A Instrument Components

A continuum source provides a broad distribution of wavelengths within a particular spectral range. This distribution is known as a spectral continuum. 

Note that the intensity reaches a maximum in the near-IR region of the spectrum ( 1200 nm in this case). 

In addition to the continuum sources just discussed, line sources are also important for use in the UV/visible region. Low-pressure mercury arc lamps are very common sources that are used in liquid chromatography detectors. The dominant line emitted by these sources is the 253.7-nm Hg line. Hollow-cathode lamps are also common line sources that are specifically used for atomic absorption spectroscopy, as discussed in Chapter 28. Lasers (see Feature 25-1) have also been used in molecular and atomic spectroscopy, both for single-wavelength and for scanning applications. Tunable dye lasers can be scanned over wavelength ranges of several hundred nanometers when more than one dye is used. 

When the gain of the active medium equals the losses in the system, laser oscillation begins. 

Population inversion is often achieved by a multi-level atomic or molecular system in which the excitation process, called pumping, is accomplished by electrical means, by optical methods, or by chemical reactions. In some cases, the population inversion can be sustained to produce a continuous wave (CW) output beam that is continuous with respect to time. In other cases, the lasing action is self terminating, so that the laser is operated in a pulsed mode to produce a repetitive pulse train or a single-shot action.  

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