How Molecules Are Detected
Molecules absorb and emit light at specific wavelengths that are characteristic of the molecule. Some substances are colored because this absorption occurs in the visible region of the electromagnetic spectrum. Unfortunately, interstellar space contains small dust grains (which we will later see are essential to the chemistry) which scatter light in the visible and ultraviolet, preventing it from reaching us. Thus, only a few of the most abundant interstellar molecules, such as H2 and CN have been detected from their visible or ultraviolet spectrum.
The vast majority of interstellar molecules have been observed using radiotelescopes. These operate much like a standard optical telescope, imaging the electromagnetic radiation to give us a picture of a region of space, but they detect microwaves and radio waves. These lie at wavelengths of > 1 mm, which is much longer than the wavelengths of visible light, and thus corresponds to much lower energy than the visible. Radio waves are hardly scattered by dust grains, and, at most wavelengths, they pass easily through the earth's atmosphere.
The 100-meter radio telescope near Bonn, Germany
Atoms and molecules emit light in the visible and ultraviolet when they make a transition from an electronic state to another electronic state at lower energy. For example, the characteristic red glow of a neon light is due to an electronic transition in the Ne atom. Molecules emit radio waves when they make a transition from a rotational state to another rotational state at lower energy. In order to emit radio waves the molecule must be polar. Thus, H2 and CO2 don't emit radio waves and can only be detected by other means.