  Observations of four comets made with the Far Ultraviolet Spectroscopic
Explorer show the rotational envelope of the (0,0) band of the CO
Hopfield-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 a
rotational temperature in the range 55-75 K. We identify the "hot" component as
coming from the dissociation of CO2 into rotationally "hot" CO, with electron
impact dissociation probably dominant over photodissociation near the nucleus.
An additional weak, broad satellite band is seen centered near the position of
the P(40) line that we attribute to CO fluorescence from a non-thermal high J
rotational population produced by photodissociation of formaldehyde into CO and
H2. This process also leaves the H2 preferentially populated in excited
vibrational levels which are identified by fluorescent H2 lines in the spectrum
excited by solar OVI 1031.9 and solar Lyman-alpha. The amount of H2 produced by
H2CO dissociation is comparable to the amount produced by photodissociation of
H2O. 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.
