Singlet-triplet anticrossings between ungerade states of H2

Singlet-triplet anticrossings in the H₂ molecule have been observed between two ungerade states, B′(3p)¹Σ_u⁺ and f(4p)³Σ_u⁺. This is the first time that an observed H₂ anticrossing has involved a state which can radiate directly to the ground state. Analysis yields accurate values for the zero field separations between two pairs of rotational and vibrational levels. It also yields a value for the Fermi contact interaction in the triplet state as well as the difference in the orbital angular momentum g factors for the two states. From linewidth measurements, we deduce a rigorous lower limit to the radiative lifetime of the B′¹Σ_u⁺ state and a (nearly equal) most reasonable value for it. It is shown that the perturbation between the two states is quite weak leading to little singlet-triplet mixing in zero field. The experimental data establish that the oscillator strength for the forbidden transition from the ground state to the f(4p)³Σ_u⁺ state is at least seven orders of magnitude smaller than that of the allowed transition to the B′¹Σ_u⁺ state.