system initial ( v, j ) k$_{T}$ =0 (cm $^{3}$s − $^{1}$) Ref. F + H 2 ( v = 0 , j = 0) 1.3 × 10 − 12 (H + HF) [11] F + HD ( v = 0 , j = 0) 2.8 × 10 − 14 (D + HF) [74] ( v = 0 , j = 0) 0.5 × 10 − 14 (H + DF) [74] F + D 2 ( v = 0 , j = 0) 2.1 × 10 − 14 (D + DF) [37] F + HCl ( v = 0 , j = 0) 1.2 × 10 − 17 (Cl + HF) [88] F + HCl ( v = 1 , j = 0) 4.0 × 10 − 15 (Cl + HF) [88] 5.3 × 10 − 15 (F + HCl) [88] F + HCl ( v = 2 , j = 0) 5.2 × 10 − 13 (Cl + HF) [88] 3.3 × 10 − 13 (F + HCl) [88] F + DCl ( v = 1 , j = 0) 4.4 × 10 − 21 (Cl + DF) [88] H + HCl ( v = 0 , j = 0) 2.4 × 10 − 19 (Cl + H 2 ) [83] ( v = 1 , j = 0) 7.2 × 10 − 14 (Cl + H 2 ) [83] ( v = 2 , j = 0) 1.9 × 10 − 11 (Cl + H 2 ) [83] H + DCl ( v = 1 , j = 0) 7.8 × 10 − 15 (Cl + HD) [83] ( v = 2 , j = 0) 1.7 × 10 − 12 (Cl + HD) [83] Cl + HD ( v = 1 , j = 0) 1.7 × 10 − 13 (D + HCl) [82] 7.1 × 10 − 16 (H + DCl) [82] 7.8 × 10 − 14 (Cl + HD) [82] Li + HF ( v = 0 , j = 0) 4.5 × 10 − 20 (H + LiF) [84] ( v = 1 , j = 0) 2.8 × 10 − 17 (H + LiF) [84] H + LiF ( v = 1 , j = 0) 3.8 × 10 − 15 (Li + HF) [85] 1.6 × 10 − 14 (H + LiF) [85] ( v = 2 , j = 0) 1.7 × 10 − 14 (Li + HF) [85] 2.7 × 10 − 13 (H + LiF) [85] TABLE III: Zero-temperature quenching rate coefficients for tunneling dominated reactions. The final arrangement has also been specified. B. Barrierless reactions 1. Collision systems of three alkali metal atoms As discussed by Hutson and Sold´ an in two recent re- views [6, 14], progress on the production of ultracold molecules and the creation of molecular Bose–Einstein condensates of alkali-metal systems have motivated the- oretical studies on ultracold atom - dimer alkali-metal collisions. Here, we give an overview of recent cal- culations for spin-polarized triatomic alkali-metal sys- tems, Li + Li$_{2}$ [94–97], Na + Na$_{2}$ [98, 99] and K + K$_{2}$ [100]. These results have all been obtained using the reactive scattering code written by Launay and Le Dourneuf [101], based on a time-independent quantum formalism. Refs. [94, 95, 97, 100] describe the details of the PESs and dynamics calculations. The quantum dynamics studies of Refs. [98, 99] have been performed using the Na$_{3}$ PES reported by Higgins et al. [102], while the results of Ref. [96] have been obtained using the Li$_{3}$ PES calculated by Colavecchia et al. [103]. Another PES for Li$_{3}$ has been constructed by Brue et al. [104]. Quantum scattering calculations show that ultracold Third, the density of states of the system plays an important role. If the density of states is small as for light systems, the typical energy spacing is rather large. reactions of alkali metal atoms with alkali metal dimers are much more efficient than tunneling driven processes. This can be explained based on the following consid- erations. First, the indistinguishability of the atoms may play a role. For a homonuclear triatomic system, the three possible arrangement channels are the same. Also the presence of identical two-body potentials in all three arrangement channels may enhance the depth of the three-body interaction potential. Consider the two- body term (additive term) of a homonuclear triatomic system at equilateral geometries. Since the distances be- tween the three identical atoms are the same, the same diatomic potential term is added three times to yield the two-body terms of the triatomic system. If the diatomic potential is deep or repulsive, the two-body term will be three times deeper or repulsive at equilateral geometries. In contrast, for triatomic systems with distinguishable atoms, the diatomic pairs are not the same. For example, for the Li + HF system, the three diatomic fragments are distinctly different. They have completely different elec- tronic structures and properties such as minima, equilib- rium distances, turning points, and the nature and range of the interaction. Thus, at an equilateral configuration for which the three diatomic distances are the same but the diatomic potential energies are different (one may be attractive while the other two may be repulsive), the overall two-body term can be weaker than the individual two-body interaction potential. This could lead to a tri- atomic interaction region that is less strong than that of a homonuclear triatomic system. Second, the topology of the PES plays a significant role. For all alkali-metal trimer systems, the minimum energy configurations arise at equilateral and collinear geometries as shown by Sold´ an et al. [105]. Furthermore, the surface is barrierless so that all atom - dimer alkali- metal collisional approaches are energetically possible. In contrast, most of the non-alkali systems discussed above are characterized by a collinear (or bend) atom - diatom approach, and dominated by a repulsive barrier in the triatomic transition-state region. The particular topol- ogy of the PES arises from the three-body interaction potential. Also, the couplings between different elec- tronic surfaces of the triatomic system give rise to coni- cal intersections, which create a repulsive barrier in the transition-state region. If the energy barrier is high and its width large, tunneling will be very inefficient leading to very small values for the reaction rate coefficients in the Wigner regime. Although reactivity can be enhanced when resonances are present, the background scattering in reactive cross sections is generally quite small. In contrast, for almost all alkali-metal trimer systems, the conical intersections arise at small interatomic distances where the PES is sufficiently repulsive that it plays no significant role at ultra-low energy and the reactivity is not influenced by such repulsive barriers. 14