RYDBERG STATES OF H-4

Energies of the 15 lowest Rydberg states of the metastable H-4 cluster have been determined using Koopman's theorem at the equilibrium geome try of the par ent ion. H-4(+) To represent the core orbitals of H-4, a 6-31 G* basis set has been used at the SCF, MP2, MP4(SDTQ), CI-SD, and CI-SDT levels. The Rydberg orbitals have been modeled using a basi s set analogous to that used to model the Rydberg orbitals of H-3 in p revious theoretical calculations. To test the validity of the calculat ions for the H-4 molecule, ab initio calculations were repeated for th e Rydberg orbitals of the H-3 molecule at the stable geometry of the H -3(+) core. Predicted transitions were within 2% of the rotational ban d spectra of H-3 observed by Herzberg. The metastable H-4 cluster form ed from charge neutralization of H-4(+) decomposes into two H-2 molecu les. Previous calculations have predicted that one of the two H-2 prod ucts will be vibrationally hot while the other will be relatively cold and that a large recoil energy of approximately 9 eV is expected for the relative kinetic energy of the two H-2 products. The present work suggests(2) that if Rydberg states are involved in the charge neutrali zation process, the recoil energy could be reduced due to radiative tr ansitions among the Rydberg states.