Observation of the 4s 3Σ+g state of H2

Abstract

A blue cw dye laser has been used to excite 14 transitions from the metastable c(2p)³Π_u state to a previously unknown electronic state of H₂, believed to be the (4s)³Σ⁺_g state. The observed transitions are all Q branches and are diagonal in vibrational quantum number. Levels with v=0 and v=1 are detected by their radiative decay, but all of the v=2 levels are predissociative with lifetimes on the order of 500 ps. Approximate molecular constants are derived from the spectra and found to be similar to those of the hydrogen molecular ion. The effective quantum number is n*=3.97, in agreement with the quantum defect found for lower triplet s states.