Tunable laser operation over a nearly continuous range of frequencies has been attained with the molecules of certain organic dyes. The molecules of these dyes have a large number of spectral lines and each of them has a characteristic spread of frequencies which is large compared to the spread of gaseous atomic spectral lines. With the overlap of these lines in the dyes, the dye laser can be tuned to produce laser action for laser spectroscopy.
A widely used dye is rhodamine 6G, commonly referred to as Rh6G. It is one of the most highly fluorescing materials known and was used by early astronauts to mark the position of their capsules when landing in the ocean. The unique properties which have made it useful in such exotic applications have also made it popular as a laser medium. Another dye used for spectroscopy is known as "ring dye" and is capable of essentially continuous tuning.
The dye laser medium is typically in liquid form and the dye is circulated continuously through the laser chamber to keep it from being limited by saturation effects. The dye may be pumped by flash lamps or by another laser such as an argon ion laser.