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displayed in Fig. 15 as functions of the atom - molecule separation. The adiabatic potential curves are computed by diagonalizing the diabatic potential energy matrix ob- tained in a basis set of rovibrational levels of the HD molecule at each value of the atom - molecule separa- tion. The Feshbach resonances labeled B, C, D, and E in Fig. 14 result from the decay of the corresponding qua- sibound states shown in Fig. 15. 

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The cross sections in Fig. 14 do not show a peak corresponding to the metastable state A. This is because the state A is too deeply bound and it is not accessible through scattering in the v = 1 , j = 0 channel. Fig. 14 also shows that the quasibound states preferentially undergo prereaction than predissociation. This is due to the enhanced coupling of the resonance states with the reactive channels compared to those with the non-reactive channels. The wavefunctions of quasibound states B and E are shown in Fig. 16 as functions of the atom - molecule separation. Although the wavefunction of the weakly bound state E extends far beyond the transition state region of the reaction, it preferentially undergoes prereaction compared to predissociation. Thus, regions of the interaction potential far away from the transition state region may have a significant effect on reactivity especially when part of the wavefunction in that region is sampled by a resonance state which is cou- pled to the reactive channel. Fig. 14 also demonstrates that since the reaction is dominated by tunneling at low t emperatures the formation of the DCl + H product which involves the tunneling of the D atom is severely s uppressed in the threshold regime. This is more clearly illustrated in Fig. 4 of Ref. [82] where the reaction cross s ection is plotted as a function of the incident kinetic e nergy. 

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Table III provides a compilation of zero-temperature quenching rate coefficients for a number of atom - diatom chemical reactions which are dominated by tunneling at low energies. 

