Operation of Chemical Lasers

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 . 

The first cw HF lasers were operated in supersonic flows. This is deemed necessary for higher mass transport, smaller back diffusion from the reaction zone, and reduced collisional deactivation prior to emission. We will omit a description of the early stages of the development and present here some 

Power levels above 1 kW are reported. The efficiency of emission of chemical energy to laser power is 16% at low SFe flow rates and approximately 10% at peak power. Fig. 19 gives a schematic representation of some of the operational features. It is intuitively obvious that, in order to have an efficient laser, it is necessary that the rate of H2 diffusion into the jet and the rate of the pumping reaction be rapid compared with the rates of collisional deactivation. The performance of a corresponding DF laser has also been investigated 78 . The ratio of DF to HF laser power is 0.7 under similar flow conditions. The observed output spectra are reproduced in Table 13. It has been suggested that the lower DF efficiency is due to vibrational deactivation by N2. The efficiency and intracavity power of HF and DF is indeed the same with He as a diluent instead of N2. The efficiency of HF lasers with He and, with N2 carrier gases is compared in Fig. 20. 

Obviously SFe as a fluorine source can be substituted by molecular fluorine. An improved version of this laser would thus employ partly dissociated F2(T 1000°K) and a diluent in the plenum section. The F2 present in the jet will permit the reaction H + F2 - HF(v) + F, v 3, AH = — 98 kcal. The chemical potential of the H2 + F2 chain reaction 

Identification vibrational band ) Line Wavelength ( i) Identification vibrational band Line Wavelength (n) 

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A very interesting application of LIF is the measurement of population distributions N(v., J. ) in rovibronic molecular levels under situations which differ from thermal equilibrium. Examples are chemical reactions of the type AB + C - AC + B, where a reaction product AC with internal energy is formed in a reactive collision between the partners AB and C. The measurement of the internal state distribution N q(v,J) can often provide useful information on the reaction paths and on the potential surfaces of the collision complex (AB)C. The fact that for some of these reactions -population invevsion has been observed allowing the operation of chemical lasers [8.71] may elucidate the importance of such studies. A better knowledge of the reaction mechanisms can help to optimize the conditions for maximum inversion. 

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