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 ¹⁴ N and ¹⁵ N released in dissociative recombination of ²⁸ N ₂ ⁺ and ²⁹ N ₂ ⁺ , respectively, for a range of ion temperatures ( T _i ) and electron temperatures ( T _e ) appropriate to the Martian exosphere. Separate calculations were carried out for each vibrational level of N ₂ ⁺ 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 ²⁸ N ₂ ⁺ and ²⁹ N ₂ ⁺ . The escape probabilities and ratios are found to be quite sensitive to the temperature of the N ₂ ⁺ ions, but less so to the electron temperature or to the assumed rotational distribution. The escape probabilities for N ₂ ⁺ (ν=0), with T _i =400 K and T _e =2000 K and an assumed escape velocity of 4.877 kms ⁻¹ , are 0.391 and 0.669 for ¹⁵ N and ¹⁴ N, 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 N ₂ 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.