A cage of sixty carbon atoms, shaped like a soccer ball, lets small molecules slip inside. Carbon buckyballs, fullerenes, pack into a crystal like oranges in a crate, leaving gaps between them. At 570 degrees Celsius and 30 atmospheres of pressure, hydrogen seeped into those gaps. Some molecules stuck to the outside of the balls like burrs to clothing. The result was a substance resembling a hydrogen-greased soccer ball.
After cooling to 20 kelvins (minus 253 degrees Celsius), the samples lit up. The light came from electrons jumping between energy levels, like a ball bouncing down a staircase. The glow was blue — shorter in wavelength than that of pure fullerene. This color shift, like the sky blushing blue at sunset, indicates that only a few hydrogen atoms attached to each ball, just a handful per molecule.
With nitrogen, things went differently. At 720 degrees Celsius, a carbon atom in the cage was replaced by a nitrogen atom, and two such cages linked into a sturdy pair. This pair glows in the infrared with an energy of 1.53 electronvolts, like a warm ember in a campfire. Its light was the brightest and set the tone for the whole glow of the new mixture.
So in one study, two different states were captured: hydrogen balls and nitrogen pairs. Both glow in extreme cold, and their light tells you unmistakably what the reaction produced. It's like telling apple varieties apart by skin color without cutting the fruit.
Fullerene C60 is a cage molecule about a nanometer across, big enough to hold an atom or a small molecule. The discovery of fullerenes earned the 1996 Nobel Prize in Chemistry.