The spectrum above was acquired the morning of Feb. 28,1997. It shows emission lones from gases in the coma of the comet, and a reflection spectrum of the sun due to sunlight reflected off of comet dust. The most prominent solar absorption feature is the H-alpha line at 656 nm The absorption feature at 686-690 nm is from oxygen in the earth's atmosphere.
Hale-Bopp minus Mars
By subtracting the reflection spectrum of Mars, the contribution from the comet dust can be removed. The resulting spectrum shows at least 10 emission lines from cometary gases. According to Rod Heyd at Arizona State University the lines in the region from 662 to 680 are very likely due to NH_2 which comes from the photo-dissociation of ammonia.. The other lines are currently unidentified.