Quantum defect theory for cold chemistry with product-quantum-state resolution

We present a formalism for cold and ultracold atom-diatom chemical reactions that combines a quantum close-coupling method at short range with quantum defect theory at long range. The method yields full state-to-state rovibrationally resolved cross sections as in standard close-coupling (CC) calculations but at a considerably less computational expense. This hybrid approach exploits the simplicity of MQDT while treating the short-range interaction explicitly using quantum CC calculations. The method, demonstrated for D + H-2 -> HD + H collisions with rovibrational quantum state resolution of the HD product, is shown to be accurate for a wide range of collision energies and initial conditions. The hybrid CC-MQDT formalism may provide an alternative approach to full CC calculations for cold and ultracold reactions.