a Comet.

High-resolution spectra of Comet Ikeya (1963a) show the C11C13(1, 0) band at X 4745 with measurable strength. Its intensity has been measured and compared with the intensity of two unresolved-triplet, rotational lines of C11C12(1, 0). The structure of the isotope band has been computed to obtain the number of lines forming the observed head. From the theory of the excitation of comet lines by resonance fluorescence, we derive the isotope ratio C11/C11 as 70 + 15. Solar-absorption features, although included, modify the excitation pattern of the C1 lines only slightly, unlike the CN band which has strongly deficient solar radiation. A feature at 4752 appears near C13C13(1, 0). Its strength is too great to be compatible with C11/C13 70, requiring an unacceptably low value. A discussion as to whether C11C13 can be de-excited by pure rotation, unlike the homonuclear C11 molecules, leads to no definite conclusion since the dipole moment of C12C11 is unknown. We believe that x 4752 is largely affected by a blend with NH1. Another slight blend, at X 4745, may require a slightly higher isotope ratio C11/C13. The C /C13 ratio is near that on the earth. Some implications for the early history of the solar system are drawn. If the solar C12/C13 ratio proves indeed to be very high the cometary C13 could have been produced by neutron irradiation.