The 15N/14N isotope fractionation in dissociative recombination of N2 +

We have carried out Monte Carlo calculations of the velocity distributions and escape fractions for 14N and 15N released in dissociative recombination of 28N2+ and 29N2+, respectively, for a range of ion temperatures (Ti) and electron temperatures (Te) appropriate to the Martian exosphere. Separate calculations were carried out for each vibrational level of N2+ for 0≤ν≤6. We have taken into account the variation in the dissociative recombination cross section with relative velocity of the ion and electron, and the differences in the vibrational and rotational energy levels of 28N2+ and 29N2+. The escape probabilities and ratios are found to be quite sensitive to the temperature of the N2+ ions, but less so to the electron temperature or to the assumed rotational distribution. The escape probabilities for N2+(ν=0), with Ti=400 K and Te=2000 K and an assumed escape velocity of 4.877 kms−1, are 0.391 and 0.669 for 15N and 14N, respectively. The ratio of the escape fractions, which is a measure of the isotope differentiation effect, is 0.58. This value is close to the ratio 0.51 computed by Wallis [1978], but the individual escape fractions are smaller than his values by about a third. Sample calculations are carried out of the time evolution of the N2 abundance and of the isotope enhancement ratio to illustrate the effect of the new computed escape probabilities. It is found that the N isotopes are still predicted to be overfractionated over the last 3.8 Gyr, but the requirements on the mechanism assumed to reduce the escape, such as the existence of a dense, early atmosphere, are reduced over those required by calculations that employ the escape fractions of Wallis.