Although the diffuse ISM is a much less forgiving environment than that found within dense molecular clouds, there are still several organic molecular species able to tough it out. An example of these is the PAHs, which are very stable organic molecules made up of only carbon and hydrogen, with each carbon having three neighbouring atoms, much like graphite. PAHs are relatively large and have a range of vibrational modes, which explains their ability to survive in harsh radiation fields that destroy most other molecules.
These molecules are very common on Earth, being standard combustion products. Some PAHs are also highly potent carcinogens, and therefore biologically interesting; they have the ability to interfere with DNA.
Other organic molecules in the diffuse ISM include aliphatic hydrocarbons and microdiamonds. Not surprisingly, all these species are also found in dense molecular clouds.