Large-angle correlations between /sup 4/He and /sup 1,2,3/H or /sup 4/He in the reactions 247 and 337 MeV /sup 40/Ar+/sup nat/Ag: Unexpected properties of the nuclear stratosphere

We have measured coincidences between /sup 4/He and /sup 1,2,3/H or a second /sup 4/He (large-angle correlations) in the reactions 247 and 337 MeV /sup 40/Ar+/sup nat/Ag. These coincidences are dominated by reactions leading to evaporation residues with spins of about 0--70(h2..pi..). Large values of the angle-integrated double and triple coincidence cross sections indicate that many particle evaporation chains involve 2 or 3 protons andor 2 or 3 alphas. The angular and energy distributions are well described by an evaporation model with Hauser-Feshbach coupling. However, one must employ effective evaporation barriers for /sup 1/H and /sup 4/He that are strongly reduced compared to empirical fusion barriers. The barrier reduction required for /sup 2/H is rather small, and for /sup 3/H no barrier reduction is needed. Evaporation calculations that include emitter deformation do not give a satisfactory explanation for these results. We suggest a new aspect of composite nucleus excitation and evaporation-like decay. A hot, very diffuse nuclear surface may be formed that promotes evaporative decay prior to its relaxation. The low barriers for /sup 1/H and /sup 4/He reflect the radial extent of this transitory, diffuse surface.