Perturbed structure of molecular hydrogen near the second dissociation limit

The region of the second dissociation limit of H₂ converging to H(1s)+H(2s,2p) has been investigated using high-resolution double-resonance spectroscopy through the E,F¹{{{Σ}}}g⁺$ state. Transitions to the very high vibrational levels of the ${B}¹Σᵤ⁺, B' ¹Σᵤ⁺, and C¹{{{Π}}}ᵤ$ states have been measured with 0.02-${cm}^{{{-}}1}$ accuracy. Numerous perturbations and unexpected transitions are observed as far as 10 ${cm}^{{{-}}1}$ below the dissociation limit. This complex structure, still not fully understood, arises from a combination of electronic, fine-structure, and hyperfine interactions.