Re: HOW DO NOBLE GASES CREATE NEON SIGNS?

Two of the most common plasma devices on the planet are the fluorescent light bulb, and its cousin,the neon sign. Since their development in the 1940's, fluorescent bulbs have become the lightingfixture of choice in offices, factories, and schools, and they are beginning to be found more widely in homes as well..

The light from fluorescent light bulbs looks white in most cases, and that white color is a combination (as it is with sunlight) of all of the colors of the visible spectrum. In the case of the fluorescent bulb, the material that is actually doing the glowing is a white powder applied to the inner wall of the bulb's long glass tube. This powder (commonly called a 'phosphor', although it may not have any phosphorus in it) is giving off the white light we see through a process called fluorescence, which is the basis of the name 'fluorescent' light bulb.

Fluorescence occurs when an atom (or molecule) absorbs energy from some source (like a photon of light, or a collision with another atom) and then releases that energy in the form of light in two or more consecutive steps. In the fluorescent bulb, high-energy ultraviolet light from within the tube is absorbed by the phosphor, which then re-radiates the energy by emitting two or three lower-energy light waves. Since the visible spectrum to which our eye is sensitive is at a lower energy than is ultraviolet (uv) radiation, we can use the fluorescing phosphor as a light source.

By changing the gas used, for example, krypton, xenon you can control the colours, hence, green and red.

A bit more scientific answer...
A neon sign produces an emission spectrum. The electric current in the tube drives electrons through the gas. These energetic electrons collide with atoms in the neon gas, which then become excited. As the atoms become de-excited (= their electrons jump down to the lower energy level), they emit photons. Since the electrons in the neon atoms are only allowed specific energy levels (regarding which noble gas is used), they all emit photons at only a few energies: the energies that correspond to the energy difference between the levels.