Lyman-α Fluorescence from the Photodissociation of H2

Photodissociation and fluorescence cross sections for H₂ have been measured from 860 to 700 Å by observing the Lyman‐α photons emitted at 90° to an incident photon beam. The photodissociation quantum efficiency was found to be unity from 850 Å to the photoionization threshold at 804 Å and about 10% at shorter wavelengths. By observing the fluorescence radiation as a function of decreasing pressure, it was determined that most of the excited H atoms were formed in the metastable $\text{2s}$ state and that the Lyman‐α emission results primarily from collisional quenching. The ratio of the collisional cross section with and without Lyman‐α emission was measured for different relative velocities and was found to increase appreciably with increasing velocity.