The photodissociation of expansion-cooled HI monomer by using 266 nm radiation yields H atoms having 12 830 and 5287 cm-1 of translational energy in the HI center-of-mass system for the I(P-2(3/2)) and I(P-2(1/2)) (i.e., I and I-*, respectively) co-fragments. Irradiating HI clusters [i.e., (HI)(n), with n=2 being the dominant cluster] with 266 nm radiation produces, among other things, some H atoms whose translational energies are peaked at 20 285 cm-1, which is 7455 cm-1 higher in energy than the more energetic of the monomer peaks. These very fast H atoms arise from sequential photodissociation within the clusters. Namely, a weakly bound I-*.(HI)(n-1) complex is first created by the photodissociation of an HI moiety within (HI)(n), and then the photodissociation of a second HI moiety [within I-*.(HI)(n-1)] produces a fast H atom that scatters from the nearby I-*, in some cases deactivating it in the process. Thus, the latter superelastically scattered H atom acquires, as translational energy, nearly all of the I-* energy (7603 cm-1). For example, for the dimer, the first dissociation event, (HI)(2)+hv-->H+I(I-*).HI, is followed by I-*.HI+hv-->H-superelastic+I-I. High quality potentials for the relevant HI excited states have been calculated recently, and coupling between (3)Pi(0)(+) (which correlates with I-*) and (1)Pi (which correlates with I) has been shown to be due to spin-rotation interaction. There is a high degree of separability between the photodissociation of the second HI moiety and the subsequent H+I-* scattering (within a given cluster). This is due mainly to the shape of the (3)Pi(0)(+) potential; specifically, it has a shallow well that persists to small r. The shape of the (3)Pi(0)(+) potential is influenced by relativity; i.e., strong spin-orbit coupling maintains the I-* spherical electron density to relatively small r. The (3)Pi(0)(+)-->(1)Pi transition probabilities are calculated for H+I-* collisions having different values of the collisional orbital angular momentum quantum number, l, by scaling the spin-rotation matrix elements by [l(l+1)](1/2) and using the Landau-Zener model to treat the electronically nonadiabatic dynamics. It is shown that large l values (l(max)=52) play a dominant role in the quenching of I-* by H. For example, the partial superelastic scattering cross section is six orders of magnitude larger for l=52 than for l=1, underscoring the dramatic role of angular momentum in this system. It is noted that HI photodissociation (which is dominated by low l) proceeds almost entirely along the diabats with little transfer of flux between them, whereas H+I-* intracluster "collisions" take place with sufficiently large l to facilitate the electronically nonadiabatic process. (C) 2003 American Institute of Physics.