Laser excitation high power

The fluorescence signal is linearly proportional to the fraction/of molecules excited. The absorption rate and the stimulated emission rate 1 2 are proportional to the laser power. In the limit of low laser power,/is proportional to the laser power, while this is no longer true at high powers 1 2 <42 j). Care must thus be taken in a laser fluorescence experiment to be sure that one is operating in the linear regime, or that proper account of saturation effects is taken, since transitions with different strengdis reach saturation at different laser powers. 

The transversely excited atmospheric-pressure (TEA) laser, inherently a pulsed device rather than a continuous laser, is another common variety of carbon dioxide laser (33,34). Carbon dioxide—TEA lasers are an important class of high-power pulsed lasers. Pulse durations are in the submicrosecond regime peak powers exceed 10 MW. 

High power pulsed lasers are used to produce plasmas and thus to sample and excite the surfaces of soHds. Improvements in minimum detectable limits and decreases in background radiation and in interelement interference effects result from the use of two lasers (99) (see Surface and interface analysis). 

Gas lasers are not unlike fluorescent light bulbs and neon signs. Gas is confined to a hollow tube, and electricity passing through it excites the atoms. The most common gas lasers use carbon dioxide, argon, and helium-neon. Gas lasers are relatively inexpensive and can produce very high-powered beams. 

The last problem of this series concerns femtosecond laser ablation from gold nanoparticles [87]. In this process, solid material transforms into a volatile phase initiated by rapid deposition of energy. This ablation is nonthermal in nature. Material ejection is induced by the enhancement of the electric field close to the curved nanoparticle surface. This ablation is achievable for laser excitation powers far below the onset of general catastrophic material deterioration, such as plasma formation or laser-induced explosive boiling. Anisotropy in the ablation pattern was observed. It coincides with a reduction of the surface barrier from water vaporization and particle melting. This effect limits any high-power manipulation of nanostructured surfaces such as surface-enhanced Raman measurements or plasmonics with femtosecond pulses. 

Since the Raman scattering is not very efficient (only one photon in 107 gives rise to the Raman effect), a high power excitation source such as a laser is needed. Also, since we are interested in the energy (wavenumber) difference between the excitation and the Stokes lines, the excitation source should be monochromatic, which is another property of many laser systems. 

The second category comprises the flash photolysis experiments using the short high power light pulses from Q-switched lasers, furthermore all investigations of time-dependent behavior of excited dye molecules, which play an important role as active material in dye lasers or as saturable absorbers in passive Q-switched giant pulse lasers. 

The nitrogen laser (X = 3200 A)supplies a high-power nanosecond ultraviolet light source which allows in many cases direct initiation of photochemical reactions. Sousa 169) excited a 10 ... 

Figure 3 shows the 10-ns Raman spectra of Ni protoporphyrin in neat piperidine (>909S 6-coordinate form). The laser excitation wavelength of 420 nm preferentially excites the 6-coordinate form which happens to dominate the absorption at this wavelength (9-11). With respect to the low power spectrum, the spectrum obtained at high power shows increased intensity in the lines at 1660 ( q)>... 

Figure 2. Transient Raman spectra of Ni(PP) (60-80 ]iM) in pyrrolidine. Traces a) and b) were obtained with 406 nm excitation at high and low power, respectively, while traces c) and d) were generated with 420 nm excitation (which i.s a compromise frequency which resonantly enhances both species to some extent) at high and low power, respectively. For low power spectra the average laser power (at 10 Hz) was. 75-1.0 mW. The beam was only slightly focused onto the sample with a cylindrical lens. High power spectra were generated with 5-6 mW of average laser power sharply focused at the sample via a spherical lens. Spectra are the unsmoothed sum of 3-5 scans at 7-9 cm" spectral resolution.

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