Search the After Effect forums for NEON. We see the topic often. The question is confused by the needs for simulation of a realworld object or to deliver an incorrect but conventionally accepted version of "neon."
Needlessly pedantic entry: Stimulated electrons inside a glass tube either excite the enclosed gas (neon is just one of many) to glow or they cause a coating on the inside of the tube to glow. The glow is not atmospheric; there is no halo around a neon tube unless there's dust or water vapor in the air.
Visit a few neon signs and take a close look at how they're glowing.
This is my standard sigfile so do not take it personally: "For crying out loud, read the freakin' manual."