Laser chemical pumping

Because of the tunabiUty, dye lasers have been widely used in both chemical and biological appHcations. The wavelength of the dye laser can be tuned to the resonant wavelength of an atomic or molecular system and can be used to study molecular stmcture as well as the kinetics of a chemical reaction. If tunabiHty is not required, a dye laser is not the preferred instmment, however, because a dye laser requires pumping with another laser and a loss of overall system efficiency results. 

A chemically pumped CO2 laser oscillating at 10 p was reported by GrossIn this system vibrationally excited COj molecules are produced by inelastic collisions with vibrationally excited DF which was formed by ultraviolet photolysis of a F2O-D2 mixture with a Xe flashlamp, producing free fluorine atoms which could react with Dj... 

F+Ha HF- +. H AH =-139.9 kj is also exothermic and can produce energy rich HF molecules. The heat of chemical reaction is distributed in various vibrational-rotational modes to give vibrationally excited HF or HC1 in large numbers. Emission from these hot molecules can be observed in the infrared region at h 3.7 (j-m. The reaction system in which partial liberation of the heat of reaction can generate excited atoms or molecules is capable of laser action (Section 3.2.1). They are known as chemical lasers. The laser is chemically pumped, without any external source of radiation. The active molecule is born in the excited state. Laser action in these systems was first observed by Pimental and Kasper in 1965. They had termed such a system as photoexplosion laser. 

Figure 11-3. Effect of the pump laser at 250 nm with a time advance of 10 ns on the one laser REMPI spectrum of the probe laser [15]. Both spectra were recorded using the same intensity for the probe laser. The pump laser in the two laser experiment resulted in a maximum depletion of 20-25% in the region between 245 and 280 nm, and an enhancement of three decades at 220 nm. See text for a detailed explanation of the experimental method. (Reproduced with permission from J. Phys. Chem. 2004, 108, 943-949. Copyright 2004 American Chemical Society.)...

The enthalpy of formation of the azide radical is 467 SkJmoR. The spin-allowed dissociation to N( D) and N2(X 1 +) is endoergic by 225kJmol, the dissociation enthalpy to N( S) - -N2(X i +) is 0.5 IkJmol. The azide radical is only stable because this spin-forbidden decomposition pathway has an appreciable energy barrier. In aqueous solution, it primarily exists as a monomer, in contrast to other halide or pseudohaUde radicals that exist as the less reactive dimers (e. g. Brs (SCN)2 ). Reaction ofthe azide radical with halogen atoms or other small molecules hke O2, NO, CO, and CO2 produces molecules in electronically excited states because of propensity rules, which can be used for chemically pumped lasers. The azide ion is also formed during high-pressure photolysis of sodium azide. 

In this chapter, I will consider chemically pumped electronic transition lasers that are based on energy transfer from 02(a) or NX(a) metastables, focusing on the energy transfer and reaction kinetics of these devices. As the... 

The potential for using NCl(a) in a chemically pumped laser was first examined by Benard et al. These investigators used pulsed CO2 laser pyrolysis of ClNs/SFe/Ar mixtures to generate high concentrations (> 10 cm ) of NCI (a). From the rate of NCI (a) decay in this system they estimated an upper limit for the NCl(a) self-annihilation rate constant of > 8 x Energy pooling between NCI (a) and NF(a)... 

Chemical lasers are pumped by reactive processes, whereas in photodissociation lasers the selective excitation of certain states and the population inversion are directly related to the decomposition of an electronically excited molecule. Photolysis has been the only source of energy input employed in dissociation lasers, although it appears quite feasible to use other energy sources, e.g. electrons, to generate excited states. Table 4 lists the chemical systems where photolysis produces laser action. It is appropriate to begin the discussion of Table 4 with the alkali-metal lasers since Schawlow and Townes in 1958 35> chose the 5 f> 3 d transitions of potassium for a first numerical illustration of the feasibility of optical amplification. These historical predictions were confirmed in 1971 by the experimental demonstration of laser action in atomic potassium, rubidium and cesium (Fig. 14). 

CSg + O2 / hv First report of a chem. CO laser, spectroscopy, chemical pumping scheme Pollack 200)... 

Earlier than with pulsed chemical lasers, the first technological breakthrough in chemical lasers occurred for continuous-wave lasers. Almost simultaneously in 1968 two groups successfully operated continuous-wave chemical lasers. One was at the Aerospace Corporation headed by T. A. Jacobs 75>, the other one at Cornell University under T. A. Cool 76>. One of these lasers was an HF laser the other was that is now called a hybrid chemical laser, being pumped by energy transfer rather than by a direct chemical reaction. This laser principle has been described in the context of pulsed chemical lasers in Section 6.5, In addition to these devices, an HF cw laser having millisecond flow duration was also demonstrated in principle in a shock tunnel. The latter employed diffusion of HC1 into a supersonic stream containing F atoms 77>. 

Figure 4.17 EKasic silicon carbide laser-structured sliding bearings used for example, in highly loaded chemical pumps, in magnetic couplings for hermetically sealed pumps and in...

Lasers, Chemical Lasers, Color Center Lasers, Dye Lasers, Free Electron Lasers, Gas Lasers, Nuclear Pumped Lasers, Optical Fiber Lasers, Semiconductor Lasers, Solid State Lasers, Ultrafast Pulse TLchnology Lasers, X-Ray... 

Another key metastable energy carrier is NCl(a A), which is isovalent with NF(a A) and 02(0 A). The discoveiy of the efficient energy transfer from NCl(a A) to atomic iodine opened the door to an all-gas-phase, chemically pumped atomic iodine laser operating at the same wavelength as COIL. Indeed, a subsonic transverse flow device... 

Other applications of F2 lasers are optical pumping of Br and OCS Raman lasers, NO laser, and solid state NdiLaFs laser. Using the high photon energy and short wavelength, photolithography, laser chemical vapor deposition (C VD), laser ablation, etc. are demonstrated. 

It is often possible to use lasers to pump and probe a chemical reaction simultaneously. In other words, it is... 

The ultimate molecular laser is, of course, one with no recourse to electrons, ions, or discharges but is purely chemically pumped. These chemical schemes have worked well for infrared lasers and for the 02( A) - iodine system. However, the... 

Davis and co-workers clearly demonstrated that the IF(B -> X) system will lase if a suitable chemical pump source can be found [18,19]. In a pulsed optically pumped laser containing 10 Torr of He they found that IF lased on the (v, J )... 

The reaction system in which partial liberation of heat of reaction can generate excited atoms or molecules are capable of laser action. This laser in known as chemical laser. The laser is chemically pumped, with out any external source of radiation. 

Figure 3.20 summarizes familiar relationships for laser operation on a Doppler-broadened spectral line. The fluorescence rate at laser threshold, KJts, is proportional to A" which necessitates a larger chemical pumping rate at visible wavelengths than is necessary for near-infrared operation. The condition that the chemical pumping rate to the laser upper state exceed the threshold fluorescence rate provides an estimate of the minimum required pump rate. This relationship is outlined in Figure 3.21 for simple atom-exchange reactions of the type A + BC -> AB(7, v, J) + C, where the letters (/, v, J) denote the states of electronic, vibrational, and rotational excitation, respectively, of the diatomic product molecule AB. The fraction,... 

As the considerations of Section 3.4.1 demonstrate, an important requirement for either pulsed or cw electronic transition chemical laser operation is the achievement of large metal atom or free-radical concentrations. Moreover, these reagent concentrations must be made available in a manner that ensures that the chemical pumping rate of the upper laser level is large enough compared with the rates of the competing processes of collisional quenching and radiative, decay that an upper level population is created sufficient to exceed laser threshold requirements. 

Figure 3.37. Schematic illustration of potential energy curves for a hypothetical chemically pumped excimer laser.

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