Laser Techniques for Molecular Tunable Lasers

For a few laser analytical techniques (molecular fluorimetry of liquids, spontaneous Raman scattering), fixed wavelength lasers are used. The following discussion is limited to the two most widely used types of tunable laser. 

Typically, dye lasers are pumped by an intense fixed-frequency laser, the most often used being nitrogen (337 nm), argon ion (488.0, 514.5 nm), XeCl excimer (308 nm). and frequency-doubled and frequency-tripled Nd-YAG (530, 353.3 nm). 

The organic dyes used for lasers can be classified on the basis of molecular structure terphe-nyls (spectral range 312-400 nm) coumarines (420 - 580nm) rhodamines (570-720 nm), oxa-zines (680-860 nm). Each class consists of several dyes with slightly modified chemical structure and, hence, slightly shifted absorption and luminescence bands. There are special dyes for UV and near IR. 

The most utilized solvents are ethanol, methanol, cyclohexane, ethylene glycol, and dioxane. For different solvents, the tuning curves of the same dye may be shifted by 10-15 nm. 

The dyes for the UV spectral range (below 380-400 nm) are not photochemically stable. In most analytical applications, intense UV radiation below 360 nm is obtained by nonlinear optics, i.e., the generation of harmonics and sum frequencies of the visible dye laser radiation in nonlinear crystals. The most widely employed crystals are KDP. ADP, LiF, LilOs, and BBO. For example, the second harmonic of a rhodamine dye laser (fundamental band near 640 nm) is a more practical way to obtain tunable UV radiation near 320 nm rather than direct lasing of an unstable UV dye laser. 

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