Intracluster superelastic scattering via sequential photodissociation in small HI clusters

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(2P3/2) and I(2P1/2) (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→Hsuperelastic+I–I. High quality potentials for the relevant HI excited states have been calculated recently, and coupling between Π0+3 (which correlates with I*) and Π1 (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 Π0+3 potential; specifically, it has a shallow well that persists to small r. The shape of the Π0+3 potential is influenced by relativity; i.e., strong spin–orbit coupling maintains the I* spherical electron density to relatively small r. The Π0+3→1Π 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 (lmax=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 t