Types of Lasers

The so-called peak power delivered by a pulsed laser is often far greater than that for a continuous one. Whereas many substances absorb radiation in the ultraviolet and infrared regions of the electromagnetic spectrum, relatively few substances are colored. Therefore, a laser that emits only visible light will not be as generally useful as one that emits in the ultraviolet or infrared ends of the spectrum. Further, witli a visible-band laser, colored substances absorb more or less energy depending on the color. Thus two identical polymer samples, one dyed red and one blue, would desorb and ionize with very different efficiencies. 

---

Apart from the obvious property of defining pulses within short time intervals, the pulsed laser radiation used in reaetion kineties studies ean have additional partieular properties (i) high mtensity, (ii) high monoehromatieity, and (iii) eoherenee. Depending on the type of laser, these properties may be more or less pronouneed. For instanee, the pulsed CO2 lasers used in IR laser ehemistry easily reaeh intensities between... 

Historically, the first type of laser to be tunable over an appreciable wavelength range was the dye laser, to be described in Section 9.2.10. The alexandrite laser (Section 9.2.1), a tunable solid state laser, was first demonstrated in 1978 and then, in 1982, the titanium-sapphire laser. This is also a solid state laser but tunable over a larger wavelength range, 670-1100 nm, than the alexandrite laser, which has a range of 720-800 nm. 

There are many types of lasers, having a wide variety of methods of constmction and based on many different classes of materials. The properties of some commercially available lasers are summarized in Table 1. Typical available characteristics are given. More detailed compilations of the properties of commercially available lasers are available (20,21). 

Other Lasers. There are two other types of lasers which as of this writing are not at the same stage of maturity as those already discussed. 

Free-Electron Lasers. The free-electron laser (EEL) directly converts the kinetic energy of a relativistic electron beam into light (45,46). Relativistic electron beams have velocities that approach the speed of light. The active medium is a beam of free electrons. The EEL, a specialized device having probably limited appHcations, is a novel type of laser with high tunabiHty and potentially high power and efficiency. 

The He-Ne laser has a stable, narrow-beam output at 632.8 nm in the red region. This type of laser typically has short term peak-to-peak fluctuation of less than 1%. Its long wavelength enables the studies of colorless materials and colored red-transmitting materials. The 632.8 nm line has a maximum output about 80 mW. 

Advances in laser technology now allow for solid-state lasers of high beam quality. These beams may be projected from a much smaller auxiliary telescope, which negates the need for optical switching and completely eliminates any main telescope fluorescence. Solid-state YAG lasers are the most common type of lasers commercially available. These lasers use a crystal as the lasing... 

Since there are a large number of different experimental laser and detection systems that can be used for time-resolved resonance Raman experiments, we shall only focus our attention here on two common types of methods that are typically used to investigate chemical reactions. We shall first describe typical nanosecond TR spectroscopy instrumentation that can obtain spectra of intermediates from several nanoseconds to millisecond time scales by employing electronic control of the pnmp and probe laser systems to vary the time-delay between the pnmp and probe pnlses. We then describe typical ultrafast TR spectroscopy instrumentation that can be used to examine intermediates from the picosecond to several nanosecond time scales by controlling the optical path length difference between the pump and probe laser pulses. In some reaction systems, it is useful to utilize both types of laser systems to study the chemical reaction and intermediates of interest from the picosecond to the microsecond or millisecond time-scales. 

Monochromatic light can also be obtained from other types of lasers solid state, gas, ion, dye. Among them argon ion laser with its many lines is an especially valuable light source used in many sensors. However, these types of lasers are expensive, the modulation of the light cannot be done internally and external modulators (e.g. choppers) should be used. Wavelengths emitted by some exemplary lasers are presented in Table 1. 

Depending on the type of laser used, this energy can come from a source of light, an electrical discharge or via chemical reactions. 

The type of laser source that can be used is exactly the same as for single-photon counting pulse fluorometry (see above). Such a laser system, which delivers pulses in the picosecond range with a repetition rate of a few MHz can be considered as an intrinsically modulated source. The harmonic content of the pulse train - which depends on the width of the pulses (as illustrated in Figure 6.11) - extends to several gigahertz. 

One final type of laser resonator, which is also applicable for molecular glasses, should be mentioned The random laser, based on coherent backscat-tering in an amplifying medium [212, 213]. In these structures, strongly scattering nanoparticles like Ti02 colloids are randomly dispersed in the amorphous films leading to self-contained optical paths and thus to the localization of optical modes. Since disordered structures are much easier to produce than ordered... 

Nd YAG and other types of lasers can be used in air to synthesize conductors and semiconductors as a second phase in SiC and AIN (Quick 1995). 

Laser radiation can be obtained nowadays over a wide spectral range from the ultraviolet to the far infrared region, covering the range of optical spectroscopy. Fignre 2.4 shows schematically the spectral zones covered by different types of lasers. Although there are some specific regions in which direct laser action is not available. 

Figure 2.4 The spectral regions covered by different types of lasers ( c. centers stands for color center lasers).

Due to the large variety of laser materials and pumping methods, it is almost impossible to catalog all the laser devices that have been demonstrated up to date. However, we can make a classification of the laser systems based on the different types of active media. We will briefly comment on the basis and properties of some specific types of laser systems, which are representative of different laser schemes. 

In this type of Laser, the active medium consists of an inert gas (X) or of a mixture of an inert gas and a halide gas (X + Y). The term excimer stands for excited dimmer which refers to a diatomic molecule of two inert gas atoms (XX) or a molecule of an inert gas atom and a halide gas atom (XY). ... 

Excimer lasers are of great importance for UV and vacuum UV (VUV) spectroscopy and photochemistry. They are also found in a wide range of applications. For example, they are used in micromachine medical devices, including refractive surgery, in photo-lithography for the microelectronics industry, for material processing, as optical pump sources for other type of lasers (dyes), and so on. More details about excimer lasers can be found in Rodhes (1979). 

In gases (atomic or ionic) the electronic energy levels of free atoms are narrow, since they are diluted systems and perturbation by the surroundings is very weak. An important fact derived from the discrete nature of the electronic levels in a gas is the high monochromaticity of the laser lines in this type of laser, compared to that of solid-medium based lasers. The high degree of coherence achievable with gas lasers is also a characteristic feature related to the narrow linewidth. 

---