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Observations of four comets made with the Far Ultraviolet SpectroscopicExplorer show the rotational envelope of the (0,0) band of the COHopfield-Birge system (C - X) at 1088 A to consist of both "cold" and "hot"components, the "cold" component accounting for ~75% of the flux and with arotational temperature in the range 55-75 K. We identify the "hot" component ascoming from the dissociation of CO2 into rotationally "hot" CO, with electronimpact dissociation probably dominant over photodissociation near the nucleus.An additional weak, broad satellite band is seen centered near the position ofthe P(40) line that we attribute to CO fluorescence from a non-thermal high Jrotational population produced by photodissociation of formaldehyde into CO andH2. This process also leaves the H2 preferentially populated in excitedvibrational levels which are identified by fluorescent H2 lines in the spectrumexcited by solar OVI 1031.9 and solar Lyman-alpha. The amount of H2 produced byH2CO dissociation is comparable to the amount produced by photodissociation ofH2O. Electron impact excitation of CO, rather than resonance fluorescence,appears to be the primary source of the observed (B - X) (0,0) band at 1151 A.

The Far Ultraviolet Spectral Signatures of Formaldehyde and Carbon Dioxide in Comets