per minute. Arc lamps come in nearly all of the noble gas types, including xenon, krypton, argon, neon, and helium, which all emit spectral lines that are very specific to the gas. The output spectrum of an arc lamp is mostly dependent on the gas type, being narrow band spectral lines very similar to a flashlamp operated at low current densities. The output is highest in the near infrared, and are usually used to pump infrared lasers such as Nd:YAG. External laser pumping A laser of a suitable type can be used to pump another laser. The pump laser's narrow spectrum allows it to be closely matched to the absorption lines of the lasing media, giving it much more efficient energy transfer than the broadband emission of flashlamps. Diode lasers pump solid state lasers and liquid dye lasers. A ring laser design is often used, especially in dye lasers. The ring laser uses three or more mirrors to reflect light in a circular path. This helps eliminate the standing wave generated by most Fabry–Pérot resonators, leading to a better use of the gain medium's energy. Other optical pumping methods Microwaves or radiofrequency EM radiation can be used to excite gas lasers. A solar-pumped laser uses solar radiation as a pump source. Electrical pumping Electric glow discharge is common in gas lasers. For example, in the helium–neon laser the electrons from the discharge collide with the helium atoms, exciting them. The excited helium atoms then collide with neon atoms, transferring energy. This allows an inverse population of neon atoms to build up. Electric current is typically used to pump laser diodes and semiconductor crystal lasers (for example germanium) Electron beams pump free electron lasers and some excimer lasers. Gas dynamic pumping Gas dynamic lasers are constructed using the supersonic flow of gases, such as carbon dioxide, to excite the molecules past threshold. The gas is pressurized and then heated to as high as 1400 kelvins. The gas is then allowed to expand rapidly through specially shaped nozzles to a very low pressure. This expansion occurs at supersonic velocities, sometimes as high as mach 4. The hot gas has many molecules in the upper excited states, while many more are in the lower states. The rapid expansion causes adiabatic cooling, which reduces the temperature to as low as 300 K. This reduction in temperature causes the molecules in the upper and lower