HF lasers

Richeboeuf, L., Pasquiers, S., Legentil, M. et al. (1998) The influence of H2 and C2H6 molecules on discharge equilibrium and F-atom production in a phototriggered HF laser using SF6, J. Phys. D Appl. Phys. 31, 373-89. 

Spencer et al. 4I0)obtained 475 watt at 3 jam in a cw HF laser based Upton the reaction ... 

Different theoretical models used to predict the extent 621 of IRMPD in CFjCHjOH induced by HF laser excitation. All models found to predict fraction of decomposed molecules to within a factor of 3—4 of the experimental determinations... 

To initiate and sustain this sequence, high flow rates of halogen atoms may be derived from the types of combustor used in chemical HF lasers. Another promising reaction for generation of NF(a) is ... 

Perhaps the most likely immediate commercial application of carbonyl fluoride, however, arises from its spectroscopic properties. Irradiation of mixtures of COF, and H, (or D,), over a wide pressure range and at ambient temperature, with the multiline output of a continuous wave CO, laser, results in the generation of excited state HF (DF) which lases [1387]. Energy transfer from the R, line (970 cm" ) of CO, (which is close in energy to the c, band of COF,) causes the dissociation of the COF, to CO and two excited state fluorine atoms which subsequentiy react with the dihydrogen (or dideuterium). However, COF, itself has been found to effect rapid vibrational de-excitation of HF [239], an observation that suggests that the COF,/H, route to the HF laser may be of limited practicality. 

UF8 + H2(D2, HD)/hv First report of a flash initiated HF laser, empirical study of experimental parameters, quenching by various additives, spectra Kompa, Pimentel Kompa, Parker, Pimentel 122>... 

CF4(CBrF3, CCIF3, CC12F2) + Ha (Da, CH4, CH3CI) / e Cla (Bra) + H2(D2)/e First report of the discharge-initiated chemical HF laser, spectra, parameter study, rotational emission of HF, HC1 Deutsch 123.193.177)... 

Chemical-laser emission spectra up to 1967 have been compiled by Patel 6 ). There is some inconsistency between HF laser spectra obtained in different laboratories and with different experimental set-ups. This is probably due in part to the absorption of several HF lines by the atmosphere inside or outside the laser cavity. However, there is additional inconsistency between the emission spectra of infrared chemiluminescence experiments and HF chemical lasers. While spontaneous luminescence predicts a peak in the v=2 - 1 transitions around /=614>, the laser emission usually... 

Table 9. Measured wavelengths, identification, and peak powers of HF laser transitions observed in flash photolysis of Hg and F2132>...

Pure rotational HF laser emission can be produced under suitable conditions, as Table 11 shows. It is not dear at present, whether there... 

HF rotational laser emission has also been obtained by Skribanovitz et al. 73> by pumping the first vibrational state of HF gas resonantly with another pulsed HF laser. Pumping the P-branch transitions of v =0 1... 

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. 

It is somewhat difficult to compare these predictions with experimental results since no really systematic experimental study has yet been published. This is due in part to difficulties in preparing mixtures of H2 and Fa of any desired composition and pressure and also to experimental limitations in the sufficiently rapid initiation of the pumping reaction. However, as far as the experimental information goes, it can be concluded that the efficiency is considerably lower than expected. For instance, in flash photolysis-initiated HF lasers a chemical efficiency of below 1% is usually found 101>. Two suggestions may be made to explain this discrepancy. One may look at it as either a chemical rate problem or a laser problem. In the first case, some unknown rate process must be assumed to reduce the build-up of excited HF. Since the formation and deactivation rates are known with some accuracy, this could only be excessive recombination or an unusually high rate of the reverse reaction 102>. Alternatively, parasitic oscillations or superradiance have been claimed to cause radiation losses in off-axis... 

Since this condition is satisfied during the decay of the HF laser emission in certain cases (fast photolysis, low pressure), the time profile of the output intensity reflects the time dependence of the pumping. The pumping term P (t) can be described solely as the formation of the inversion by the reaction as long as losses due to vibrational deactivation are negligible. 

Comprehensive studies have been carried out on the MPD of hexafluoroacetone as a function of laser fluence, frequency and substrate pressure, and the influence of collisional effects on the formation of CF2 in the MPD of CF2CFCI and CFjHCl has been experimentally measured and theoretically modelled. Fluence dependences of the HF laser-induced decomposition of 2,2,2-trifluoroethanol, and the 9.4 pm CO2 laser MPD of C2F3C1 have been reported, and triethylphosphite joins the increasing list of molecules dissociated by CO2 laser radiation. ... 

Fig. 7. Time evolution of the vibrational distribution of the frictionless (no relaxation) HF laser. Lasing terminates (arrow) when the system is still energy-rich.

Although the formidable difficulties associated with isotope separation schemes based on photochemical vibrational excitation plus chemical reaction continue to attract considerable attention, some earlier hopes appear to have been dashed. An experiment performed in 1970 by Mayer et al. has been much quoted. They reported irradiating mixtures of CHjOH and CD3OD in the presence of Bts with lines from an HF laser that are absorbed only by CH3OH. Product analysis indicated the selective depletion of the CH3OH. This observation was interpreted in terms of a selective reaction between vibrationally excited CH3OM and Bra, nnd it appeared to point the way to an economic method for the production of heavy water. However, the results of a careful re-examination of this system have just... 

The first chemical laser was demonstrated in 1965 by Kasper and Pimentel by initiating a hydrogen-chlorine explosion with a flashlamp. By 1967, lasers based on the reaction of atomic fluorine with molectrlar hydrogen and other molectrles had been developed. Several key experiments were performed in 1969, and lasing was achieved in an HF supersonic diSusion flame by Spencer and coworkers. Subsequent development oftheHF chemical laser was rapid and by 1984 HF lasers with powers exceeding 1 MW had been developed. 

FIGURE 3 (a) Simulated spectrum of an HF laser showing several discrete laser wavelengths, (b) The transitions between ro-vibrational levels are labeled according to spectroscopic notation. 

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