Strong isotope effect in ultracold collision of N2+ (ν=1, j=0) with He: A case study of virtual-state scattering

A comparative study of the vibrational deactivation of 14N2+ (ν=1, j=0) in collision with 3He and 4He atoms is investigated. Close-coupling calculations are carried out in the 10-6-2000-cm-1 kinetic energy range. At very low kinetic energy, it is found that the cross sections for the collision involving 3He is strongly enhanced by the presence of a virtual state associated with the van der Waals well. The change of mass from 3He to 4He also erases a strong shape resonance at 5x10-3 cm-1 supported by the van der Waals well. As a result, the law of variation of the associated vibrational quenching rate coefficient is found to be very different for these two systems. This behavior is further analyzed by drawing the real part of the scattering length as a function of the variable mass of a fictitious He atom. Two zero energy resonances are identified for a mass of, respectively, about 3.18 and 3.85 atomic mass units