Cold and ultracold NH–NH collisions: The field-free case

Abstract

We present elastic and inelastic spin-changing cross sections for cold and ultracold NH(X 3Σ−) + NH(X 3Σ−) collisions, obtained from full quantum scattering calculations on an accurate ab initio quintet potential-energy surface. Although we consider only collisions in zero field, we focus on the cross sections relevant for magnetic trapping experiments. It is shown that evaporative cooling of both fermionic 14NH and bosonic 15NH is likely to be successful for hyperfine states that allow s-wave collisions. The calculated cross sections are very sensitive to the details of the interaction potential, due to the presence of (quasi)bound state resonances. The remaining inaccuracy of the ab initio potential-energy surface therefore gives rise to an uncertainty in the numerical cross-section values. However, based on a sampling of the uncertainty range of the ab initio calculations, we conclude that the exact potential is likely to be such that the elastic-to-inelastic cross-section ratio is sufficiently large to achieve efficient evaporative cooling. This likelihood is only weakly dependent on the size of the channel basis set used in the scattering calculations.

Efficient method for quantum calculations of molecule-molecule scattering properties in a magnetic field

“…As in the previous studies of 15 NH-15 NH collisions [37,48,49], we assume that all three spin states of the 15 NH-15 NH complex are described by the nonreactive quintet surface, i.e. [50].…”

“…However, additional recoupling transformation required to bring the Hamiltonian matrix defined in Ref. [48] to the basis set representation (9) can be readily obtained by a simple modification of Eq. (30) in Ref.…”

“…Note that the total angular momentum basis set used in Ref. [48] is based on a different coupling scheme. However, additional recoupling transformation required to bring the Hamiltonian matrix defined in Ref.…”

“…Ref. [48] also gives the expressions for the matrix elements of the operators (3) for zero magnetic field and the matrix elements of the magnetic dipole -dipole interaction (3) in a SF total angular momentum basis. Note that the total angular momentum basis set used in Ref.…”

“…The basis set representation (9) is most suitable for molecule -molecule scattering problems in the absence of external fields [48] but it can also be used for computations of cross sections for molecular collisions in external fields. The matrix elements of the operatorl 2 in the SF representation (9) are particularly simple:…”