CO, laser

EIOs), backward wave oscillators (BWOs) or magnetrons are available. Their spectral characteristics may be favourable however, they typically require highly stabilized high-voltage power supplies. Still higher frequencies may be obtained using far-infrared gas lasers pumped for example by a CO- laser [49]. 

Legay-Sommaire N and Legay F 1980 Observation of a strong vibrational population inversion by CO laser exoitation of pure solid oarbon monoxide IEEE J. Quantum Electron. 16 308-14... 

Another feature of many (though not all) lasers is a cooling system to dissipate energy wasted m the form of heat. Although some are much more efficient than others, all lasers waste significant amounts of energy. For example, a CO, laser (the most popu-... 

Epidermal growths such as actinic keratosis, lentigines or thin seborrheic keratoses can all be treated effectively with 25-35% TCA peels. Thicker epidermal growths or growths involving the dermis will be more resistant to treatment such as hypertrophic actinic keratoses and thicker seborrheic keratoses and may even be resistant to a medium-depth peel. Resistant lesions many times are best treated with a combination of a medium-depth chemical peel and other modalities such as manual dermasanding or CO, laser. 

The pump source in the dispersive experiment is typically a nanosecond laser the probe source can be broadband IR light from a globar or tunable IR light from a CO laser or a semiconductor diode laser. Although CO and diode lasers can produce... 

Four different laboratories have built IR kinetic spectrometers for use with organometallic compounds. A fundamental feature of all these spectrometers is that the detector is AC coupled. This means that the spectrometers only measure changes in IR absorption. Thus, in the time-resolved IR spectrum, bands due to parent compounds destroyed by the flash appear as negative absorptions, bands due to photoproducts appear as positive absorptions, and static IR absorptions, due to solvents, for example, do not register at all. The important features of these spectrometers are listed in Fig. 2. Since three spectrometers have a line-tunable CO laser as the monochromatic light source, we begin with the CO laser. Then we look in more detail at spectrometers designed for gas phase and solution experiments. 

Northwestern c.w. CO laser (2060-1800 cm 1) excimer later InSb 30 nsec... 

The IR probe radiation, provided by a liquid N2-cooled line-tunable CO laser, is passed through the cell collinearly or almost collinearly with the uv photolysis radiation. The probe beam is then directed onto an IR detector using a filter for either selectively blocking the photolysis beam or... 

Fig. 6. Schematic diagram of the Nottingham apparatus for IR kinetic measurements on solutions. Solid lines represent the light path, broken lines the electrical connections. L = Line tunable CO laser, S = sample cell, F = flash lamp, P = photodiode, D = fast MCT IR detector, T = transient digitizer, O = oscilloscope, and M = microcomputer. Nonfocussing optics were used throughout, and the IR laser beam was heavily attenuated by a variable path cell V, filled with liquid methanol, placed immediately in front of the detector. [Reproduced with permission from Moore et al. (61).]...

Stability may not be as much of a problem as with a diode source. However, there are problems with this method as well. The range of tunability is limited by the absorption properties of the nonlinear crystal which generates the difference frequency. At present, tunability is limited to wavenumbers >2500 cm-1 and conversion efficiencies are low. Typical laser powers in the CH2 experiments (82) were 20 n W (compared to the power of the CO lasers, 10 mW-1 W). This produces a situation where IR detectors, particularly fast ones, may be close to or background noise limited. However, it is clear that more applications of this technique will appear in the future. 

The photolysis of Cr(CO)6 also provides evidence for the formation of both CO (69) and Cr(CO) species (91,92) in vibrationally excited states. Since CO lasers operate on vibrational transitions of CO, they are particularly sensitive method for detecting vibrationally excited CO. It is still not clear in detail how these vibrationally excited molecules are formed during uv photolysis. For Cr(CO)6 (69,92), more CO appeared to be formed in the ground state than in the first vibrational excited state, and excited CO continued to be formed after the end of the uv laser pulse. Similarly, Fe(CO) and Cr(CO) fragments were initially generated with IR absorptions that were shifted to long wavelength (75,91). This shift was apparently due to rotationally-vibrationally excited molecules which relaxed at a rate dependent on the pressure of added buffer gas. 

Thus, the region 2100-1830 cm 1 can be covered. This allows us to monitor CO(v,J) by resonance absorption and various M(CO)n [n = 3-6] as a result of near coincidences between the CO laser lines and the carbonyl stretching vibrations of these species. The temporal response of the detection system is ca. 100 ns and is limited by the risetime of the InSb detector. Detection limits are approximately 10 5 torr for CO and M(CO)n. The principal limitation of our instrumentation is associated with the use of a molecular, gas discharge laser as an infrared source. The CO laser is line tuneable laser lines have widths of ca. lO cm 1 and are spaced 3-4 cm 1 apart. Thus, spectra can only be recorded point-by-point, with an effective resolution of ca. 4 cm 1. As a result, band maxima (e.g. in the carbonyl stretching... 

Figure 1. Time resolved absorption of the CO laser P 1,0(10) line following the 351 nm photolysis of W(CO)g. [W(CO)g] = 0.025 toir, [He] = 4.0 toir.

Conclusions. Time-resolved CO laser absorption spectroscopy can provide information useful in characterizing the primary photochemical channels in gas-phase transition metal carbonyls. We have found that product vibrational energy distributions indicate that W(CO)g and Cr(CO>6 dissociate via different... 

Under conditions similar to that used with CO, laser-evaporated Be atoms and C02 react during condensation in excess argon to form CO, ArBeO, BeOBe, and two new molecules in a 29 1 ratio that display CO and BeO stretching absorptions, identified as OC-BeO and the isocarbonyl CO-BeO (Equation (3)). 

Even more complex experiments have been performed on matrix isolated Fe(C0)4, generated by UV photolysis of Fe(C0)5. Isotopic labelling coupled with CW-CO laser pumping (65) of the CO stretching vibrations ( 1900 cnrl) showed that the rearrangement mode of Fe(C0)4 follows an inverse Berry pseudo-rotation as shown in Figure 8. 

Figure 8, The observed IR-laser induced isomerizations of Fe(12C160) 13C180)x species in an Ar matrix. X represents the 13C180 group, and the numbers represent the wavenumbers of the CO laser lines that induce the particular isomerizations.

Figure 9. OA signal vs. step-tuned CO laser wavelength for 100 ppM NO, in nitrogen (b) OA sigruil vs. step tuned CO laser wavelength for 1000 ppM NH, in...

Figure 10. Steptuned COg/CO laser-OA pollution detection spectrometer...

The photolysis of CH2CO at 193 nm has also been studied by Fujimoto et al. (50). A CO laser was used to probe the vibrational excitation of the CO produced. The product CO contains an average vibrational energy of 6.4 kcal/mol. The vibrational population distribution corresponds to about 4000 K in good agreement with Sonobe and Rosenfeld (49,50). 

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