Lasers: tunable emission

Figure 39. OH - X II chemiluminescence from the reaction of hot H atoms with NjO under bulk conditions, (a) Shows vibrationally-resolved emission from 193 nm HBr photolysis (h) is for 220 nm HI photolysis (i.e., doubled dye laser tunable photolysis source) and lower resolution. The peaks labeled a and b were used to obtain the data shown in Figure 40.

It would be premature, however, to conclude that the extremely narrow (20-30 kHz) linewidths achieved with broad-area, homojunction solitary PbSi xSex tunable diode lasers can be obtained with all other lead-salt compounds and diode structures as well. Figure 6 shows some of the most frequently used lead-salt semiconductors which can provide a class of tunable lasers with emission wavelengths anywhere between about 2 1/2 um and 30 um. The figure illustrates the dependence of laser wavelengths on composition of the lead-salt compounds. The lower left portion of the figure shows PbEuSeTe which can be used to cover the 2.6 to 6.6 pm wavelength range. This is a new material recently developed at the... 

When lasers first emerged from the laboratory and became commercially available in the late 1960s, they have found application in diverse areas. Spectrometric analysis was among the technology s earliest applications. Unlike established continuum sources, the monochromatic, coherent beams from lasers delivered high energy density. The result was a quantum leap in spatial and temporal resolution, sensitivity and speed of acquisition. For more than 40 years, spectrometric techniques implemented sources with narrower wavebands, shorter pulses and tunable emission. 

Gas lasers became available to date thanks to reasonable prices. Their restrictions are the same as those of gas discharge lamps. Generally, with lasers the emission lines are more narrow-band. Tunable dye lasers would be the ideal light source. However, they are too sophisticated and too expensive. Furthermore, the dyes used are not sufficiently stable due to their light sensitivity. 

Liquid dyes lasers tunable in the visible range have been known since the middle of the sixties. They are based on organic colorants dissolved in various solvents. The light absorption from the pump source brings the dye molecule to its excited singlet state, and the emission to the terminal vibrational state can then be turned by using appropriate resonant cavities to produce laser emission in the spectral range of fluorescent emission. 

The most widely employed optical method for the study of chemical reaction dynamics has been laser-induced fluorescence. This detection scheme is schematically illustrated in the left-hand side of figure B2.3.8. A tunable laser is scanned tlnough an electronic band system of the molecule, while the fluorescence emission is detected. This maps out an action spectrum that can be used to detemiine the relative concentrations of the various vibration-rotation levels of the molecule. 

A progression with v = 2, illustrated in Figure 7.18, can be observed only in emission. Its observation could result from a random population of v levels or it could be observed on its own under rather special conditions involving monochromatic excitation from v" = 0 to if = 2 with no collisions occurring before emission. This kind of excitation could be achieved with a tunable laser. 

The potential of a tunable dye laser should not be overlooked. A tunable dye laser, employing an organic dye as lasing material allows one to choose any suitable excitation line within a particular region. This is in contrast to the case of a gas ion laser which has a limited number of emission lines at fixed wavelength. Nevertheless, a tunable dye laser has significant drawbacks such as poor resolution imposed by the dye laser linewidth (1.2 cm-1) and a continuous background spectrum which requires the use of a tunable filter 15-18). 

The general principle of detection of free radicals is based on the spectroscopy (absorption and emission) and mass spectrometry (ionization) or combination of both. An early review has summarized various techniques to detect small free radicals, particularly diatomic and triatomic species.68 Essentially, the spectroscopy of free radicals provides basic knowledge for the detection of radicals, and the spectroscopy of numerous free radicals has been well characterized (see recent reviews2-4). Two experimental techniques are most popular for spectroscopy studies and thus for detection of radicals laser-induced fluorescence (LIF) and resonance-enhanced multiphoton ionization (REMPI). In the photochemistry studies of free radicals, the intense, tunable and narrow-bandwidth lasers are essential for both the detection (via spectroscopy and photoionization) and the photodissociation of free radicals. 

Fundamentally, the properties of laser light are concomitants of its coherence, which is in turn a consequence of the nature of stimulated emission. Most of these properties, especially brightness, monochromaticity, directionality, polarization, and coherence itself, are useful (for many applications, indis-pensible) in a spectroscopic light source. The spectroscopic potential of lasers was recognized even before they were invented. Actual applications remained very specialized until tunable lasers were devised. 

Hill, J. C. Majkowski, R. F. "Time-Resolved Measurement of Vehicle Sulfate and Methane Emissions with Tunable Diode Lasers" SAE Paper No. 800510, 1980. 

The optical features of a center depend on the type of dopant, as well as on the lattice in which it is incorporated. For instance, Cr + ions in AI2O3 crystals (the ruby laser) lead to sharp emission lines at 694.3 nm and 692.8 nm. However, the incorporation of the same ions into BeAl204 (the alexandrite laser) produces a broad emission band centered around 700 nm, which is used to generate tunable laser radiation in a broad red-infrared spectral range. 

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