Fluorescent molecular hydrogen line emission in the far-red

Over 30 lines from the v = 3–0, 4–1, 7–3, and 8–4 vibrational series of the hydrogen molecule have been observed from λ = 7600–8800 Å in the reflection nebula NGC 2023, originating from levels up to 41,000 K above ground. This is the first time that H2 has been observed in the optical CCD regime, and these are the highest excitation lines from the ground electronic state that have yet been detected. The spectrum shows the characteristics expected of UV fluorescence, although in detail there are some differences from model predictions. Emission from newly formed molecules in v = 4 may have been detected. Strong [C I] 8727-Å forbidden line emission is also observed, coincident with the H2 emission, and arising from the same photodissociation region gas.