H(2s) and H(2p) branching ratio in the photodissociation of H2 near threshold

The excitation spectrum for producing Lyman-α fluorescence by photodissociating H2 has been obtained with 0.12 Å bandwidth in the dissociation threshold region. At 839 Å, the peak in the fluorescence cross section, one observes Lyman-α emission from several continuums and from the J=1 and J=2 predissociated rotational levels of the D 1Π+u(v′=3) state. At this wavelength the branching ratio σ (2s)/[σ (2s)+σ (2p)] was obtained using a quenching field method. It was determined that 57% of the excited atoms are produced in the metastable 2 2S1/2 state. From a study of the fluorescence signal versus pressure a cross section of 0.9×10−14 cm2 was derived from the chemi-ionization reaction H2+H(2s) →H+3+e−.